size hpack dynamic table for the 33-byte minimum entry

[ubeddulla]

What problem does this PR solve?

Issue Number:

Problem Summary:
IndexTable::Init sizes the HPACK dynamic-table ring buffer as max_size / (32 + 2), assuming every entry is at least 34 bytes. But AddHeader() only requires a non-empty name, so a header with an empty value costs name(1) + value(0) + 32 = 33 bytes (rfc7541 section 4.1). With the default 4096-byte table that leaves room for 124 such entries before eviction fires, while the queue only holds 120, so the 121st AddHeader() from a peer trips CHECK(!_header_queue.full()). That is a fatal log plus stack trace on every crafted HEADERS block, and a process abort when -crash_on_fatal_log is set, all driven straight from HTTP/2 header bytes.

What is changed and the side effects?

Changed:
Provision the queue for the real 33-byte minimum entry (max_size / (32 + 1)) so eviction always fires before the ring buffer fills. Added a regression test that decodes 121 one-byte-name/empty-value indexed headers, which previously tripped the CHECK.

Side effects:

• Performance effects: negligible, a few more slots in the dynamic-table ring buffer.

• Breaking backward compatibility: none.

[Copilot]

Pull request overview

This PR hardens brpc’s HTTP/2 HPACK dynamic table implementation by sizing the dynamic-table header ring buffer for the RFC7541 minimum 33-byte entry, preventing a peer from triggering a fatal CHECK(!_header_queue.full()) via many small indexed headers.

Changes:

• Adjust IndexTable::Init capacity calculation from assuming 34-byte minimum entries to the correct 33-byte minimum.
• Add a regression unit test that decodes 121 minimal (1-byte name, empty value) “literal with incremental indexing” headers to ensure decode does not abort.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/brpc/details/hpack.cpp	Fixes dynamic-table queue sizing to avoid ring-buffer overflow before eviction for 33-byte minimum entries.
test/brpc_hpack_unittest.cpp	Adds regression coverage for decoding many minimal indexed headers without triggering fatal checks.

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[ubeddulla]

Good catch. Pushed a guard that keeps num_headers at 1 when max_size is below 33 (including the max_size == 0 disable case), so we don't hit malloc(0). Nothing actually lands in that slot since entry_size > _max_size still holds in AddHeader. Added a test that Init(0) succeeds.