Fix memory leaks in PL/Python.

Previously, plpython was in the habit of allocating a lot of stuff in
TopMemoryContext, and it was very slipshod about making sure that stuff
got cleaned up; in particular, use of TopMemoryContext as fn_mcxt for
function calls represents an unfixable leak, since we generally don't
know what the called function might have allocated in fn_mcxt.  This
results in session-lifespan leakage in certain usage scenarios, as for
example in a case reported by Ed Behn back in July.

To fix, get rid of all the retail allocations in TopMemoryContext.
All long-lived allocations are now made in sub-contexts that are
associated with specific objects (either pl/python procedures, or
Python-visible objects such as cursors and plans).  We can clean these
up when the associated object is deleted.

I went so far as to get rid of PLy_malloc completely.  There were a
couple of places where it could still have been used safely, but on
the whole it was just an invitation to bad coding.

Haribabu Kommi, based on a draft patch by Heikki Linnakangas;
some further work by me

1 files changed, 11 insertions, 4 deletions

diff --git a/src/pl/plpython/plpy_subxactobject.c b/src/pl/plpython/plpy_subxactobject.c
index 2e7ec4fdab4..81fb3a3a4ab 100644
--- a/src/pl/plpython/plpy_subxactobject.c
+++ b/src/pl/plpython/plpy_subxactobject.c
@@ -8,6 +8,7 @@
 
 #include "access/xact.h"
 #include "executor/spi.h"
+#include "utils/memutils.h"
 
 #include "plpython.h"
 
@@ -132,16 +133,22 @@ PLy_subtransaction_enter(PyObject *self, PyObject *unused)
 	subxact->started = true;
 	oldcontext = CurrentMemoryContext;
 
-	subxactdata = PLy_malloc(sizeof(*subxactdata));
+	subxactdata = (PLySubtransactionData *)
+		MemoryContextAlloc(TopTransactionContext,
+						   sizeof(PLySubtransactionData));
+
 	subxactdata->oldcontext = oldcontext;
 	subxactdata->oldowner = CurrentResourceOwner;
 
 	BeginInternalSubTransaction(NULL);
-	/* Do not want to leave the previous memory context */
-	MemoryContextSwitchTo(oldcontext);
 
+	/* Be sure that cells of explicit_subtransactions list are long-lived */
+	MemoryContextSwitchTo(TopTransactionContext);
 	explicit_subtransactions = lcons(subxactdata, explicit_subtransactions);
 
+	/* Caller wants to stay in original memory context */
+	MemoryContextSwitchTo(oldcontext);
+
 	Py_INCREF(self);
 	return self;
 }
@@ -204,7 +211,7 @@ PLy_subtransaction_exit(PyObject *self, PyObject *args)
 
 	MemoryContextSwitchTo(subxactdata->oldcontext);
 	CurrentResourceOwner = subxactdata->oldowner;
-	PLy_free(subxactdata);
+	pfree(subxactdata);
 
 	/*
 	 * AtEOSubXact_SPI() should not have popped any SPI context, but just in