Create a stack of pl/python "execution contexts".

This replaces the former global variable PLy_curr_procedure, and provides
a place to stash per-call-level information.  In particular we create a
per-call-level scratch memory context.

For the moment, the scratch context is just used to avoid leaking memory
from datatype output function calls in PLyDict_FromTuple.  There probably
will be more use-cases in future.

Although this is a fix for a pre-existing memory leakage bug, it seems
sufficiently invasive to not want to back-patch; it feels better as part
of the major rearrangement of plpython code that we've already done as
part of 9.2.

Jan Urbański

src/pl/plpython/plpy_procedure.h

@@ -45,8 +45,4 @@ extern PLyProcedure *PLy_procedure_get(Oid fn_oid, bool is_trigger);
 extern void PLy_procedure_compile(PLyProcedure *proc, const char *src);
 extern void PLy_procedure_delete(PLyProcedure *proc);
 
-
-/* currently active plpython function */
-extern PLyProcedure *PLy_curr_procedure;
-
 #endif	/* PLPY_PROCEDURE_H */