Return 400 on a malformed request envelope instead of 500 (#19)

[phillipclapham]

Closes #19.

The /ldp/messages route didn't wrap request.json() / model_validate, so a malformed request envelope — bad JSON, or a body that fails envelope validation — surfaced as a 500 rather than a 400.

This parses the envelope defensively:

• bad JSON (JSONDecodeError) or a bad encoding (UnicodeDecodeError) → 400
• a well-encoded body that isn't a valid envelope (ValidationError) → 400
• anything else (a server/transport fault, or a bug inside handle_message) still propagates as 500 — it's ours, not the peer's, and stays observable

The 400 body is a structured LdpError (transport / MALFORMED_ENVELOPE). The bundled client calls raise_for_status() before parsing, so the non-envelope error shape doesn't affect it. (Happy to switch to a non-retryable category if you'd prefer for client errors.)

To make the route testable without binding a socket, the Starlette app moves into a _build_app() helper; run() is otherwise unchanged.

Tests cover: malformed JSON → 400, invalid envelope → 400, valid envelope still dispatches, a SESSION_PROPOSE→SESSION_ACCEPT round-trip confirming PayloadMode serializes over the wire, identity route unaffected. Starlette is in the [server] extra (not [dev]), so the route tests pytest.importorskip it — they skip rather than fail collection in a dev-only env. (Minor: TestClient emits a StarletteDeprecationWarning about httpx2 on starlette 1.2.1 — left your deps untouched, flagging in case you'd rather pin httpx2.)

Summary by CodeRabbit

• Bug Fixes

  • Strengthened input validation and error handling for incoming requests, ensuring malformed data is properly rejected with clear error messages to improve server reliability.

• Tests

  • Added comprehensive test coverage for request validation, error handling, and endpoint functionality to ensure robust behavior across various scenarios.

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: cc486b85-9a6f-43bd-9656-0b75f0bae1c8

📥 Commits

Reviewing files that changed from the base of the PR and between c59c1fe and a2be01c.

📒 Files selected for processing (2)

• sdk/python/src/ldp_protocol/delegate.py
• sdk/python/tests/test_delegate.py

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📝 Walkthrough

Walkthrough

The LdpDelegate HTTP server logic is refactored: app construction moves from run() into a new _build_app() method. A _bad_request() helper is added to return structured 400 responses with LdpError.transport, and POST /ldp/messages now explicitly catches JSON decode and envelope validation errors rather than letting them propagate as 500s. Tests covering these paths are added.

Envelope Parsing Hardening

Layer / File(s)	Summary
_build_app(), _bad_request(), and hardened /ldp/messages parsing sdk/python/src/ldp_protocol/delegate.py	Adds json/TYPE_CHECKING imports and a TYPE_CHECKING-guarded Starlette import; factors app construction into _build_app(); defines _bad_request() returning a JSONResponse with LdpError.transport(...) at status 400; wraps request.json() for JSONDecodeError/UnicodeDecodeError and LdpEnvelope model validation for ValidationError, both mapping to the structured 400 response.
run() wiring and TestHttpRouteEnvelopeParsing sdk/python/src/ldp_protocol/delegate.py, sdk/python/tests/test_delegate.py	run() delegates app construction to self._build_app() before starting uvicorn; TestHttpRouteEnvelopeParsing uses Starlette TestClient to assert malformed JSON, invalid envelopes, valid dispatch, and unaffected /ldp/identity behavior.

Sequence Diagram(s)

sequenceDiagram
  participant Client
  participant _build_app as POST /ldp/messages
  participant _bad_request
  participant handle_message

  Client->>"POST /ldp/messages": request with body
  alt malformed JSON / Unicode error
    "POST /ldp/messages"->>_bad_request: JSONDecodeError or UnicodeDecodeError
    _bad_request-->>Client: 400 · LdpError MALFORMED_ENVELOPE
  else envelope schema invalid
    "POST /ldp/messages"->>_bad_request: ValidationError
    _bad_request-->>Client: 400 · LdpError MALFORMED_ENVELOPE
  else valid LdpEnvelope
    "POST /ldp/messages"->>handle_message: LdpEnvelope
    handle_message-->>"POST /ldp/messages": LdpEnvelope response
    "POST /ldp/messages"-->>Client: 200 · model_dump(by_alias=True)
  end

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Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🐇 Hoppity-hop through the HTTP lane,
Bad JSON now bounces with status 400 plain.
_bad_request catches each mangled byte,
No more 500 gremlins prowling the night.
The envelope's guarded, the route is sound—
This rabbit's code review is homeward bound! 🌙

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title directly and clearly describes the main change: returning HTTP 400 status for malformed request envelopes instead of 500, which matches the primary objective of the PR and the linked issue #19.
Linked Issues check	✅ Passed	The PR implementation fully addresses issue #19: it wraps envelope parsing to return HTTP 400 with structured LDP errors for malformed JSON/validation failures instead of unhandled 500 errors, directly matching the stated objectives.
Out of Scope Changes check	✅ Passed	All changes are scoped to the objectives: delegate.py refactors the app builder and hardens POST /ldp/messages parsing, and test_delegate.py adds tests validating malformed envelope handling and envelope dispatch, with no unrelated modifications.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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