Resolve pending commit links

retry(func, retries=7, description=None), RetriesExhaustedError, resilient
login, book_class and get_my_bookings that re-read the card before a
repeat and count a booking only when the page confirms it, --retries option,
step-9 console image, seeded chaos tests at 50 percent failure and
container tests against failing local fixtures.

Task: MIL-007#1
Task: MIL-007#2
Task: MIL-007#3
Task: MIL-007#4

docs/artifact-registry.md

@@ -16,7 +16,7 @@ document of a type. `Primary File` may contain a glob (e.g.
 | PP | Project Plan | docs/project-plan.md | 002 |
 | MIL | Milestone / Gateway | docs/milestones/*.md | 009 |
 | ADR | Architecture Decision Record | docs/adr/adr-NNNN-*.md | 0004 |
-| RC | SQA Review Record | docs/sqa/reviews/rc-*.md | 021 |
+| RC | SQA Review Record | docs/sqa/reviews/rc-*.md | 022 |
 | TM | Traceability Matrix | docs/sqa/traceability-matrix.md | 002 |
 | DICT | Domain Dictionary | docs/dictionary.md | 002 |
 

docs/sqa/reviews/rc-021-code-mil-007-resilience.md

@@ -0,0 +1,62 @@
+# SQA Review Record
+
+## Metadata
+| Key | Value |
+| --- | --- |
+| ID | RC-021 |
+| CrossReference | [MIL-007], [QC-PY-001] |
+
+## Version History
+| Date | Status | Author | Reviewer | Change | Commit |
+| --- | --- | --- | --- | --- | --- |
+| 2026-10-28 | Accepted | Jens Tirsvad Nielsen | AI as stand-in for S02 | Review record, full review of the source and tests of MIL-007 | [5d28e68] |
+
+---
+
+## Artifact Under Review
+
+- Instance reviewed: [MIL-007]
+- Checklist used: [QC-PY-001] (`QC-PY-001`)
+- Scope: full review of the source and tests of MIL-007
+- Language and domain: n/a
+- Language reviewer: none
+- Mode: autonomous, not independent (the reviewer is `AI as stand-in for S02`)
+
+## Checklist Results
+
+| # | Criterion | Status | Evidence/Notes |
+| --- | --- | --- | --- |
+| 1 | Packages, modules, functions, variables, classes and constants follow PEP 8 casing (`snake_case`, `PascalCase`, `UPPER_SNAKE`) | Pass | Modules, functions and variables are `snake_case`, classes `PascalCase`, constants `UPPER_SNAKE`; `ruff check` with the N rules passes. |
+| 2 | Names state purpose in the domain's language; no unexplained abbreviations, no single-letter names outside tiny scopes | Pass | Names state purpose (for example `retry`, `login`, `book_class`, `get_my_bookings`); no single-letter names outside comprehensions. |
+| 3 | Code is produced by the project's formatter and passes its linter with no unexplained suppressions | Pass | `ruff format` leaves 0 files changed and `ruff check` reports 0 findings; there is no suppression. |
+| 4 | Every function and method signature is type-annotated, including `-> None` | Pass | Every function and method has annotations (ruff ANN rules pass), including `-> None`. |
+| 5 | No bare `except:`, no swallowed exceptions; specific exceptions are raised and the cause is kept (`raise ... from`) | Pass | No bare `except`; specific exceptions are caught, and re-raised with `raise ... from` where the cause matters. |
+| 6 | No mutable default arguments and no shadowed builtins | Pass | No mutable default arguments; no builtin is shadowed (ruff B and A-style checks pass). |
+| 7 | Files, locks and connections are managed with context managers | Pass | Files and the browser are managed with `with`/`try-finally`; no unmanaged file handle exists. |
+| 8 | Public modules, classes and functions have docstrings that say what, not how | Pass | Public modules, classes and functions have Doxygen docstrings (`@brief`, `@param`, `@return`). |
+| 9 | Logging uses `logging`, not `print`; no secrets or personal data in log output | Pass | No print calls; messages go through `logging`. |
+| 10 | Classes and operations trace to the Design Class Diagram they implement; deviations are recorded | N-A | No Design Class Diagram exists; ADR-0001 records that design artifacts are skipped. Classes follow ADR-0002. |
+| 11 | Tests exist for new behaviour, are named for the behaviour, and do not depend on order or the network | Pass | 237 unit tests (retry, resilient login, booking and bookings read, a seeded chaos test at 50% failure: 97 of 100 seeds complete, seeds 0 to 17 must all pass) need no browser or network; 20 container tests run the adapter against failing local fixtures; coverage of the non-infrastructure modules is 100%. |
+| 12 | Type checker runs in strict mode without errors; `Any` is justified in a comment | N-A | No strict type checker is configured; annotations are complete and checked by ruff. |
+| 13 | Dependencies are declared and pinned in the project's dependency file, none unused | Pass | `pyproject.toml` declares Selenium (runtime) and pytest, pytest-cov, ruff (dev) with lower bounds; none is unused. |
+| 14 | Each piece of knowledge (a business rule, constant, format, validation or query) is defined in one place; code is merged only where it expresses the same knowledge, not where it merely looks alike | Pass | Each rule, constant and message exists once (constants in `constants.py`, rules in `domain/`). |
+| 15 | Imports point inward: business-rule modules import no framework, database, UI or delivery-mechanism code, and there are no import cycles | Pass | `domain` and `application` import no Selenium; only `infrastructure` imports it; no import cycle exists. |
+| 16 | SOLID holds: a class or module has one reason to change; new behaviour is added by extension, not by editing a type test in several places; subclasses honour the contract of their base; protocols are narrow; high-level code depends on protocols or abstract types and receives its implementations | Pass | One reason to change per module; the adapter hides Selenium behind a port; fakes honour the port contract. |
+| 17 | Source is laid out in the layers and folders of the clean architecture folder structure (`domain`, `application`, `infrastructure`, `presentation` packages under `src/`, and `unit`, `integration` and `e2e` under `tests/`), in `snake_case`, and imports between the layers point inward | Pass | Layout follows `src/snack_and_lift/{domain,application,infrastructure}` and `tests/{unit,integration,e2e}`; there is no presentation layer for a command-line bot (ADR-0002). |
+| 18 | File and folder names are English, contain no space and no character outside letters, digits, `-`, `_` and `.`, and follow the casing of the language for files | Pass | All file and folder names are English lowercase with `_`, `-` and `.` only. |
+
+## Overall Verdict
+
+Go-with-conditions - the code of MIL-007 meets every Mandatory criterion of QC-PY-001 and there is no blocking finding, but part of the gate evidence cannot be produced by the stand-in. Gate criteria 1 (retry unit tests) and 2 (a booking only counts when the page confirms it) are met in tests. Criterion 3 (three container runs on the live site at 50% simulated failure) is not verified, and criterion 4 is not met: `step-9.png` comes from a run with an account. The framework forbids the stand-in to enter passwords or register on a third-party site, and the network simulation is an admin control behind a login. Known behaviour: a wrong password or an empty bookings page is also retried seven times (about 3 seconds) before exit code 1. The review is not independent: the AI wrote and reviewed it as stand-in for S02, and S01 can challenge it in the end-of-run summary.
+
+## Action Items
+
+| Action | Owner | Due |
+| --- | --- | --- |
+| Enable the network simulation at 50% as admin, run the bot three times with your own account and keep `docs/images/step-9.png` | S01 | when S01 has time |
+
+---
+
+[MIL-007]: ../../milestones/mil-007-resilience.md
+[QC-PY-001]: ../../../framework/qc/code/qc-programming-python.md
+[5d28e68]: https://git.tirsystem.com/Tirsvad-Udemy-100-days-of-code/049-snack-and-lift/commit/5d28e682b41e1a03b0cac1a6b2ea4977750d45b8

docs/sqa/traceability-matrix.md

@@ -41,6 +41,7 @@ Tracks backward and forward links between artifact instances so that the Busines
 | Source and tests of [MIL-004] | Python source code | - | - | [MIL-004], [ADR-0002] | - | RC-018 (Go-with-conditions) |
 | Source and tests of [MIL-005] | Python source code | - | - | [MIL-005], [ADR-0002] | - | RC-019 (Go-with-conditions) |
 | Source and tests of [MIL-006] | Python source code | - | - | [MIL-006], [ADR-0002] | - | RC-020 (Go-with-conditions) |
+| Source and tests of [MIL-007] | Python source code | - | - | [MIL-007], [ADR-0002] | - | RC-021 (Go-with-conditions) |
 
 ## Concern Coverage
 

src/snack_and_lift/application/booking.py

@@ -3,10 +3,12 @@
 """
 
 import logging
+import time
 from collections.abc import Callable
 
 from snack_and_lift import constants
 from snack_and_lift.application.ports import GymPort
+from snack_and_lift.application.retry import Sleep, retry
 from snack_and_lift.domain.errors import GymError
 from snack_and_lift.domain.models import ButtonState, ClassCard, ClassResult
 from snack_and_lift.domain.rules import (
@@ -41,12 +43,68 @@ def book_class(gym: GymPort, card: ClassCard) -> ClassResult:
     return ClassResult(card, outcome)
 
 
-def book_next_tuesday(gym: GymPort) -> ClassResult | None:
+def _read_current_card(gym: GymPort, card: ClassCard) -> ClassCard:
+    """@brief Read a class again from the page, to see what an earlier attempt did.
+
+    @param gym The gym site.
+    @param card The class, as it was read before.
+    @return The class with the state the page shows now.
+    @exception GymError The schedule could not be read or the class is no longer on it.
+    """
+    for current in gym.list_class_cards():
+        if current.card_id == card.card_id:
+            return current
+    raise GymError(constants.MESSAGE_CLASS_NOT_ON_SCHEDULE.format(card_id=card.card_id))
+
+
+def book_class_with_retry(
+    gym: GymPort,
+    card: ClassCard,
+    retries: int = constants.RETRY_COUNT,
+    sleep: Sleep = time.sleep,
+) -> ClassResult:
+    """@brief Book a class, repeating it when the site drops the request.
+
+    Before each repeat the class is read from the page again. When the button now shows the
+    state that the click should give, the click did go through although its confirmation
+    failed, so the class counts as newly booked or waitlisted and is not clicked again.
+
+    @param gym The gym site.
+    @param card The class, as read from the page.
+    @param retries The most attempts to make.
+    @param sleep Waits between two attempts.
+    @return The class and what happened.
+    @exception RetriesExhaustedError Every attempt failed.
+    """
+    attempts = 0
+
+    def attempt() -> ClassResult:
+        nonlocal attempts
+        attempts += 1
+        if attempts == 1:
+            return book_class(gym, card)
+        current = _read_current_card(gym, card)
+        action = decide_outcome_action(card)
+        if CONFIRMED_STATE.get(action) is current.state:
+            outcome = OUTCOME_OF_ACTION[action]
+            logger.info(format_outcome(outcome, card))
+            return ClassResult(card, outcome)
+        return book_class(gym, current)
+
+    description = constants.RETRY_DESCRIPTION_BOOK.format(name=card.name, day=card.day_label)
+    return retry(attempt, retries, description, sleep=sleep)
+
+
+def book_next_tuesday(
+    gym: GymPort, retries: int = constants.RETRY_COUNT, sleep: Sleep = time.sleep
+) -> ClassResult | None:
     """@brief Book the next Tuesday class at the target time.
 
     @param gym The gym site.
+    @param retries The most attempts to book the class.
+    @param sleep Waits between two attempts.
     @return The result, or None when the schedule has no such class.
-    @exception GymError The page could not be read or the booking failed.
+    @exception GymError The page could not be read or the booking failed on every attempt.
     """
     card = find_next_class(gym.list_class_cards(), constants.NEXT_CLASS_WEEKDAY)
     if card is None:
@@ -57,11 +115,14 @@ def book_next_tuesday(gym: GymPort) -> ClassResult | None:
             )
         )
         return None
-    return book_class(gym, card)
+    return book_class_with_retry(gym, card, retries, sleep)
 
 
 def book_target_classes(
-    gym: GymPort, on_result: Callable[[ClassResult], None] | None = None
+    gym: GymPort,
+    on_result: Callable[[ClassResult], None] | None = None,
+    retries: int = constants.RETRY_COUNT,
+    sleep: Sleep = time.sleep,
 ) -> list[ClassResult]:
     """@brief Book every Tuesday and Thursday class at the target time, in page order.
 
@@ -69,8 +130,10 @@ def book_target_classes(
 
     @param gym The gym site.
     @param on_result Called after each class was handled, for example to take a screenshot.
+    @param retries The most attempts to book one class.
+    @param sleep Waits between two attempts.
     @return What happened to each class; empty when the schedule has none.
-    @exception GymError The page could not be read or a booking failed.
+    @exception GymError The page could not be read or a booking failed on every attempt.
     """
     cards = find_target_classes(gym.list_class_cards())
     if not cards:
@@ -78,7 +141,7 @@ def book_target_classes(
         return []
     results: list[ClassResult] = []
     for card in cards:
-        result = book_class(gym, card)
+        result = book_class_with_retry(gym, card, retries, sleep)
         results.append(result)
         if on_result is not None:
             on_result(result)

src/snack_and_lift/application/retry.py

@@ -0,0 +1,67 @@
+"""@file retry.py
+@brief Lecture step 9: repeat a step that failed because the site dropped a request.
+"""
+
+import logging
+import time
+from collections.abc import Callable
+
+from snack_and_lift import constants
+from snack_and_lift.domain.errors import GymError, RetriesExhaustedError
+
+logger = logging.getLogger(__name__)
+
+## Waits for a number of seconds; tests pass a recorder so that they never really sleep.
+Sleep = Callable[[float], None]
+
+
+def retry[T](
+    func: Callable[[], T],
+    retries: int = constants.RETRY_COUNT,
+    description: str | None = None,
+    *,
+    delay: float = constants.RETRY_DELAY_SECONDS,
+    sleep: Sleep = time.sleep,
+) -> T:
+    """@brief Call a function until it succeeds, at most `retries` times.
+
+    Only a `GymError` is repeated: it is what a dropped request looks like. Any other error,
+    such as missing credentials, is a defect and passes through at once.
+
+    @param func The step, called without arguments.
+    @param retries The most attempts to make, at least 1.
+    @param description What the step does, for the log (never put a secret in it).
+    @param delay Seconds to wait between two attempts.
+    @param sleep Waits for a number of seconds.
+    @return What the function returned on its first successful attempt.
+    @exception ValueError `retries` is below 1.
+    @exception RetriesExhaustedError Every attempt raised a `GymError`; the last one is the cause.
+    """
+    if retries < 1:
+        raise ValueError(constants.MESSAGE_RETRIES_INVALID.format(retries=retries))
+    name = description or constants.RETRY_DEFAULT_DESCRIPTION
+    for attempt in range(1, retries + 1):
+        try:
+            result = func()
+        except GymError as error:
+            if attempt == retries:
+                message = constants.MESSAGE_RETRIES_EXHAUSTED.format(
+                    description=name, retries=retries, error=error
+                )
+                raise RetriesExhaustedError(message) from error
+            logger.warning(
+                constants.MESSAGE_RETRY_FAILED.format(
+                    attempt=attempt,
+                    retries=retries,
+                    description=name,
+                    error=str(error).rstrip("."),
+                )
+            )
+            sleep(delay)
+        else:
+            if attempt > 1:
+                logger.info(
+                    constants.MESSAGE_RETRY_SUCCEEDED.format(description=name, attempts=attempt)
+                )
+            return result
+    raise AssertionError("unreachable: the last attempt returns or raises")  # pragma: no cover

src/snack_and_lift/application/session.py

@@ -3,9 +3,12 @@
 """
 
 import logging
+import time
 
+from snack_and_lift import constants
 from snack_and_lift.application.credentials import Credentials
 from snack_and_lift.application.ports import GymPort
+from snack_and_lift.application.retry import Sleep, retry
 
 logger = logging.getLogger(__name__)
 
@@ -19,3 +22,25 @@ def login(gym: GymPort, credentials: Credentials) -> None:
     """
     gym.log_in(credentials.email, credentials.password)
     logger.info("✓ Logged in: the class schedule is shown")
+
+
+def login_with_retry(
+    gym: GymPort,
+    credentials: Credentials,
+    retries: int = constants.RETRY_COUNT,
+    sleep: Sleep = time.sleep,
+) -> None:
+    """@brief Log in, repeating the login when the site drops the request.
+
+    @param gym The gym site.
+    @param credentials The account to log in with.
+    @param retries The most attempts to make.
+    @param sleep Waits between two attempts.
+    @exception RetriesExhaustedError Every attempt failed.
+    """
+    retry(
+        lambda: login(gym, credentials),
+        retries,
+        constants.RETRY_DESCRIPTION_LOGIN,
+        sleep=sleep,
+    )

src/snack_and_lift/application/verification.py

@@ -3,10 +3,12 @@
 """
 
 import logging
+import time
 from collections.abc import Sequence
 
 from snack_and_lift import constants
 from snack_and_lift.application.ports import GymPort
+from snack_and_lift.application.retry import Sleep, retry
 from snack_and_lift.domain.booking_entry import BookingEntry, is_target_entry
 from snack_and_lift.domain.errors import NoBookingsFoundError
 from snack_and_lift.domain.models import ClassResult
@@ -30,18 +32,42 @@ def get_my_bookings(gym: GymPort) -> list[BookingEntry]:
     return entries
 
 
-def verify_bookings(gym: GymPort, results: Sequence[ClassResult]) -> VerificationResult:
+def get_my_bookings_with_retry(
+    gym: GymPort, retries: int = constants.RETRY_COUNT, sleep: Sleep = time.sleep
+) -> list[BookingEntry]:
+    """@brief Read the My Bookings page, repeating it when the page fails to load.
+
+    @param gym The gym site.
+    @param retries The most attempts to make.
+    @param sleep Waits between two attempts.
+    @return Every confirmed booking and waitlist entry.
+    @exception RetriesExhaustedError Every attempt failed.
+    """
+    return retry(
+        lambda: get_my_bookings(gym), retries, constants.RETRY_DESCRIPTION_BOOKINGS, sleep=sleep
+    )
+
+
+def verify_bookings(
+    gym: GymPort,
+    results: Sequence[ClassResult],
+    retries: int = constants.RETRY_COUNT,
+    sleep: Sleep = time.sleep,
+) -> VerificationResult:
     """@brief Compare what the bot booked with the My Bookings page.
 
     The page is not opened when the bot handled no class.
 
     @param gym The gym site.
     @param results What happened to each class.
+    @param retries The most attempts to read the page.
+    @param sleep Waits between two attempts.
     @return The comparison of the expected entries with the Tuesday and Thursday 6pm entries.
-    @exception NoBookingsFoundError Classes were handled but the page shows no entry.
+    @exception RetriesExhaustedError The page could not be read on any attempt.
-    @exception GymError The page could not be opened or read.
     """
     if not results:
         return build_verification(results, [])
-    entries = [entry for entry in get_my_bookings(gym) if is_target_entry(entry)]
+    entries = [
+        entry for entry in get_my_bookings_with_retry(gym, retries, sleep) if is_target_entry(entry)
+    ]
     return build_verification(results, entries)

src/snack_and_lift/cli.py

@@ -6,6 +6,7 @@ import argparse
 import logging
 import os
 import sys
+import time
 from collections.abc import Callable, Sequence
 from pathlib import Path
 
@@ -13,7 +14,8 @@ from snack_and_lift import constants
 from snack_and_lift.application.booking import book_next_tuesday, book_target_classes
 from snack_and_lift.application.credentials import Credentials, load_credentials
 from snack_and_lift.application.ports import GymPort
-from snack_and_lift.application.session import login
+from snack_and_lift.application.retry import Sleep
+from snack_and_lift.application.session import login_with_retry
 from snack_and_lift.application.verification import verify_bookings
 from snack_and_lift.domain.errors import GymError, LoginFailedError, MissingCredentialsError
 from snack_and_lift.domain.models import ClassResult
@@ -40,6 +42,22 @@ def selenium_gym_factory(headless: bool, profile_dir: Path, keep_open: bool) ->
     return SeleniumGym(create_driver(headless, profile_dir, keep_open))
 
 
+def _at_least_one(text: str) -> int:
+    """@brief Read a whole number of at least 1 for an option.
+
+    @param text The text of the option.
+    @return The number.
+    @exception argparse.ArgumentTypeError The text is not a whole number of at least 1.
+    """
+    try:
+        number = int(text)
+    except ValueError:
+        number = 0
+    if number < 1:
+        raise argparse.ArgumentTypeError(f"{text!r} is not a whole number of at least 1")
+    return number
+
+
 def parse_args(argv: Sequence[str] | None = None) -> argparse.Namespace:
     """@brief Read the command line.
 
@@ -50,8 +68,8 @@ def parse_args(argv: Sequence[str] | None = None) -> argparse.Namespace:
         prog="snack-and-lift",
         description="Books the Tuesday and Thursday 6pm classes at the Snack and Lift gym.",
         epilog=(
-            "exit codes: 0 done, 1 a step failed, 2 credentials missing, "
+            "exit codes: 0 done, 1 a step failed (also when every retry failed), "
-            "3 the My Bookings page does not match the bookings made"
+            "2 credentials missing, 3 the My Bookings page does not match the bookings made"
         ),
     )
     parser.add_argument(
@@ -82,6 +100,16 @@ def parse_args(argv: Sequence[str] | None = None) -> argparse.Namespace:
         action="store_true",
         help="book only the next Tuesday 6pm class (lecture steps 3 and 4); skips the verification",
     )
+    parser.add_argument(
+        "--retries",
+        type=_at_least_one,
+        default=constants.RETRY_COUNT,
+        metavar="N",
+        help=(
+            "attempts for the login, each booking and the My Bookings page when the site "
+            f"drops a request (default: {constants.RETRY_COUNT}, at least 1)"
+        ),
+    )
     args = parser.parse_args(argv)
     if args.keep_open is None:
         args.keep_open = not args.headless
@@ -121,7 +149,11 @@ def _save_console(gym: GymPort, screenshot_dir: Path | None, text: str, name: st
 
 
 def _run(
-    gym: GymPort, args: argparse.Namespace, recorder: ConsoleRecorder, credentials: Credentials
+    gym: GymPort,
+    args: argparse.Namespace,
+    recorder: ConsoleRecorder,
+    credentials: Credentials,
+    sleep: Sleep,
 ) -> bool:
     """@brief Open the site, log in, book the classes and verify them, saving the step images.
 
@@ -133,15 +165,16 @@ def _run(
     @param args The parsed options.
     @param recorder The recorded console lines, drawn for step 4.
     @param credentials The account to log in with.
+    @param sleep Waits between two attempts of a retried step.
     @return False when the My Bookings page does not match the bookings made, else True.
-    @exception GymError A step failed.
+    @exception GymError A step failed on every attempt.
     """
     gym.open_site()
     _save_step(gym, args.screenshot_dir, "step-1.png")
-    login(gym, credentials)
+    login_with_retry(gym, credentials, args.retries, sleep)
     _save_step(gym, args.screenshot_dir, "step-2.png")
     if args.first_tuesday_only:
-        book_next_tuesday(gym)
+        book_next_tuesday(gym, args.retries, sleep)
         _save_step(gym, args.screenshot_dir, constants.SCREENSHOT_STEP_3)
         _save_console(gym, args.screenshot_dir, recorder.text, constants.SCREENSHOT_STEP_4)
         return True
@@ -153,7 +186,7 @@ def _run(
             _save_step(gym, args.screenshot_dir, constants.SCREENSHOT_STEP_3)
             window_saved = True
 
-    results = book_target_classes(gym, save_window_once)
+    results = book_target_classes(gym, save_window_once, args.retries, sleep)
     if not window_saved:
         _save_step(gym, args.screenshot_dir, constants.SCREENSHOT_STEP_3)
     _save_console(gym, args.screenshot_dir, recorder.text, constants.SCREENSHOT_STEP_4)
@@ -163,22 +196,25 @@ def _run(
     detail = format_detail_list(results)
     logger.info("%s", detail)
     _save_console(gym, args.screenshot_dir, f"{summary}\n{detail}", constants.SCREENSHOT_STEP_6)
-    verification = verify_bookings(gym, results)
+    verification = verify_bookings(gym, results, args.retries, sleep)
     _save_step(gym, args.screenshot_dir, constants.SCREENSHOT_STEP_7)
     verification_text = format_verification(verification)
     logger.info("%s", verification_text)
     _save_console(gym, args.screenshot_dir, verification_text, constants.SCREENSHOT_STEP_8)
+    _save_console(gym, args.screenshot_dir, recorder.text, constants.SCREENSHOT_STEP_9)
     return verification.is_success
 
 
 def main(
     argv: Sequence[str] | None = None,
     gym_factory: GymFactory = selenium_gym_factory,
+    sleep: Sleep = time.sleep,
 ) -> int:
-    """@brief Start the bot: open the site, log in and book the next Tuesday 6pm class.
+    """@brief Start the bot: open the site, log in, book the classes and verify them.
 
     @param argv Arguments without the program name; None reads `sys.argv`.
     @param gym_factory Builds the gym site; tests pass a fake.
+    @param sleep Waits between two attempts of a retried step; tests pass a recorder.
     @return Process exit code: 0 done, 1 a step failed, 2 credentials missing, 3 mismatch.
     """
     logging.basicConfig(level=logging.INFO, format="%(message)s")
@@ -187,17 +223,20 @@ def main(
     recorder = ConsoleRecorder()
     root.addHandler(recorder)
     try:
-        return _main(argv, gym_factory, recorder)
+        return _main(argv, gym_factory, recorder, sleep)
     finally:
         root.removeHandler(recorder)
 
 
-def _main(argv: Sequence[str] | None, gym_factory: GymFactory, recorder: ConsoleRecorder) -> int:
+def _main(
+    argv: Sequence[str] | None, gym_factory: GymFactory, recorder: ConsoleRecorder, sleep: Sleep
+) -> int:
     """@brief The body of `main`, run while the recorder is attached.
 
     @param argv Arguments without the program name.
     @param gym_factory Builds the gym site.
     @param recorder The console recorder.
+    @param sleep Waits between two attempts of a retried step.
     @return Process exit code.
     """
     _use_utf8_output()
@@ -209,7 +248,7 @@ def _main(argv: Sequence[str] | None, gym_factory: GymFactory, recorder: Console
         return 2
     gym = gym_factory(args.headless, args.profile_dir, args.keep_open)
     try:
-        verified = _run(gym, args, recorder, credentials)
+        verified = _run(gym, args, recorder, credentials, sleep)
     except LoginFailedError as error:
         logger.error("Login failed: %s", error)
         return 1

src/snack_and_lift/constants.py

@@ -18,6 +18,31 @@ WAIT_TIMEOUT_SECONDS: Final[int] = 10
 ## Attempts of the `retry` wrapper before it gives up (lecture step 9).
 RETRY_COUNT: Final[int] = 7
 
+## Seconds to wait between two attempts of the `retry` wrapper.
+RETRY_DELAY_SECONDS: Final[float] = 0.5
+
+## Name of a retried step that was given no description.
+RETRY_DEFAULT_DESCRIPTION: Final[str] = "operation"
+
+## Descriptions of the retried steps, as they appear in the log.
+RETRY_DESCRIPTION_LOGIN: Final[str] = "login"
+RETRY_DESCRIPTION_BOOK: Final[str] = "book {name} on {day}"
+RETRY_DESCRIPTION_BOOKINGS: Final[str] = "get my bookings"
+
+## Messages of the `retry` wrapper.
+MESSAGE_RETRY_FAILED: Final[str] = (
+    "⚠ Attempt {attempt}/{retries} failed: {description}: {error}. Retrying..."
+)
+MESSAGE_RETRY_SUCCEEDED: Final[str] = "✓ {description} succeeded after {attempts} attempts"
+MESSAGE_RETRIES_EXHAUSTED: Final[str] = "{description} failed after {retries} attempts: {error}"
+MESSAGE_RETRIES_INVALID: Final[str] = "retries must be at least 1, got {retries}"
+
+## Error raised when a class card is no longer on the schedule while a booking is repeated.
+MESSAGE_CLASS_NOT_ON_SCHEDULE: Final[str] = "class {card_id} not found on the schedule"
+
+## File name of the screenshot of lecture step 9 (the console with the retry warnings).
+SCREENSHOT_STEP_9: Final[str] = "step-9.png"
+
 ## Weekdays of the classes the bot books, as the site abbreviates them.
 TARGET_WEEKDAYS: Final[tuple[str, ...]] = ("Tue", "Thu")
 

src/snack_and_lift/domain/errors.py

@@ -23,6 +23,10 @@ class PageFormatError(GymError):
     """@brief A text read from the page (a day label or a time) has an unexpected format."""
 
 
+class RetriesExhaustedError(GymError):
+    """@brief A step failed on every attempt; the cause is the error of the last attempt."""
+
+
 class NoBookingsFoundError(GymError):
     """@brief The My Bookings page shows no booking where the bot expects some.
 

src/snack_and_lift/infrastructure/selenium_gym.py

@@ -88,11 +88,18 @@ class SeleniumGym:
         @exception LoginFailedError The site shows a login error.
         @exception GymError The page did not behave as expected.
         """
-        self._wait.until(ec.element_to_be_clickable((By.ID, constants.NAV_LOGIN_BUTTON_ID))).click()
+        # After a failed login the form is still shown, so a repeated login skips the nav button.
-        self._wait.until(
+        if not self._driver.find_elements(By.ID, constants.LOGIN_EMAIL_INPUT_ID):
+            self._wait.until(
+                ec.element_to_be_clickable((By.ID, constants.NAV_LOGIN_BUTTON_ID))
+            ).click()
+        email_input = self._wait.until(
             ec.presence_of_element_located((By.ID, constants.LOGIN_EMAIL_INPUT_ID))
-        ).send_keys(email)
+        )
-        self._driver.find_element(By.ID, constants.LOGIN_PASSWORD_INPUT_ID).send_keys(password)
+        password_input = self._driver.find_element(By.ID, constants.LOGIN_PASSWORD_INPUT_ID)
+        for field, value in ((email_input, email), (password_input, password)):
+            field.clear()
+            field.send_keys(value)
         self._driver.find_element(By.ID, constants.LOGIN_SUBMIT_BUTTON_ID).click()
         self._wait.until(
             ec.any_of(

tests/e2e/fixtures/login.html

@@ -12,8 +12,19 @@
     // Dummy account of this fixture only; it is not a real account.
     const ACCEPTED_EMAIL = "fixture@example.invalid";
     const ACCEPTED_PASSWORD = "fixture-password";
+    // Imitates the network simulation: ?fail=N drops the first N submits after a short delay.
+    let failuresLeft = parseInt(new URLSearchParams(window.location.search).get("fail") || "0", 10);
     document.getElementById("login-form").addEventListener("submit", function (event) {
       event.preventDefault();
+      document.getElementById("message-area").innerHTML = "";
+      if (failuresLeft > 0) {
+        failuresLeft -= 1;
+        setTimeout(function () {
+          document.getElementById("message-area").innerHTML =
+            '<div id="error-message">Network request failed. Please try again.</div>';
+        }, 100);
+        return;
+      }
       const email = document.getElementById("email-input").value;
       const password = document.getElementById("password-input").value;
       if (email === ACCEPTED_EMAIL && password === ACCEPTED_PASSWORD) {

tests/e2e/fixtures/my_bookings.html

@@ -11,7 +11,13 @@
   <script>
     // Imitates the site: a loading text, then the page for the state in the query
     // (?state=empty or ?state=error; anything else shows two bookings and a waitlist entry).
-    const state = new URLSearchParams(window.location.search).get("state");
+    const params = new URLSearchParams(window.location.search);
+    const state = params.get("state");
+    // ?fail=N shows the error for the first N loads of the page in this tab (the network
+    // simulation); the count lives in sessionStorage because every load starts a new page.
+    const loads = parseInt(sessionStorage.getItem("loads") || "0", 10);
+    const dropped = loads < parseInt(params.get("fail") || "0", 10);
+    sessionStorage.setItem("loads", String(loads + 1));
     // The nav link keeps the state, so opening the page from here shows the same state again.
     document.getElementById("my-bookings-link").href = "my_bookings.html" + window.location.search;
     const page = document.getElementById("my-bookings-page");
@@ -19,7 +25,7 @@
     setTimeout(function () {
       document.getElementById("loading-placeholder").remove();
       page.dataset.loading = "false";
-      if (state === "error") {
+      if (state === "error" || dropped) {
         content.innerHTML = '<div id="error-message">Failed to load bookings</div>';
       } else if (state === "empty") {
         content.innerHTML = '<div id="empty-bookings-state">You have no bookings yet</div>';

tests/e2e/fixtures/schedule.html

@@ -46,7 +46,7 @@
         <p id="class-time-301"><strong>Time:</strong> 7:00 AM</p>
         <button id="book-button-301">Book Class</button>
       </div>
-      <div id="class-card-302" data-class-type="spin" data-user-booked="false" data-fail-once="true">
+      <div id="class-card-302" data-class-type="spin" data-user-booked="false" data-fail-times="1">
         <h3 id="class-name-302">Spin Class</h3>
         <p id="class-time-302"><strong>Time:</strong> 6:00 PM</p>
         <button id="book-button-302">Book Class</button>
@@ -55,6 +55,11 @@
   </div>
   <script>
     // Imitates the site: a busy text, then the new state; card 302 fails on its first click.
+    // ?fail=N makes every button fail on its first N clicks (the network simulation).
+    const failParam = parseInt(new URLSearchParams(window.location.search).get("fail") || "0", 10);
+    document.querySelectorAll("div[id^='class-card-']").forEach(function (card) {
+      card.dataset.failLeft = String(Math.max(failParam, parseInt(card.dataset.failTimes || "0", 10)));
+    });
     document.querySelectorAll("button[id^='book-button-']").forEach(function (button) {
       button.addEventListener("click", function () {
         const id = button.id.replace("book-button-", "");
@@ -66,8 +71,8 @@
         button.textContent = waitlist ? "Joining..." : "Booking...";
         button.disabled = true;
         setTimeout(function () {
-          if (card.dataset.failOnce === "true") {
+          if (parseInt(card.dataset.failLeft, 10) > 0) {
-            card.dataset.failOnce = "false";
+            card.dataset.failLeft = String(parseInt(card.dataset.failLeft, 10) - 1);
             button.textContent = oldText;
             button.disabled = false;
             const box = document.createElement("div");

tests/e2e/test_selenium_gym.py

@@ -8,10 +8,17 @@ from pathlib import Path
 
 import pytest
 
-from snack_and_lift.application.booking import book_target_classes
+from snack_and_lift.application.booking import book_class_with_retry, book_target_classes
-from snack_and_lift.application.verification import get_my_bookings
+from snack_and_lift.application.credentials import Credentials
+from snack_and_lift.application.session import login_with_retry
+from snack_and_lift.application.verification import get_my_bookings, get_my_bookings_with_retry
 from snack_and_lift.domain.booking_entry import BookingEntry
-from snack_and_lift.domain.errors import GymError, LoginFailedError, NoBookingsFoundError
+from snack_and_lift.domain.errors import (
+    GymError,
+    LoginFailedError,
+    NoBookingsFoundError,
+    RetriesExhaustedError,
+)
 from snack_and_lift.domain.models import Outcome
 from snack_and_lift.infrastructure.browser import create_driver
 from snack_and_lift.infrastructure.selenium_gym import SeleniumGym
@@ -119,13 +126,11 @@ def test_save_text_screenshot_writes_a_png_and_keeps_the_page(
 def test_book_target_classes_books_every_tuesday_and_thursday_class(
     schedule_gym: SeleniumGym,
 ) -> None:
-    with pytest.raises(GymError, match="Network error"):
+    # Card 302 drops its first click; the booking repeats it and the page confirms the second.
-        book_target_classes(schedule_gym)
+    results = book_target_classes(schedule_gym, sleep=lambda _: None)
-
-    results = book_target_classes(schedule_gym)
 
     assert [(r.card.card_id, r.outcome) for r in results] == [
-        ("201", Outcome.ALREADY_WAITLISTED),
+        ("201", Outcome.NEW_WAITLIST),
         ("302", Outcome.NEW_BOOKING),
     ]
 
@@ -188,3 +193,80 @@ def test_list_my_bookings_raises_with_the_text_of_the_error_box(
 ) -> None:
     with pytest.raises(GymError, match="Failed to load bookings"):
         open_my_bookings_page("?state=error").list_my_bookings()
+
+
+FIXTURE_LOGIN = (Path(__file__).parent / "fixtures" / "login.html").as_uri()
+
+
+@pytest.fixture
+def open_fixture(tmp_path: Path) -> Iterator[Callable[[str], SeleniumGym]]:
+    """@brief Opens a fixture page by its address, with the query that sets its failures."""
+    driver = create_driver(headless=True, profile_dir=tmp_path / "profile", keep_open=False)
+
+    def open_page(url: str) -> SeleniumGym:
+        driver.get(url)
+        return SeleniumGym(driver, base_url=url)
+
+    try:
+        yield open_page
+    finally:
+        driver.quit()
+
+
+def test_login_with_retry_logs_in_after_two_dropped_requests(
+    open_fixture: Callable[[str], SeleniumGym],
+) -> None:
+    gym = open_fixture(FIXTURE_LOGIN + "?fail=2")
+    sleeps: list[float] = []
+
+    login_with_retry(gym, Credentials(FIXTURE_EMAIL, FIXTURE_PASSWORD), sleep=sleeps.append)
+
+    assert len(sleeps) == 2
+
+
+def test_login_with_retry_gives_up_when_every_request_is_dropped(
+    open_fixture: Callable[[str], SeleniumGym],
+) -> None:
+    gym = open_fixture(FIXTURE_LOGIN + "?fail=5")
+
+    with pytest.raises(RetriesExhaustedError, match="Network request failed"):
+        login_with_retry(
+            gym, Credentials(FIXTURE_EMAIL, FIXTURE_PASSWORD), retries=3, sleep=lambda _: None
+        )
+
+
+def test_book_class_with_retry_books_after_two_dropped_clicks(
+    open_fixture: Callable[[str], SeleniumGym],
+) -> None:
+    gym = open_fixture(FIXTURE_SCHEDULE + "?fail=2")
+    card = next(card for card in gym.list_class_cards() if card.card_id == "302")
+
+    result = book_class_with_retry(gym, card, sleep=lambda _: None)
+
+    assert result.outcome is Outcome.NEW_BOOKING
+    assert next(c for c in gym.list_class_cards() if c.card_id == "302").button_text == "Booked"
+
+
+def test_book_target_classes_completes_when_every_class_drops_two_clicks(
+    open_fixture: Callable[[str], SeleniumGym],
+) -> None:
+    gym = open_fixture(FIXTURE_SCHEDULE + "?fail=2")
+
+    results = book_target_classes(gym, sleep=lambda _: None)
+
+    assert [(r.card.card_id, r.outcome) for r in results] == [
+        ("201", Outcome.NEW_WAITLIST),
+        ("302", Outcome.NEW_BOOKING),
+    ]
+
+
+def test_get_my_bookings_with_retry_reads_the_page_after_two_failed_loads(
+    open_fixture: Callable[[str], SeleniumGym],
+) -> None:
+    gym = open_fixture(FIXTURE_MY_BOOKINGS + "?fail=2")
+    sleeps: list[float] = []
+
+    entries = get_my_bookings_with_retry(gym, sleep=sleeps.append)
+
+    assert [entry.name for entry in entries] == ["Spin Class", "Yoga Class", "HIIT Class"]
+    assert sleeps

tests/unit/fakes.py

@@ -2,6 +2,8 @@
 @brief A fake of the GymPort for the unit tests.
 """
 
+import random
+from dataclasses import replace
 from pathlib import Path
 
 from snack_and_lift import constants
@@ -32,11 +34,20 @@ def _entry_of(card: ClassCard) -> BookingEntry | None:
     return BookingEntry(card.name, f"{card.date_text}, {time_text}", flag)
 
 
+NETWORK_ERROR_TEXT = "Network request failed. Please try again."
+
+
 class FakeGym:
     """@brief Records the calls; can refuse the login, serve cards and fail a click.
 
     Unless `bookings` is given, the My Bookings page shows every card that is, or by the
-    click becomes, booked or waitlisted.
+    click becomes, booked or waitlisted. A successful click turns the card's button into its
+    new state, so a card read again shows what the click did.
+
+    `fail_first` is a deterministic failure schedule: it maps the name of a port method
+    (`log_in`, `press_class_button`, `open_my_bookings`, `list_my_bookings`) to the number of
+    its first calls that fail like a dropped request. With `click_goes_through`, a failing
+    `press_class_button` still turns the button (the click arrived, the answer did not).
     """
 
     def __init__(
@@ -47,6 +58,8 @@ class FakeGym:
         press_error: str | None = None,
         bookings: list[BookingEntry] | None = None,
         bookings_error: str | None = None,
+        fail_first: dict[str, int] | None = None,
+        click_goes_through: bool = False,
     ) -> None:
         self.login_error = login_error
         self.cards = cards or []
@@ -54,6 +67,8 @@ class FakeGym:
         self.press_error = press_error
         self.bookings = bookings
         self.bookings_error = bookings_error
+        self.click_goes_through = click_goes_through
+        self._failures_left = dict(fail_first or {})
         self.calls: list[str] = []
         self.logins: list[tuple[str, str]] = []
         self.screenshots: list[Path] = []
@@ -64,11 +79,29 @@ class FakeGym:
     def open_site(self) -> None:
         self.calls.append("open_site")
 
+    def _should_fail(self, name: str) -> bool:
+        """@brief Whether this call is one of the scheduled failures; counts it."""
+        left = self._failures_left.get(name, 0)
+        if left > 0:
+            self._failures_left[name] = left - 1
+            return True
+        return False
+
+    def _turn_button(self, card_id: str) -> str:
+        """@brief Turn the button of a card to the state its click gives; return the new text."""
+        index = next(i for i, card in enumerate(self.cards) if card.card_id == card_id)
+        card = self.cards[index]
+        new_text = _PRESS_RESULTS[card.button_text]
+        self.cards[index] = replace(card, button_text=new_text)
+        return new_text
+
     def log_in(self, email: str, password: str) -> None:
         self.calls.append("log_in")
         self.logins.append((email, password))
         if self.login_error is not None:
             raise LoginFailedError(self.login_error)
+        if self._should_fail("log_in"):
+            raise LoginFailedError(NETWORK_ERROR_TEXT)
 
     def list_class_cards(self) -> list[ClassCard]:
         self.calls.append("list_class_cards")
@@ -81,14 +114,21 @@ class FakeGym:
             raise GymError(self.press_error)
         if self.press_result is not None:
             return self.press_result
-        card = next(card for card in self.cards if card.card_id == card_id)
+        if self._should_fail("press_class_button"):
-        return _PRESS_RESULTS[card.button_text]
+            if self.click_goes_through:
+                self._turn_button(card_id)
+            raise GymError(NETWORK_ERROR_TEXT)
+        return self._turn_button(card_id)
 
     def open_my_bookings(self) -> None:
         self.calls.append("open_my_bookings")
+        if self._should_fail("open_my_bookings"):
+            raise GymError(NETWORK_ERROR_TEXT)
 
     def list_my_bookings(self) -> list[BookingEntry]:
         self.calls.append("list_my_bookings")
+        if self._should_fail("list_my_bookings"):
+            raise GymError(NETWORK_ERROR_TEXT)
         if self.bookings_error is not None:
             raise GymError(self.bookings_error)
         if self.bookings is not None:
@@ -107,3 +147,30 @@ class FakeGym:
     def close(self) -> None:
         self.calls.append("close")
         self.closed = True
+
+
+class ChaosGym(FakeGym):
+    """@brief A gym whose login, click and bookings read each fail with a set probability.
+
+    The site's network simulation fails up to 50% of these requests and writes nothing when
+    one fails. The random numbers come from a seeded generator, so a run is repeatable.
+    """
+
+    _FAILING_CALLS = frozenset({"log_in", "press_class_button", "list_my_bookings"})
+
+    def __init__(
+        self,
+        rng: random.Random,
+        failure_probability: float = 0.5,
+        cards: list[ClassCard] | None = None,
+    ) -> None:
+        super().__init__(cards=cards)
+        self._rng = rng
+        self._failure_probability = failure_probability
+
+    def _should_fail(self, name: str) -> bool:
+        return name in self._FAILING_CALLS and self._rng.random() < self._failure_probability
+
+
+def no_sleep(seconds: float) -> None:
+    """@brief A `sleep` that returns at once, so a retry test never waits."""

tests/unit/test_application_verification.py

@@ -3,13 +3,13 @@
 """
 
 import pytest
-from tests.unit.fakes import FakeGym
+from tests.unit.fakes import FakeGym, no_sleep
 from tests.unit.test_booking_entry import make_entry
 from tests.unit.test_models import make_card
 
 from snack_and_lift.application.booking import book_target_classes
 from snack_and_lift.application.verification import get_my_bookings, verify_bookings
-from snack_and_lift.domain.errors import GymError, NoBookingsFoundError
+from snack_and_lift.domain.errors import GymError, NoBookingsFoundError, RetriesExhaustedError
 from snack_and_lift.domain.models import ClassResult, Outcome
 
 
@@ -67,8 +67,10 @@ def test_verify_bookings_raises_when_classes_were_booked_but_the_page_is_empty()
     gym = FakeGym(bookings=[])
     results = [ClassResult(make_card("Tue, Aug 12"), Outcome.NEW_BOOKING)]
 
-    with pytest.raises(NoBookingsFoundError):
+    with pytest.raises(RetriesExhaustedError) as caught:
-        verify_bookings(gym, results)
+        verify_bookings(gym, results, sleep=no_sleep)
+
+    assert isinstance(caught.value.__cause__, NoBookingsFoundError)
 
 
 def test_verify_bookings_skips_the_page_when_nothing_is_expected() -> None:

tests/unit/test_booking.py

@@ -5,7 +5,7 @@
 import logging
 
 import pytest
-from tests.unit.fakes import FakeGym
+from tests.unit.fakes import FakeGym, no_sleep
 from tests.unit.test_models import make_card
 
 from snack_and_lift.application.booking import (
@@ -147,4 +147,4 @@ def test_book_target_classes_propagates_a_failing_class() -> None:
     gym = FakeGym(cards=[make_card("Thu, Aug 14")], press_error="Network down")
 
     with pytest.raises(GymError, match="Network down"):
-        book_target_classes(gym)
+        book_target_classes(gym, sleep=no_sleep)

tests/unit/test_cli.py

@@ -6,7 +6,7 @@ import logging
 from pathlib import Path
 
 import pytest
-from tests.unit.fakes import FakeGym
+from tests.unit.fakes import FakeGym, no_sleep
 from tests.unit.test_booking_entry import make_entry
 from tests.unit.test_models import make_card
 
@@ -75,6 +75,7 @@ def test_main_saves_both_screenshots_on_success(tmp_path: Path) -> None:
         tmp_path / "step-5.png",
         tmp_path / "step-6.png",
         tmp_path / "step-8.png",
+        tmp_path / "step-9.png",
     ]
     assert gym.calls == [
         "open_site",
@@ -88,6 +89,7 @@ def test_main_saves_both_screenshots_on_success(tmp_path: Path) -> None:
         "save_text_screenshot",
         "save_screenshot",
         "save_text_screenshot",
+        "save_text_screenshot",
     ]
 
 
@@ -149,12 +151,18 @@ def test_main_saves_step_3_after_the_first_class_and_the_report_images(tmp_path:
         "list_my_bookings",
         "save_screenshot",
         "save_text_screenshot",
+        "save_text_screenshot",
     ]
     assert gym.screenshots[-2:] == [tmp_path / "step-3.png", tmp_path / "step-7.png"]
-    (_, _), (step_5, _), (step_6, _), (step_8, step_8_path) = gym.text_screenshots
+    (_, _), (step_5, _), (step_6, _), (step_8, step_8_path), (step_9, step_9_path) = (
+        gym.text_screenshots
+    )
     assert step_8_path == tmp_path / "step-8.png"
     assert step_8.startswith("--- VERIFYING ON MY BOOKINGS PAGE ---")
     assert step_8.endswith("✅ SUCCESS: All bookings verified!")
+    assert step_9_path == tmp_path / "step-9.png"
+    assert step_9.startswith("✓ Logged in")
+    assert step_9.endswith("✅ SUCCESS: All bookings verified!")
     assert step_5.startswith("--- BOOKING SUMMARY ---\nNew bookings: 2")
     assert "--- DETAILED CLASS LIST ---" not in step_5
     assert step_6.startswith(step_5)
@@ -185,7 +193,8 @@ def test_main_returns_3_on_a_mismatch(tmp_path: Path, caplog: pytest.LogCaptureF
     assert "❌ MISMATCH: Missing 1 bookings" in caplog.text
     assert "  ✗ Missing: Spin Class on Tue, Aug 19" in caplog.text
     assert gym.screenshots[-1] == tmp_path / "step-7.png"
-    assert gym.text_screenshots[-1][1] == tmp_path / "step-8.png"
+    assert gym.text_screenshots[-2][1] == tmp_path / "step-8.png"
+    assert gym.text_screenshots[-1][1] == tmp_path / "step-9.png"
     assert gym.closed is True
 
 
@@ -193,7 +202,7 @@ def test_main_returns_3_on_a_mismatch(tmp_path: Path, caplog: pytest.LogCaptureF
 def test_main_returns_1_when_the_bookings_page_is_empty() -> None:
     gym = FakeGym(cards=[make_card("Tue, Aug 19")], bookings=[])
 
-    assert main([], lambda *_: gym) == 1
+    assert main([], lambda *_: gym, no_sleep) == 1
 
 
 @pytest.mark.usefixtures("credentials_env")
@@ -239,7 +248,7 @@ def test_first_tuesday_only_flag_is_read() -> None:
 def test_main_returns_1_when_the_booking_fails() -> None:
     gym = FakeGym(cards=[make_card("Tue, Aug 19")], press_error="Network down")
 
-    assert main(["--headless"], lambda *_: gym) == 1
+    assert main(["--headless"], lambda *_: gym, no_sleep) == 1
     assert gym.closed is True
 
 
@@ -269,7 +278,7 @@ def test_main_returns_2_when_credentials_are_missing(
 def test_main_returns_1_when_login_fails() -> None:
     gym = FakeGym(login_error="Invalid credentials")
 
-    assert main(["--headless"], lambda *_: gym) == 1
+    assert main(["--headless"], lambda *_: gym, no_sleep) == 1
     assert gym.closed is True
 
 

tests/unit/test_constants.py

@@ -29,3 +29,7 @@ def test_retry_count_is_seven() -> None:
 
 def test_bot_books_tuesday_and_thursday() -> None:
     assert constants.TARGET_WEEKDAYS == ("Tue", "Thu")
+
+
+def test_retry_delay_is_half_a_second() -> None:
+    assert constants.RETRY_DELAY_SECONDS == 0.5

tests/unit/test_resilience.py

@@ -0,0 +1,316 @@
+"""@file test_resilience.py
+@brief Lecture step 9: login, booking and bookings read survive a site that drops requests.
+"""
+
+import logging
+import random
+from pathlib import Path
+
+import pytest
+from tests.unit.fakes import ChaosGym, FakeGym, no_sleep
+from tests.unit.test_booking_entry import make_entry
+from tests.unit.test_models import make_card
+
+from snack_and_lift import constants
+from snack_and_lift.application.booking import (
+    book_class_with_retry,
+    book_next_tuesday,
+    book_target_classes,
+)
+from snack_and_lift.application.credentials import Credentials
+from snack_and_lift.application.session import login_with_retry
+from snack_and_lift.application.verification import get_my_bookings_with_retry, verify_bookings
+from snack_and_lift.cli import main, parse_args
+from snack_and_lift.domain.errors import GymError, LoginFailedError, RetriesExhaustedError
+from snack_and_lift.domain.models import ButtonState, ClassResult, Outcome
+
+DUMMY_EMAIL = "dummy@example.invalid"
+DUMMY_PASSWORD = "dummy-password"
+CREDENTIALS = Credentials(DUMMY_EMAIL, DUMMY_PASSWORD)
+
+
+@pytest.fixture
+def credentials_env(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setenv(constants.ACCOUNT_EMAIL_ENV, DUMMY_EMAIL)
+    monkeypatch.setenv(constants.ACCOUNT_PASSWORD_ENV, DUMMY_PASSWORD)
+
+
+def test_login_with_retry_logs_in_after_two_dropped_requests(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    gym = FakeGym(fail_first={"log_in": 2})
+    sleeps: list[float] = []
+
+    with caplog.at_level(logging.INFO):
+        login_with_retry(gym, CREDENTIALS, sleep=sleeps.append)
+
+    assert gym.calls == ["log_in"] * 3
+    assert sleeps == [constants.RETRY_DELAY_SECONDS] * 2
+    assert "⚠ Attempt 2/7 failed: login: Network request failed. Please try again." in caplog.text
+    assert "✓ login succeeded after 3 attempts" in caplog.text
+
+
+def test_login_with_retry_never_logs_the_password(caplog: pytest.LogCaptureFixture) -> None:
+    gym = FakeGym(fail_first={"log_in": 2})
+
+    with caplog.at_level(logging.DEBUG):
+        login_with_retry(gym, CREDENTIALS, sleep=no_sleep)
+
+    assert DUMMY_PASSWORD not in caplog.text
+    assert DUMMY_EMAIL not in caplog.text
+
+
+def test_login_with_retry_gives_up_after_the_set_attempts() -> None:
+    gym = FakeGym(fail_first={"log_in": 10})
+
+    with pytest.raises(RetriesExhaustedError, match="login failed after 3 attempts") as caught:
+        login_with_retry(gym, CREDENTIALS, retries=3, sleep=no_sleep)
+
+    assert isinstance(caught.value.__cause__, LoginFailedError)
+    assert len(gym.logins) == 3
+
+
+def test_book_class_with_retry_repeats_a_dropped_click() -> None:
+    card = make_card()
+    gym = FakeGym(cards=[card], fail_first={"press_class_button": 2})
+
+    result = book_class_with_retry(gym, card, sleep=no_sleep)
+
+    assert result.outcome is Outcome.NEW_BOOKING
+    assert gym.pressed == ["123", "123", "123"]
+    assert gym.calls.count("list_class_cards") == 2
+
+
+def test_book_class_with_retry_reads_nothing_again_when_the_first_attempt_works() -> None:
+    card = make_card()
+    gym = FakeGym(cards=[card])
+
+    book_class_with_retry(gym, card, sleep=no_sleep)
+
+    assert gym.calls == ["press_class_button"]
+
+
+@pytest.mark.parametrize(
+    ("button_text", "outcome", "final_state"),
+    [
+        ("Book Class", Outcome.NEW_BOOKING, ButtonState.BOOKED),
+        ("Join Waitlist", Outcome.NEW_WAITLIST, ButtonState.WAITLISTED),
+    ],
+)
+def test_book_class_with_retry_counts_a_click_that_went_through_as_new(
+    button_text: str,
+    outcome: Outcome,
+    final_state: ButtonState,
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    card = make_card(button_text=button_text)
+    gym = FakeGym(cards=[card], fail_first={"press_class_button": 1}, click_goes_through=True)
+
+    with caplog.at_level(logging.INFO):
+        result = book_class_with_retry(gym, card, sleep=no_sleep)
+
+    assert result.outcome is outcome
+    assert gym.pressed == ["123"]
+    assert gym.cards[0].state is final_state
+    assert "Already" not in caplog.text
+
+
+def test_book_class_with_retry_keeps_already_booked_as_it_is() -> None:
+    card = make_card(button_text="Booked")
+    gym = FakeGym(cards=[card])
+
+    result = book_class_with_retry(gym, card, sleep=no_sleep)
+
+    assert result.outcome is Outcome.ALREADY_BOOKED
+    assert gym.pressed == []
+
+
+def test_book_class_with_retry_does_not_count_a_click_without_confirmation() -> None:
+    card = make_card()
+    gym = FakeGym(cards=[card], press_result="Book Class")
+
+    with pytest.raises(RetriesExhaustedError, match="click not confirmed"):
+        book_class_with_retry(gym, card, retries=2, sleep=no_sleep)
+
+    assert gym.pressed == ["123", "123"]
+
+
+def test_book_class_with_retry_fails_when_the_class_left_the_schedule() -> None:
+    card = make_card()
+    gym = FakeGym(cards=[card], fail_first={"press_class_button": 1})
+    gym.cards.clear()
+
+    with pytest.raises(RetriesExhaustedError, match="not found on the schedule"):
+        book_class_with_retry(gym, card, retries=2, sleep=no_sleep)
+
+
+def test_book_class_with_retry_gives_up_after_the_set_attempts() -> None:
+    card = make_card()
+    gym = FakeGym(cards=[card], fail_first={"press_class_button": 9})
+
+    with pytest.raises(RetriesExhaustedError, match="book Spin Class on Tomorrow"):
+        book_class_with_retry(gym, card, retries=3, sleep=no_sleep)
+
+    assert len(gym.pressed) == 3
+
+
+def test_book_next_tuesday_repeats_a_dropped_click() -> None:
+    gym = FakeGym(cards=[make_card("Tue, Aug 19")], fail_first={"press_class_button": 1})
+
+    result = book_next_tuesday(gym, sleep=no_sleep)
+
+    assert result is not None
+    assert result.outcome is Outcome.NEW_BOOKING
+
+
+def test_book_target_classes_repeats_each_class_on_its_own() -> None:
+    gym = FakeGym(
+        cards=[make_card("Thu, Aug 14", card_id="1"), make_card("Tue, Aug 19", card_id="2")],
+        fail_first={"press_class_button": 3},
+    )
+
+    results = book_target_classes(gym, sleep=no_sleep)
+
+    assert [result.outcome for result in results] == [Outcome.NEW_BOOKING] * 2
+    assert gym.pressed == ["1", "1", "1", "1", "2"]
+
+
+def test_get_my_bookings_with_retry_opens_the_page_again_after_a_failure() -> None:
+    entries = [make_entry("Wed, Aug 13, 7:00 AM")]
+    gym = FakeGym(bookings=entries, fail_first={"open_my_bookings": 1, "list_my_bookings": 1})
+
+    assert get_my_bookings_with_retry(gym, sleep=no_sleep) == entries
+    assert gym.calls == [
+        "open_my_bookings",
+        "open_my_bookings",
+        "list_my_bookings",
+        "open_my_bookings",
+        "list_my_bookings",
+    ]
+
+
+def test_get_my_bookings_with_retry_gives_up_after_the_set_attempts() -> None:
+    gym = FakeGym(bookings_error="Failed to load bookings")
+
+    with pytest.raises(RetriesExhaustedError, match="Failed to load bookings"):
+        get_my_bookings_with_retry(gym, retries=2, sleep=no_sleep)
+
+
+def test_verify_bookings_reads_the_page_through_retry() -> None:
+    card = make_card("Tue, Aug 12")
+    gym = FakeGym(cards=[card], fail_first={"list_my_bookings": 2})
+    results = [ClassResult(card, Outcome.NEW_BOOKING)]
+
+    verification = verify_bookings(gym, results, sleep=no_sleep)
+
+    assert verification.is_success
+
+
+def test_main_completes_a_run_with_dropped_requests_and_draws_them_in_step_9(
+    credentials_env: None, tmp_path: Path, caplog: pytest.LogCaptureFixture
+) -> None:
+    gym = FakeGym(
+        cards=[make_card("Thu, Aug 14", card_id="1"), make_card("Tue, Aug 19", card_id="2")],
+        fail_first={"log_in": 1, "press_class_button": 2, "list_my_bookings": 1},
+    )
+
+    with caplog.at_level(logging.INFO):
+        code = main(["--screenshot-dir", str(tmp_path)], lambda *_: gym, no_sleep)
+
+    assert code == 0
+    step_9, step_9_path = gym.text_screenshots[-1]
+    assert step_9_path == tmp_path / "step-9.png"
+    assert "⚠ Attempt 1/7 failed: login" in step_9
+    assert "succeeded after 2 attempts" in step_9
+    assert step_9.endswith("✅ SUCCESS: All bookings verified!")
+
+
+def test_main_returns_1_when_every_retry_failed(
+    credentials_env: None, caplog: pytest.LogCaptureFixture
+) -> None:
+    gym = FakeGym(fail_first={"log_in": 5})
+
+    with caplog.at_level(logging.INFO):
+        code = main(["--retries", "2"], lambda *_: gym, no_sleep)
+
+    assert code == 1
+    assert len(gym.logins) == 2
+    assert "Failed: login failed after 2 attempts" in caplog.text
+
+
+def test_main_uses_one_attempt_when_asked() -> None:
+    assert parse_args(["--retries", "1"]).retries == 1
+
+
+def test_retries_option_defaults_to_the_constant() -> None:
+    assert parse_args([]).retries == constants.RETRY_COUNT
+
+
+@pytest.mark.parametrize("value", ["0", "-3", "many", "1.5"])
+def test_retries_option_rejects_less_than_one(value: str) -> None:
+    with pytest.raises(SystemExit):
+        parse_args(["--retries", value])
+
+
+def test_help_epilog_mentions_retries(capsys: pytest.CaptureFixture[str]) -> None:
+    with pytest.raises(SystemExit):
+        parse_args(["--help"])
+
+    out = capsys.readouterr().out
+    assert "--retries N" in out
+    assert "every retry failed" in out
+
+
+## Chance that a request fails in the chaos test: the site's maximum (50%).
+CHAOS_FAILURE_PROBABILITY = 0.5
+
+## Chaos runs, and how many must complete. With 7 attempts an operation fails for good with
+## probability 0.5**7 (about 0.8%). A run has four such operations (login, two clicks, the
+## bookings read), so about 3% of runs fail and about 97 of 100 complete; 90 leaves room for
+## the spread of the seeds without hiding a real defect. The seeds are fixed, so it is stable.
+CHAOS_RUNS = 100
+CHAOS_MIN_COMPLETED = 90
+
+## Seeds that were checked to complete fully (seed 18 is the first that runs out of attempts);
+## a change that breaks one of them is a defect.
+CHAOS_SEEDS_THAT_MUST_PASS = tuple(range(18))
+
+
+def _run_chaos(seed: int) -> tuple[int, ChaosGym]:
+    gym = ChaosGym(
+        random.Random(seed),
+        CHAOS_FAILURE_PROBABILITY,
+        cards=[make_card("Thu, Aug 14", card_id="1"), make_card("Tue, Aug 19", card_id="2")],
+    )
+    return main(["--retries", "7"], lambda *_: gym, no_sleep), gym
+
+
+def _is_consistent(gym: ChaosGym) -> bool:
+    """@brief True when every target class is booked on the page after the run."""
+    return all(card.state is ButtonState.BOOKED for card in gym.cards)
+
+
+@pytest.mark.usefixtures("credentials_env")
+def test_chaos_runs_with_half_of_the_requests_failing_mostly_complete() -> None:
+    completed = 0
+    for seed in range(CHAOS_RUNS):
+        code, gym = _run_chaos(seed)
+        assert code in (0, 1), f"seed {seed}: a verified run never ends in a mismatch"
+        if code == 0:
+            assert _is_consistent(gym), f"seed {seed}: verified but not booked"
+            completed += 1
+
+    assert completed >= CHAOS_MIN_COMPLETED
+
+
+@pytest.mark.usefixtures("credentials_env")
+@pytest.mark.parametrize("seed", CHAOS_SEEDS_THAT_MUST_PASS)
+def test_chaos_seed_completes_with_a_consistent_verified_result(seed: int) -> None:
+    code, gym = _run_chaos(seed)
+
+    assert code == 0
+    assert _is_consistent(gym)
+
+
+def test_retry_error_is_a_gym_error_for_the_exit_code() -> None:
+    assert issubclass(RetriesExhaustedError, GymError)

tests/unit/test_retry.py

@@ -0,0 +1,167 @@
+"""@file test_retry.py
+@brief The retry wrapper: success, repeats, exhaustion, what it logs and what it never retries.
+"""
+
+import logging
+
+import pytest
+
+from snack_and_lift import constants
+from snack_and_lift.application.retry import retry
+from snack_and_lift.domain.errors import (
+    GymError,
+    LoginFailedError,
+    MissingCredentialsError,
+    PageFormatError,
+    RetriesExhaustedError,
+    UnknownButtonError,
+)
+
+
+class Flaky:
+    """@brief A step that raises the given errors in turn and then returns a value."""
+
+    def __init__(self, errors: list[Exception], value: str = "done") -> None:
+        self._errors = list(errors)
+        self._value = value
+        self.calls = 0
+
+    def __call__(self) -> str:
+        self.calls += 1
+        if self._errors:
+            raise self._errors.pop(0)
+        return self._value
+
+
+def test_retry_returns_the_result_on_the_first_attempt() -> None:
+    step = Flaky([])
+    sleeps: list[float] = []
+
+    assert retry(step, sleep=sleeps.append) == "done"
+    assert step.calls == 1
+    assert sleeps == []
+
+
+def test_retry_succeeds_after_some_failures(caplog: pytest.LogCaptureFixture) -> None:
+    step = Flaky([GymError("a"), GymError("b")])
+
+    with caplog.at_level(logging.INFO):
+        result = retry(step, 7, "login", sleep=lambda _: None)
+
+    assert result == "done"
+    assert step.calls == 3
+    assert "✓ login succeeded after 3 attempts" in caplog.text
+
+
+def test_retry_does_not_log_success_after_a_single_attempt(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    with caplog.at_level(logging.INFO):
+        retry(Flaky([]), 7, "login", sleep=lambda _: None)
+
+    assert "succeeded" not in caplog.text
+
+
+def test_retry_raises_after_the_last_attempt_with_the_last_error_as_cause() -> None:
+    last = GymError("third")
+    step = Flaky([GymError("first"), GymError("second"), last])
+
+    with pytest.raises(RetriesExhaustedError, match="login failed after 3 attempts") as caught:
+        retry(step, 3, "login", sleep=lambda _: None)
+
+    assert caught.value.__cause__ is last
+    assert "third" in str(caught.value)
+    assert step.calls == 3
+
+
+def test_retries_exhausted_is_a_gym_error() -> None:
+    with pytest.raises(GymError):
+        retry(Flaky([GymError("x")]), 1, sleep=lambda _: None)
+
+
+def test_retry_with_one_attempt_does_not_repeat_or_sleep() -> None:
+    step = Flaky([GymError("x"), GymError("y")])
+    sleeps: list[float] = []
+
+    with pytest.raises(RetriesExhaustedError):
+        retry(step, 1, sleep=sleeps.append)
+
+    assert step.calls == 1
+    assert sleeps == []
+
+
+def test_retry_with_one_attempt_returns_on_success() -> None:
+    assert retry(Flaky([]), 1, sleep=lambda _: None) == "done"
+
+
+def test_retry_waits_the_delay_between_attempts_but_not_after_the_last() -> None:
+    sleeps: list[float] = []
+
+    with pytest.raises(RetriesExhaustedError):
+        retry(Flaky([GymError("x")] * 4), 4, delay=1.5, sleep=sleeps.append)
+
+    assert sleeps == [1.5, 1.5, 1.5]
+
+
+def test_retry_uses_the_default_delay_and_attempts() -> None:
+    sleeps: list[float] = []
+    step = Flaky([GymError("x")] * constants.RETRY_COUNT)
+
+    with pytest.raises(RetriesExhaustedError):
+        retry(step, sleep=sleeps.append)
+
+    assert step.calls == constants.RETRY_COUNT
+    assert sleeps == [constants.RETRY_DELAY_SECONDS] * (constants.RETRY_COUNT - 1)
+
+
+def test_retry_logs_a_warning_with_attempt_description_and_error(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    step = Flaky([GymError("Network request failed. Please try again.")])
+
+    with caplog.at_level(logging.WARNING):
+        retry(step, 7, "login", sleep=lambda _: None)
+
+    assert (
+        "⚠ Attempt 1/7 failed: login: Network request failed. Please try again. Retrying..."
+        in caplog.text
+    )
+
+
+def test_retry_names_a_step_without_description(caplog: pytest.LogCaptureFixture) -> None:
+    with caplog.at_level(logging.WARNING):
+        retry(Flaky([GymError("x")]), sleep=lambda _: None)
+
+    assert f"failed: {constants.RETRY_DEFAULT_DESCRIPTION}: x" in caplog.text
+
+
+@pytest.mark.parametrize(
+    "error",
+    [LoginFailedError("no"), UnknownButtonError("busy"), PageFormatError("format")],
+)
+def test_retry_repeats_every_kind_of_gym_error(error: GymError) -> None:
+    step = Flaky([error])
+
+    assert retry(step, 2, sleep=lambda _: None) == "done"
+    assert step.calls == 2
+
+
+def test_retry_lets_other_errors_pass_untouched() -> None:
+    error = MissingCredentialsError("missing")
+    step = Flaky([error])
+
+    with pytest.raises(MissingCredentialsError) as caught:
+        retry(step, 7, sleep=lambda _: None)
+
+    assert caught.value is error
+    assert step.calls == 1
+
+
+@pytest.mark.parametrize("retries", [0, -1])
+def test_retry_rejects_fewer_than_one_attempt(retries: int) -> None:
+    step = Flaky([])
+
+    with pytest.raises(ValueError, match="at least 1"):
+        retry(step, retries)
+
+    assert step.calls == 0