fix(pdu): retain unknown fast-path input eventFlags bits instead of dropping the session - #1847
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Marc-André Moreau (mamoreau-devolutions) merged 1 commit intoAug 30, 2026
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…ropping the session The fast-path input decoder rejected the whole event stream when a keyboard or synchronize event carried an eventFlags bit outside the defined set, and a decode error there is session-fatal mid-use: input events arrive continuously, so a single unknown flag bit from a client dropped a live session with work in progress. The strictness was also internally inconsistent: the slow-path decoders for the very same events already tolerate unknown bits. Slow-path keyboard flags (scan_code.rs), synchronize toggle flags (sync.rs) and unicode keyboard flags (unicode.rs) all decode with from_bits_retain, so the identical keystroke was tolerated on one path and connection-fatal on the other. The spec's only mandated strictness in this area is about event types, not flags: [MS-RDPBCGR] 3.3.5.8.2 says the server SHOULD drop the connection when an event structure does not match one of the known types. That check is the existing eventCode rejection, which stays exactly as it is. The eventFlags tables (2.2.8.1.2.2.1 and 2.2.8.1.2.2.5) carry no receiver-validation language. Switch all three flag sites (scancode, sync, unicode) to from_bits_retain, the crate-wide policy since Devolutions#1144. The retained bits fit the 5-bit header field, so re-encoding reproduces the wire value, and the server-side consumers read these flags with contains(), so retained bits flow through harmlessly. This continues the interop line of Devolutions#1489, Devolutions#1458, Devolutions#1837, Devolutions#1843, Devolutions#1844, Devolutions#1845 and Devolutions#1846, and is the last flag-site conversion of that series.
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…ce policy (#1850) STYLE.md's error handling section documents the message-formatting conventions but predates two rounds of change in the machinery itself, and it says nothing about the decode-tolerance convention that this month's merges made the most exercised rule in ironrdp-pdu. This adds three subsections, plus two mirroring bullets in AGENTS.md's Key Style Conventions. Choosing an error kind: the structured DecodeErrorKind/EncodeErrorKind variants carry byte offsets and are constructed through the *_err! macros with in: src; Other is the deliberate offset-less kind for non-stream failures (constructors, conversions, state validation). Error types: typed errors rather than anyhow in library crates, and no proc-macro error derives in core-tier crates, hand-rolling Display and Error per ironrdp-error's own Location-capturing Source pattern. Decoding unknown values: the taxonomy the recent interop line converged on. Peer advertisements decode with from_bits_retain (unknown bits kept, never fatal, byte-identical re-encode); negotiation outputs the receiver must implement stay strict; structural discriminants stay strict per the specifications' own drop-the-connection language. Precedent: #1144 established retain crate-wide, #1489, #1458, #1536, #1541, #1837, and #1843 through #1847 applied the taxonomy field by field, and #1845 documents the strict half in-code at the server-selected encryption method. Deliberately out of scope: connector-layer error types, which #1751 and the sequence-contract stack are actively reshaping; nothing here describes them. Testing guidance is also untouched; that conversation belongs with the in-flight test-centralization series. `cargo xtask check fmt/lints/tests/typos/locks` all pass (docs-only change).
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The fast-path input decoder rejects the whole event stream when a keyboard or synchronize event carries an eventFlags bit outside the defined set, and a decode error there is session-fatal mid-use: input events arrive continuously, so a single unknown flag bit from a client drops a live session with work in progress.
The strictness is also internally inconsistent: the slow-path decoders for the very same events already tolerate unknown bits. Slow-path keyboard flags (scan_code.rs), synchronize toggle flags (sync.rs) and unicode keyboard flags (unicode.rs) all decode with from_bits_retain, so the identical keystroke is tolerated on one path and connection-fatal on the other.
The spec's only mandated strictness in this area is about event types, not flags: [MS-RDPBCGR] 3.3.5.8.2 says the server SHOULD drop the connection when an event structure does not match one of the known types. That check is the existing eventCode rejection, which stays exactly as it is. The eventFlags tables (2.2.8.1.2.2.1 and 2.2.8.1.2.2.5) carry no receiver-validation language.
All three flag sites (scancode, sync, unicode) switch to from_bits_retain, the crate-wide policy since #1144. The retained bits fit the 5-bit header field, so re-encoding reproduces the wire value, and the server-side consumers read these flags with contains(), so retained bits flow through harmlessly.
This continues the interop line of #1489, #1458, #1837, #1843, #1844, #1845 and #1846, and it is the last flag-site conversion of that series.
Tests: a keyboard event and a synchronize event, each with an undefined bit in the 5-bit field, decode successfully, retain the bit, and re-encode byte-identically. Both fail with the strict from_bits restored.
cargo xtask check fmt/lints/tests/typos/locksall pass.