Fix xdebug transform calling WRITE_READY when no data consumed

[bryancall]

The xdebug probe-full-json transform was calling TSContCall(WRITE_READY) even when no data was available to consume (towrite == 0). This caused unnecessary callbacks to the transform, and under certain conditions could lead to a tight loop that would starve other transactions on the same ET_NET thread.

The Bug

When using X-Debug: probe-full-json=nobody (or other probe-full-json variants), if the origin server was slow to send body data, the transform would:

1. Be called with WRITE_READY event
2. See no data available (towrite == 0)
3. Still call TSVIOReenable and TSContCall(WRITE_READY) back to upstream
4. Get called again immediately (step 1)

This creates a tight loop that can cause:

• High CPU usage on the affected ET_NET thread
• Starvation of other transactions on the same thread
• Requests appearing to "hang"

The Fix

Add a check to only call TSVIOReenable and TSContCall when towrite > 0, meaning we actually consumed data. If no data was available, we simply return and wait for the upstream to call us again when more data arrives.

Testing

Added a new autest (x_probe_full_json_slow_origin) that uses a slow origin server with chunked encoding to verify the transform handles delayed body data correctly.

[Copilot]

Pull request overview

This PR fixes a bug in the xdebug plugin's probe-full-json transform where it would create a tight loop when the origin server was slow to send body data. The transform was incorrectly calling TSContCall(WRITE_READY) even when no data was consumed (towrite == 0), causing high CPU usage and potential starvation of other transactions.

Key Changes:

• Added a conditional check to only reenable and notify upstream when data has been consumed (towrite > 0)
• Added comprehensive test coverage with a slow origin server simulation to verify the fix

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
plugins/xdebug/xdebug_transforms.cc	Added conditional check to prevent unnecessary callbacks when no data is consumed
tests/gold_tests/pluginTest/xdebug/x_probe_full_json_slow_origin/x_probe_full_json_slow_origin.test.py	New test to verify the transform handles delayed body data correctly
tests/gold_tests/pluginTest/xdebug/x_probe_full_json_slow_origin/slow-body-server.sh	Custom slow server implementation to simulate delayed chunked response
tests/gold_tests/pluginTest/xdebug/x_probe_full_json_slow_origin/verify_no_loop.sh	Log analysis script to detect infinite loop behavior

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