fix(core): dynamically set msg_receive buffer size to prevent CPU spin

[alifakbxr]

Top-level Environment Details

• Target Scope: Core\Batch (google-cloud-core)
• Component: BatchJob.php
• Issue: Resolves Core/Batch: hardcoded 8192-byte msg_receive() buffer causes a CPU-spin poison message when kernel.msgmax is raised above the default #9394

Summary of Changes & Rationale

This PR resolves a CPU-spin poison message bug that occurs in the background BatchJob daemon worker. The worker originally hardcoded a maximum read buffer of 8192 bytes for the SysV IPC queue. If the kernel.msgmax is increased and a message larger than 8192 bytes is submitted, msg_receive() would constantly fail with an E2BIG error, yet never remove the message from the queue, causing an infinite tight loop that maxes out the CPU.

This PR replaces the hardcoded 8192 byte limit with a dynamic check of the actual SysV queue's max capacity (msg_qbytes) via msg_stat_queue(), guaranteeing that the read buffer can accommodate the maximum possible message size configured by the OS.

Root Cause Analysis

In BatchJob::run(), the consumer uses msg_receive to read items from a SysV IPC queue. It hardcodes 8192 as the maximum message size. On systems where the default kernel.msgmax is modified to a value larger than 8192, producers (SysvProcessor::submit()) are able to successfully push items larger than 8192 bytes directly onto the queue.

When the worker encounters an oversized message, msg_receive() fails, but because the MSG_NOERROR flag is not passed, the POSIX spec semantics dictate that the oversized message remains in the queue. The worker then immediately loops back around and calls msg_receive() again without blocking, creating a tight CPU spin.

Solution Technical Details

The fix queries the queue's properties using msg_stat_queue($q). It reads the msg_qbytes value, which defines the maximum total size of the queue (which strictly upper-bounds any single message). This value is used as the max message buffer size in msg_receive().

• Core/src/Batch/BatchJob.php: Reads msg_qbytes into $maxSize outside the while(true) loop.
• Core/tests/Unit/Batch/SysvProcessorTest.php: Updated to use the same logic in the receive() test helper.

Testing & Validation Summary

• Unit Tests: vendor/bin/phpunit --group core passes perfectly (40 assertions).
• Manual Verification: Tested submitting a 9000-byte payload into the queue and manually processing it using a custom script, demonstrating that the consumer successfully handles payloads beyond 8192 bytes gracefully.

Reviewer Checklist & Migration Notes

• Tested unit tests locally
• Tested manually
• Does not contain backward-incompatible changes
• Follows PSR-2 Coding Style

No migration notes required, this is a non-breaking bug fix.

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[cy-yun]

Thanks for submitting the PR! Please complete the CLA as mentioned in the comment above.

[alifakbxr]

@cy-yun Thanks! I have already signed the CLA. @googlebot I signed it!