Report a function code mismatch before a length mismatch - #625
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When a reply arrives for a request that has already timed out, and its transaction identifier has since come round again, that reply is matched to whichever request now holds the identifier. Identifiers wrap at 256 and entries in _transactions are never removed, so the identifier stops being unique in time and the two requests become indistinguishable. The mismatch itself is not addressed here, but the message is. The length check ran before the function code check, so the caller was told about a byte count when the frame simply belonged to a different request: before Data length error, expected 7 got 6 after Unexpected data error, expected code 3 got 1 Reported in yaacov#573, where the reproduction is described.
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Included review availability: Your plan provides up to 4 included reviews per hour; 2 remain after this review. 📝 WalkthroughWalkthroughThe response handler now reports unexpected function codes before checking response length. The test port simulates this case, and a regression test verifies the error priority. ChangesResponse validation
Estimated code review effort: 2 (Simple) | ~10 minutes Merge Risk: ⚪ Minimal · up to The change reports function-code mismatches before length mismatches without altering request handling or transaction allocation, and adds coverage for the corrected message; no actionable merge-blocking risk remains. 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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Reordering two checks can quietly swallow a case that used to be reported correctly, so I walked every error path in
The one worth checking is unit 2: a reply that really is short still reports the length error, so the length check has not been shadowed. It only yields when the function code already says the frame belongs to a different request. On bounds: the minimum-length guard at |
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Inline comments:
In `@ports/testport.js`:
- Around line 308-314: Update the case 7 comment near the buffer.slice call to
state that this simulated late response intentionally has a mismatched length,
replacing the inaccurate “length to match” description while preserving the
code.
In `@test/test.js`:
- Around line 105-117: Update the assertions in the “should report a code
mismatch rather than a length error” test to verify the concrete function-code
mismatch, confirming that the error reports expected code 3 and received code 1,
while retaining the existing error-type and non-length-error checks.
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Both fixed. The comment said the reply had "a length to match" while the code drops a byte — it now says one byte short, which is what unit 7 actually does. The assertion was only proving the absence of a length error, so it would have passed on any other unexpected-data message. It now asserts the exact string, including both function codes: Lint clean, 168 tests passing. |
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Addresses the misleading message in #573, where the reproduction and the underlying cause are written up.
What happens
A reply that arrives after its request has timed out is matched to whichever request now holds the same transaction identifier. Identifiers wrap at
MAX_TRANSACTIONS(256) and entries in_transactionsare never removed, so an identifier stops being unique in time and two requests become indistinguishable to_onReceive.Reproduced with a device that answers late:
What this changes
Only the message. The length check sat ahead of the function code check, so a frame belonging to another request failed on its byte count first, and the caller was pointed at the wrong thing:
Moving the function code check first says what actually happened. The reordering is two blocks swapped; no logic is altered.
Tests
testportgains unit 7, answering with a different function code and a length to match, which stands in for a stale reply. One test asserts the code mismatch is reported and the length error is not. Existing suite passes (168 tests, lint clean).What this deliberately does not do
It does not stop the mismatch. Preventing it means changing how identifiers are allocated — removing a transaction once it settles and holding its identifier back until the socket has been quiet, or refusing to reuse one whose transaction is still outstanding. Both are design decisions about the port's contract rather than a fix a passer-by should pick, so I have left them alone and described the options in #573.
If you would rather have the prevention as well, or would rather this waited for it, I am happy either way.
Summary by CodeRabbit
Bug Fixes
Tests