Prior to posing it as a final patch contribution or opening a JIRA ticket, I wanted to propose this idea.

The attached patch adds what amounts to compile-time checking for unmatched DBUG_ENTER/DBUG_LEAVE (DBUG_RETURN, DBUG_VOID_RETURN) by introducing a variable in DBUG_ENTER which is only used in DBUG_LEAVE. This allows any compiler which can robustly detect unused variables to detect the mismatch at compile-time. There is already a run-time check for this case, but it is somewhat limited as it requires _db_return_ to be called in order to detect the mismatch, and this is in practice not always the case. Particularly three cases allow this to escape detection:

1. Some instrumented functions are called only from non-instrumented functions.
   
2. Some instrumented functions are called only from non-returning functions, such as abort or die functions.
3. Some instrumented functions are called from functions who call my_end or my_thread_end (making cs unavailable) before returning.