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_main.h

@@ -10,4 +10,19 @@
 /* the interpreter's globals dict */
 extern PyObject *PLy_interp_globals;
 
+/*
+ * A stack of PL/Python execution contexts. Each time user-defined Python code
+ * is called, an execution context is created and put on the stack. After the
+ * Python code returns, the context is destroyed.
+ */
+typedef struct PLyExecutionContext
+{
+	PLyProcedure	*curr_proc;		/* the currently executing procedure */
+	MemoryContext	scratch_ctx;	/* a context for things like type I/O */
+	struct PLyExecutionContext *next;	/* previous stack level */
+} PLyExecutionContext;
+
+/* Get the current execution context */
+extern PLyExecutionContext *PLy_current_execution_context(void);
+
 #endif	/* PLPY_MAIN_H */