Avoid leaking memory while evaluating arguments for a table function.

tglsfdc · tglsfdc · commit d297c91d4379 · 2014-06-19T22:14:00.000-04:00
ExecMakeTableFunctionResult evaluated the arguments for a function-in-FROM
in the query-lifespan memory context.  This is insignificant in simple
cases where the function relation is scanned only once; but if the function
is in a sub-SELECT or is on the inside of a nested loop, any memory
consumed during argument evaluation can add up quickly.  (The potential for
trouble here had been foreseen long ago, per existing comments; but we'd
not previously seen a complaint from the field about it.)  To fix, create
an additional temporary context just for this purpose.

Per an example from MauMau.  Back-patch to all active branches.
diff --git a/src/backend/executor/execQual.c b/src/backend/executor/execQual.c
@@ -1935,6 +1935,7 @@ ExecMakeFunctionResultNoSets(FuncExprState *fcache,
 Tuplestorestate *
 ExecMakeTableFunctionResult(ExprState *funcexpr,
 							ExprContext *econtext,
+							MemoryContext argContext,
 							TupleDesc expectedDesc,
 							bool randomAccess)
 {
@@ -2013,13 +2014,19 @@ ExecMakeTableFunctionResult(ExprState *funcexpr,
 		/*
 		 * Evaluate the function's argument list.
 		 *
-		 * Note: ideally, we'd do this in the per-tuple context, but then the
-		 * argument values would disappear when we reset the context in the
-		 * inner loop.  So do it in caller context.  Perhaps we should make a
-		 * separate context just to hold the evaluated arguments?
+		 * We can't do this in the per-tuple context: the argument values
+		 * would disappear when we reset that context in the inner loop.  And
+		 * the caller's CurrentMemoryContext is typically a query-lifespan
+		 * context, so we don't want to leak memory there.  We require the
+		 * caller to pass a separate memory context that can be used for this,
+		 * and can be reset each time through to avoid bloat.
 		 */
+		MemoryContextReset(argContext);
+		oldcontext = MemoryContextSwitchTo(argContext);
 		fcinfo.flinfo = &(fcache->func);
 		argDone = ExecEvalFuncArgs(&fcinfo, fcache->args, econtext);
+		MemoryContextSwitchTo(oldcontext);
+
 		/* We don't allow sets in the arguments of the table function */
 		if (argDone != ExprSingleResult)
 			ereport(ERROR,
diff --git a/src/backend/executor/nodeFunctionscan.c b/src/backend/executor/nodeFunctionscan.c
@@ -25,6 +25,7 @@
 #include "executor/nodeFunctionscan.h"
 #include "funcapi.h"
 #include "utils/builtins.h"
+#include "utils/memutils.h"
 
 
 static TupleTableSlot *FunctionNext(FunctionScanState *node);
@@ -64,6 +65,7 @@ FunctionNext(FunctionScanState *node)
 		node->tuplestorestate = tuplestorestate =
 			ExecMakeTableFunctionResult(node->funcexpr,
 										node->ss.ps.ps_ExprContext,
+										node->argcontext,
 										node->tupdesc,
 										node->eflags & EXEC_FLAG_BACKWARD);
 	}
@@ -217,6 +219,19 @@ ExecInitFunctionScan(FunctionScan *node, EState *estate, int eflags)
 	ExecAssignResultTypeFromTL(&scanstate->ss.ps);
 	ExecAssignScanProjectionInfo(&scanstate->ss);
 
+	/*
+	 * Create a memory context that ExecMakeTableFunctionResult can use to
+	 * evaluate function arguments in.  We can't use the per-tuple context for
+	 * this because it gets reset too often; but we don't want to leak
+	 * evaluation results into the query-lifespan context either.  We just
+	 * need one context, because we evaluate each function separately.
+	 */
+	scanstate->argcontext = AllocSetContextCreate(CurrentMemoryContext,
+												  "Table function arguments",
+												  ALLOCSET_DEFAULT_MINSIZE,
+												  ALLOCSET_DEFAULT_INITSIZE,
+												  ALLOCSET_DEFAULT_MAXSIZE);
+
 	return scanstate;
 }
 
diff --git a/src/include/executor/executor.h b/src/include/executor/executor.h
@@ -188,6 +188,7 @@ extern Datum GetAttributeByName(HeapTupleHeader tuple, const char *attname,
 				   bool *isNull);
 extern Tuplestorestate *ExecMakeTableFunctionResult(ExprState *funcexpr,
 							ExprContext *econtext,
+							MemoryContext argContext,
 							TupleDesc expectedDesc,
 							bool randomAccess);
 extern Datum ExecEvalExprSwitchContext(ExprState *expression, ExprContext *econtext,
diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
@@ -1240,6 +1240,7 @@ typedef struct SubqueryScanState
  *		tupdesc				expected return tuple description
  *		tuplestorestate		private state of tuplestore.c
  *		funcexpr			state for function expression being evaluated
+ *		argcontext			memory context to evaluate function arguments in
  * ----------------
  */
 typedef struct FunctionScanState
@@ -1249,6 +1250,7 @@ typedef struct FunctionScanState
 	TupleDesc	tupdesc;
 	Tuplestorestate *tuplestorestate;
 	ExprState  *funcexpr;
+	MemoryContext argcontext;
 } FunctionScanState;
 
 /* ----------------
