fix(pdu): set COMPRESSION_USED on the FastPath update header when compressed#1382
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Benoît Cortier (CBenoit) merged 1 commit intoJun 26, 2026
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…pressed FastPathUpdatePdu::encode wrote the update header byte before setting the COMPRESSION_USED bit, so a PDU encoded with compression flags emitted the trailing compression flags byte but never marked it in the header. The decoder keys on that bit to decide whether to consume the compression byte, so a compressed FastPath update did not round-trip: decode read the length field from the wrong offset and failed with NotEnoughBytes. Set the bit before writing the header, and add a round-trip test for the compressed case.
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Pull request overview
Fixes FastPath update PDU encoding so the update header correctly advertises the presence of the trailing compression flags byte when compression_flags is set, restoring encode/decode round-tripping for compressed FastPath updates in ironrdp-pdu.
Changes:
- Set the
COMPRESSION_USEDbit on the FastPath update header before writing the header byte inFastPathUpdatePdu::encode. - Add a regression test ensuring a compressed FastPath update PDU round-trips through encode/decode.
Reviewed changes
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| File | Description |
|---|---|
| crates/ironrdp-pdu/src/basic_output/fast_path/mod.rs | Fixes header bit emission so decode consumes the compression flags byte correctly. |
| crates/ironrdp-pdu/src/basic_output/fast_path/tests.rs | Adds a round-trip test covering the compressed FastPath update case. |
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Benoît Cortier (CBenoit)
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Jun 26, 2026
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Good catch! Thank you!
Benoît Cortier (CBenoit)
merged commit 3f96d00
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Devolutions:master
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David T. Martel (David-Martel)
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Jul 4, 2026
…pressed (Devolutions#1382) (cherry picked from commit 3f96d00)
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FastPathUpdatePdu::encodewrites the update header byte before setting theCOMPRESSION_USEDbit, so a PDU encoded withcompression_flagsset emits the trailing compression flags byte but never marks it in the header.FastPathUpdatePdu::decodekeys on that bit (bits 6..7 of the update header) to decide whether to consume the compression flags byte. With the bit clear, it skips that byte and reads the data length from the wrong offset, so a compressed FastPath update does not round-trip through encode/decode.This is currently latent: nothing in the workspace encodes a compressed fast-path output PDU (the server only decompresses, on the client side), so the path is never exercised. It surfaces as soon as anything does (it turned up while writing a bulk-decompression round-trip test).
The fix sets the bit on the header before it is written, and adds a round-trip test (
compressed_update_round_trips) covering the compressed case.