Fix cache test reenable loop in VC_EVENT_READ_READY handler

[bryancall]

Problem

The test_cache_Populated_Cache unit test was failing with "Subprocess aborted" showing thousands of repeated reenable calls on the same CacheVC object. The Jenkins CI logs showed an infinite loop of reenable messages:

https://ci.trafficserver.apache.org/job/master/job/os_build/47228/console

Root Cause

In src/iocore/cache/unit_tests/main.cc, the CacheReadTest::read_event handler was calling process_event() inside the while loop that consumes data blocks:

case VC_EVENT_READ_READY: {
  while (this->_reader->block_read_avail()) {
    // ... consume block ...
    this->process_event(event);  // ❌ Called for EVERY block
  }
  break;
}

This meant reenable() was being called for every block of data, which is incorrect. When reading from a populated cache with many blocks, this resulted in excessive reenables and triggered the test abort.

The Fix

Move the process_event() call outside the while loop so reenable() is only called once per event after all available data has been consumed:

case VC_EVENT_READ_READY: {
  while (this->_reader->block_read_avail()) {
    // ... consume block ...
  }
  this->process_event(event);  // ✅ Called once after consuming all data
  break;
}

This matches the pattern used in CacheTestSM (the production regression test code) and follows standard async I/O patterns: consume all available data, then signal readiness for more.

Testing

• ✅ test_cache_Cache - Passed (fresh cache)
• ✅ test_cache_Populated_Cache - Passed (previously failing)

The fix reduces reenable calls from thousands to just a handful per test run.

[Copilot]

Pull request overview

This PR fixes an infinite loop bug in the test_cache_Populated_Cache unit test caused by excessive reenable() calls. The fix moves the process_event() call from inside the data consumption loop to after the loop, ensuring proper async I/O handling that aligns with the pattern used in production code (CacheTestSM).

Key Changes:

• Relocated process_event(event) call to execute once per VC_EVENT_READ_READY event (after consuming all available data) instead of once per data block
• This prevents thousands of redundant reenable calls when reading from a populated cache with multiple blocks

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[bryancall]

[approve ci rocky]