avoid undefined shift in hpack DecodeInteger on crafted integer

[ubeddulla]

What problem does this PR solve?

Issue Number:

Problem Summary:
DecodeInteger keeps consuming HPACK continuation octets while tmp < MAX_HPACK_INTEGER. A run of 0x80 octets carries a zero payload, so tmp never grows and that guard never trips, while the shift amount m keeps climbing; once m reaches 64 the << m on a uint64_t is undefined behavior. UBSan reports shift exponent 70 is too large for 64-bit type. It is reachable from any HTTP/2 or gRPC peer via a crafted HEADERS frame whose index is encoded as 0xFF followed by continuation octets.

What is changed and the side effects?

Changed:
Reject the over-long encoding once m reaches 32, before the shift can leave the type width. A valid integer below MAX_HPACK_INTEGER needs only a handful of continuation octets, so no well-formed input is turned away. A regression test in brpc_hpack_unittest.cpp decodes 0xFF plus a long run of 0x80 and checks it is rejected.

Side effects:

• Performance effects: one extra comparison per continuation octet, no measurable change.

• Breaking backward compatibility: none.

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

[Copilot]

Pull request overview

This PR hardens HPACK integer decoding in src/brpc/details/hpack.cpp to avoid undefined behavior from shifting a uint64_t by 64+ bits when parsing crafted, over-long continuation sequences, and adds a regression test to ensure such inputs are rejected.

Changes:

• Add a guard in DecodeInteger to reject over-long continuation encodings before the shift amount can grow to an undefined range.
• Add a unit test that feeds 0xFF followed by many 0x80 continuation octets and asserts decode failure.

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	Adds a continuation-length/shift guard in HPACK integer decoding to prevent undefined shifts on malformed inputs.
test/brpc_hpack_unittest.cpp	Adds a regression test covering a crafted over-long HPACK integer continuation sequence.

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