okhttp: optimize HPACK to index :path and fix dynamic table eviction#12907
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okhttp: optimize HPACK to index :path and fix dynamic table eviction#12907AgraVator wants to merge 2 commits into
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…bugs (grpc#12819) Allow the :path pseudo-header to be added to the HPACK dynamic table in the gRPC-Java OkHttp transport, resolving the original goal of grpc#12819 and redoing reverted commit 397d3e7 (grpc#12799, grpc#12820). Background & Root Cause: In gRPC over HTTP/2, :path headers represent static service/method names (e.g., /package.Service/Method), which are constant across RPC calls. Previously, indexing :path was reverted (grpc#12820) because high dynamic table churn uncovered latent bugs in legacy OkHttp Hpack.java: 1. Off-by-one NPE loop bound in evictToRecoverBytes: When bytesToRecover exceeded total dynamic table size, the loop condition (j >= nextDynamicTableIndex) inspected the unallocated nextDynamicTableIndex slot containing null, throwing a NullPointerException. Fixed by changing the loop bound to (j > nextDynamicTableIndex). 2. Reference leak post-eviction: System.arraycopy shifted active entries rightward, but left stale Header/ByteString references in vacated array slots. Fixed by setting vacated array slots to null via Arrays.fill(). 3. Case sensitivity normalization: Header entries inserted into dynamicTable are now guaranteed to store lowercased name keys, preventing failed dynamic table header lookups. 4. Table Size Setting Handling: Works in conjunction with grpc#12818 (SETTINGS_HEADER_TABLE_SIZE handling). Fixes grpc#12819 Related: grpc#12799, grpc#12818, grpc#12820, b/514688016
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Background & Problem Statement
In gRPC over HTTP/2, the
:pathpseudo-header represents static RPC service/method names (e.g.,/package.Service/Method), which remain constant across calls on long-lived channels.Indexing
:pathin HPACK's dynamic table reduces header transmission overhead from ~40+ bytes per RPC down to a single 1-byte indexed header field (0x80 | index).PR #12799 (commit
397d3e7214) originally enabled:pathindexing, but was reverted in PR #12820 (commit52f2cd5982) because continuous dynamic table insertions and evictions uncovered latent bugs in OkHttp's legacyHpack.javaimplementation during internal Google testing (b/514688016).This PR resolves tracking issue #12819 by fixing the underlying
Hpack.javadynamic table bugs and safely re-enabling:pathindexing.Changes Included in This PR
Fix Off-by-One NPE Loop Bound in
evictToRecoverBytes:Hpack.ReaderandHpack.Writer, whenbytesToRecoverexceeded total table byte count,j >= nextDynamicTableIndexevaluated slotnextDynamicTableIndex(which containsnull), throwing aNullPointerException.j > nextDynamicTableIndex.Garbage Collection Memory Leak Cleanup:
System.arraycopyshifted active elements rightward during eviction, but left staleHeader/ByteStringreferences in vacated array slots.Arrays.fill(dynamicTable, oldNext + 1, oldNext + 1 + entriesToEvict, null);post-arraycopyto sever hard references and enable GC.Normalized Header Casing in Dynamic Table:
dynamicTableare now guaranteed to store lowercasednamekeys (new Header(name, value)), preventing dynamic table key lookup mismatches.Re-enabled
:pathPseudo-Header Indexing:Writer.writeHeadersto allow both:authorityand:pathto be incrementally indexed in the HPACK dynamic table.Unit Tests:
HpackTest.javato test:pathindexing reuse (pseudoHeaderIndexingForPathAndAuthority) and addedevictToRecoverBytesDoesNotNpeWhenBytesToRecoverExceedsTable.Issue References
b/514688016