Sentry Backend Bug Pattern Review

Find bugs in Sentry backend code by checking for the patterns that cause the most real production errors.

This skill encodes patterns from 638 real production issues (393 resolved, 220 unresolved, 25 ignored) generating over 27 million error events across 65,000+ affected users. These are not theoretical risks -- they are the actual bugs that ship most often, with known fixes from resolved issues.

Scope

Review the code provided by the user, Warden, or the current branch diff. If the user does not provide a target, review the current branch diff. Start from the changed hunk or file, then read outward only as needed to confirm the behavior.

1. Analyze the changed code against the pattern checks below.
2. Use Read and Grep to trace data flow beyond the initial diff when needed. Follow function calls, callers, serializers, tasks, and ORM boundaries until the behavior is confirmed.
3. Report only HIGH and MEDIUM confidence findings.

Step 1: Classify the Code

Determine what you are reviewing and load the relevant reference.

If the code spans multiple categories, load all relevant references.

Step 2: Check for Top Bug Patterns

These are ordered by combined frequency and impact from real production data.

Check 1: Metric Subscription Query Errors -- 113 issues, 3,035,640 events

Alert and metric subscriptions referencing tags or functions that do not exist in the target dataset. These fire continuously once created.

Red flags:

• Creating Snuba subscriptions with SubscriptionData using user-provided query strings without validation
• Referencing transaction.duration in p95/p99 functions on the metrics dataset (it is a string type there)
• Using custom tag names (e.g., customerType) as filter dimensions without checking they exist
• Calling resolve_apdex_function without verifying the dataset supports threshold parameters

Safe patterns:

• Validate query fields against dataset schema before subscription creation
• Wrap _create_in_snuba calls with try/except SubscriptionError and mark subscription as invalid
• Use IncompatibleMetricsQuery checks before building metric subscription queries

Check 2: Missing Record / Stale Reference -- 81 issues, 1,403,592 events

Code calls .get() on a Django model assuming the record exists, but it has been deleted, merged, or never created.

Red flags:

• Model.objects.get(id=some_id) without try/except for DoesNotExist
• Detector.objects.get(id=detector_id) in workflow engine without handling deletion
• Environment.objects.get(name=env_name) in monitor/cron consumers
• Subscription.objects.get(id=sub_id) in billing tasks
• Using Group.objects.get() with IDs from Snuba query results (groups may be deleted/merged)
• Chained lookups where second .get() fails

Safe patterns:

• Model.objects.filter(...).first() with a None check
• try/except DoesNotExist that returns a graceful fallback (404, skip, log)
• Queryset .exists() check before .get()
• In API endpoints: return 404 for DoesNotExist, 400 for validation errors. Never suggest returning 500 intentionally.

Not a bug — do not flag:

• Infrastructure invariants: .get() enforcing a deployment precondition (e.g., "default org must exist in single-org mode") should crash — a 500 signals misconfiguration, not a code defect.
• Already validated by parent: If the endpoint base class validates the object (e.g., OrganizationEndpoint resolves the org), don't flag .get() on related records unless there's a genuine race or deletion window. Read the endpoint's parent class before reporting.
• Configuration lookups: Code that loads required config objects (get_default(), settings-based lookups) is expected to fail hard if the config is wrong.

Check 3: Search Query Validation -- 57 issues, 2,001,330 events

InvalidSearchQuery from user-provided or subscription-stored query filters referencing invalid values, deleted issues, or unresolved tags.

Red flags:

• by_qualified_short_id_bulk() called with short IDs from stored subscriptions that reference deleted/renamed projects
• _issue_filter_converter that calls Group.objects.get() on user-provided issue short IDs
• resolve_tag_key() called without checking the tag exists in the target dataset
• Passing unvalidated query strings from alert rule subscriptions to Snuba

Safe patterns:

• Validate short IDs before passing to by_qualified_short_id_bulk() -- handle Group.DoesNotExist
• Wrap _issue_filter_converter in try/except and return empty results for invalid filters
• Pre-validate tag existence against dataset schema

Check 4: Value Validation Errors -- 45 issues, 1,000,624 events

ValueError from insufficient input validation: unpacking errors, invalid enum values, missing expected objects, and Pydantic/DRF validation failures.

Red flags:

• SentryAppInstallation.objects.get() in action builders assuming exactly 1 result exists
• AlertRuleWorkflow lookups by ID without handling DoesNotExist
• Tuple unpacking (a, b, c = value.split(":")) on strings that may have fewer separators
• Integer enum lookups without catching ValueError (e.g., DetectorPriorityLevel(value))

Safe patterns:

• Validate expected count before .get(): use .filter() and check .count()
• Wrap tuple unpacking in try/except ValueError or validate length first
• Use try: EnumClass(value) except ValueError: for user-facing enum conversions
• In API endpoints: return 400 for ValueError and validation failures.

Check 5: Type Errors -- 28 issues, 740,106 events

Wrong types passed to functions: iterating over non-iterables, invalid dict keys, None where object expected.

Red flags:

• orjson.dumps(payload) where payload dict may have non-string keys
• list(setting) where setting could be an int (from project.get_option())
• result["key"] = value where result could be None
• Iterating over a value from DB/config that could be int or None instead of list

Safe patterns:

• Type-check before iteration: isinstance(value, (list, str)) before list(value)
• Validate dict keys before JSON serialization: ensure all keys are strings
• None-guard before subscript assignment: if result is not None:
• Defensive option reads with type checking
• In API endpoints: return 400 for type mismatches from user input.

Check 6: Internal API Request Errors -- 71 issues, 829,753 events

ApiError from internal Sentry API calls between services, often caused by stale subscription parameters.

Red flags:

• api.client.get() calls in metric alert chart rendering without handling 400 responses
• Internal API calls that forward stale subscription queries (referencing removed tags/metrics)
• fetch_metric_alert_events_timeseries() in incidents/charts.py without ApiError handling

Safe patterns:

• Wrap internal API calls with try/except ApiError and return graceful fallback
• Validate query parameters before making internal API requests
• Handle 400/500 responses explicitly in chart/visualization code

Check 7: Database Constraint Violations -- 22 issues, 2,962,198 events

IntegrityError and DataError from integer overflow, foreign key violations, and unique constraint violations.

Red flags:

• Incrementing times_seen counter without bounding (integer overflow at ~2.1 billion)
• Deleting MonitorCheckIn records without handling FK constraints from MonitorIncident
• UPDATE on GroupOpenPeriod date ranges where lower bound can exceed upper bound
• save() on models with unique constraints without handling IntegrityError

Safe patterns:

• Cap integer fields before update: min(value, 2_147_483_647) for 32-bit int columns
• Use CASCADE or handle FK constraints before bulk deletion
• Validate date range bounds before saving
• try/except IntegrityError with fallback to get() or update_or_create()

Check 8: Data Parsing & Deserialization -- 19 issues, 272,812 events

JSONDecodeError and ZstdError from parsing external data or stored compressed data.

Red flags:

• json.loads(response.body) without catching JSONDecodeError
• zstd.decompress(data) without handling corrupt frame descriptors
• Parsing VSTS/Azure DevOps webhook bodies that may be truncated
• Assuming API responses are always valid JSON (can be HTML error pages)

Safe patterns:

• Always wrap JSON/compression in try/except with graceful fallback
• Check content-type header before parsing
• Validate response status before parsing body

Check 9: Missing Key Access (KeyError) -- 25 issues, 155,020 events

Accessing dictionary keys or HTTP headers without existence check.

Red flags:

• request.META["HTTP_X_GITLAB_TOKEN"] in webhook handlers (header may be absent)
• request.META["HTTP_X_EVENT_KEY"] in Bitbucket webhook handler
• Dict key lookups with HANDLERS[event_type] without checking the event type is registered
• Tuple unpacking from header values with variable-length splits

Safe patterns:

• request.META.get("HTTP_X_GITLAB_TOKEN") with None check
• HANDLERS.get(event_type) with fallback for unknown event types
• Validate required headers at the top of webhook handler before processing

Check 10: Concurrency and Runtime Bugs -- 23 issues, 38,443 events

Dictionary mutation during iteration, shared mutable state, and unimplemented code paths.

Red flags:

• for key in self._dict: while another thread modifies it (RuntimeError)
• Publishing to shared KafkaPublisher dict that grows unbounded
• dict.pop() or dict[key] = value on a dict being iterated in another thread
• Missing NotImplementedError handlers for new search expression types

Safe patterns:

• dict.copy() before iteration
• Use threading.Lock for shared mutable state
• Implement all code paths before enabling features
• Use list(dict.keys()) for safe iteration when mutation is needed

Check 11: Logic Correctness -- not pattern-based

After checking all known patterns above, reason about the changed code itself:

• Does every code path return the correct type?
• Are all branches of conditionals handled (especially else / default cases)?
• Can any input (None, empty list, 0, empty string) cause unexpected behavior?
• Are there off-by-one errors in loops, slices, or range checks?
• If this code runs concurrently, is shared state protected?

Only report if you can trace a specific input that triggers the bug. Do not report theoretical concerns.

Not a bug — do not flag:

• assert statements enforcing infrastructure invariants — Sentry does not run with python -O, so assertions are always active. Crashing on a violated invariant is intentional.
• Speculative input concerns (e.g., "this URL could be too long", "this header could be malformed") unless you can show the input actually reaches the code path unvalidated. Check for existing validation (host checks, schema validation, DRF serializers) before reporting.

If no checks produced a potential finding, stop and report zero findings. Do not invent issues to fill the report. An empty result is the correct output when the code has no bugs matching these patterns.

Each code location should be reported once under the most specific matching pattern. Do not flag the same line under multiple checks.

Step 3: Report Findings

For each finding, provide the evidence the review harness needs:

• precise location
• severity and confidence
• concrete triggering input or state
• root cause and consequence
• a matching production precedent when available
• a concrete code fix, preferably as a unified diff when the harness supports it

Fix suggestions must include actual code. Never suggest a comment or docstring as a fix.

When suggesting fixes in API endpoints, use appropriate HTTP status codes (404 for not found, 400 for bad input, 409 for conflicts). Never suggest returning 500 intentionally.

Do not prescribe your own output format — the review harness controls the response structure.