The Debug Sync facility allows placement of synchronization points in the code.

They can be activated by statements that set the debug_sync system variable. An active synchronization point can emit a signal or wait for a signal to be emitted by another thread. This waiting times out after 300 seconds by default. The --debug-sync-timeout=N option for mysql-test-run.pl changes that timeout to N seconds. A timeout of zero disables the facility altogether, so that synchronization points will not emit or wait for signals, even if activated.

The purpose of the timeout is to avoid a complete lockup in test cases. If for some reason the expected signal is not emitted by any thread, the execution of the affected statement will not block forever. A warning shows up when the timeout happens. That makes a difference in the test result so that it will not go undetected.

For test cases that require the Debug Sync facility, include the following line in the test case file:

--source include/have_debug_sync.inc

For a description of the Debug Sync facility and how to use synchronization points, see MySQL Source Code Documentation: Thread Synchronization.