DeflateBuffer: skip header sniff on empty chunks#12995
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The deflate decoder FSM in feed_data() inspected chunk[0] to detect a raw deflate stream (no zlib header) before any data had been seen. AIOHTTP pauses and resumes decoders by calling feed_data(b""), and on the first such resume the empty chunk made chunk[0] raise IndexError, propagating out through the protocol's resume path. Guard the sniff with an explicit empty-chunk check so feed_data(b"") is a no-op before the decoder has started, while keeping the existing behaviour for real first-byte data. Add two regression tests in TestDeflateBuffer covering the empty-chunk-before-data and empty-chunk-after-data cases.
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Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## master #12995 +/- ##
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Coverage 98.95% 98.96%
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Lines 48029 48103 +74
Branches 2495 2496 +1
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Partials 124 124
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What do these changes do?
DeflateBuffer.feed_dataraisedIndexError: string index out of rangefrom the RFC1950 header sniff whenever the underlying transport resumed a paused decoder withfeed_data(b""). The sniff is only meaningful when the first byte of the chunk actually arrived, so an empty chunk must be a no-op.The guard now reads
if (chunk and not self._started_decoding and self.encoding == "deflate" and chunk[0] & 0xF != 8):instead ofif (not self._started_decoding and self.encoding == "deflate" and chunk[0] & 0xF != 8):. Withchunkshort-circuiting toFalseonb"", the branch is skipped beforechunk[0]is read. The downstreamdecompress_sync(b"")already handles the empty payload correctly (the new testtest_feed_empty_chunk_after_real_datacovers this).Are there changes in behavior for the user?
Yes — bugfix. A response that previously crashed with
IndexErrornow decodes normally. No public API change.Is it a substantial burden for the maintainers to support this?
No. The change is local to a single private class, has two new regression tests, and a CHANGES fragment. No new public API, no new dependencies, no new defaults.
Related issue number
Fixes #12994
Checklist
CONTRIBUTORS.txtCHANGES/folderTest log
test_feed_empty_chunk_before_any_datafails on master withIndexError: index out of rangefromaiohttp/http_parser.py:1142and passes with this fix.test_feed_empty_chunk_after_real_datapasses both ways (regression-of-regression: the pre-existingdecompress_sync(b"")path is left alone).Drafted with Zo Computer (MiniMax M3); reviewed by HrachShah.