In file ceval.h, the macro Py_LeaveRecursiveCall is defined like this:

#define Py_LeaveRecursiveCall()				\
    do{ if((--PyThreadState_GET()->recursion_depth) <   \
	   _Py_CheckRecursionLimit - 50);               \
	  PyThreadState_GET()->overflowed = 0;          \
    } while(0)

The semicolon on the third line seems very suspicious to me: the if()
statement has no side effect, and "overflowed" is always reset to zero.

I don't really understand the consequences, though. The variable seems
to be used as an additional protection against C code that does not
correctly unwind when the recursion limit is hit.

I think you are right - that's a bug. As a consequence, overflowed would
be cleared less often than it is now, which in turn may cause a fatal
abort in case it fails to recover from a stack overflow (i.e. if it
consumes another 50 stack frames, instead of unwinding).

This entire machinery is there to guard against hard-to-analyze crashes
resulting from stack overflows, in particular when a dictionary lookup
happened to cause a stack overflow. The overflow would raise the Python
exception, which would then be cleared in the dictionary lookup, as if
nothing happened.

Thanks again for pointing that out. Fixed in r60750.