AST (Asyncronous System Trap)

What is an Asyncronous System Trap?: This is similar to Unixes signals or OS interrupts. Technically this is callbacks from the Lock Manager into the Engine, called when somebody acquires a lock at a level not compatible with the current level. In this case all processes who own the lock receive notification and should take appropriate action to release the lock (immediately or deferred-ASAP).

The main trick with AST’s handlers is that AST is sent asynchronously and the handler must take care to work correctly. Since v.2.5 all ATS handlers run in a separate thread. Before v.2.5 it works using signals on Unixes and threads on Windows.

P.S. the issue with old glibc and Firebird 2.5 is that since v.2.5 we abandoned the usage of old System V semaphores (do you remember semaphores are exhausted error? now you can forget it completely) and started to use pthread mutexes. And old glibc have bugs in the pthread mutex implementation.

Source: https://www.firebirdnews.org