Fix transient memory leak for SRFs in FROM.

In a9c35cf8 I changed ExecMakeTableFunctionResult() to dynamically
allocate the FunctionCallInfo used to call the SRF. Unfortunately I
did not account for the fact that the surrounding memory context has
query lifetime, leading to a leak till the end of the query.

In most cases the leak is fairly inconsequential, but if the
FunctionScan is done many times in the query, the leak can add
up. This happens e.g. if the function scan is on the inner side of a
nested loop, due to a lateral join.
EXPLAIN SELECT sum(f) FROM generate_series(1, 100000000) g(i), generate_series(i, i+1) f;
quickly shows the leak.

Instead of explicitly freeing the FunctionCallInfo it seems better to
make sure all the per-set temporary state in
ExecMakeTableFunctionResult() is cleaned up wholesale. Currently
that's probably just the FunctionCallInfo allocation, but since
there's some initialization work, and since there's already an
appropriate context, this seems like a more robust approach.

Bug: #16112
Reported-By: Ben Cornett
Author: Andres Freund
Reviewed-By: Tom Lane
Discussion: https://postgr.es/m/16112-4448bbf55a404189%40postgresql.org
Backpatch: 12, a9c35cf8

src/backend/executor/execSRF.c

@@ -114,10 +114,23 @@ ExecMakeTableFunctionResult(SetExprState *setexpr,
 	ReturnSetInfo rsinfo;
 	HeapTupleData tmptup;
 	MemoryContext callerContext;
-	MemoryContext oldcontext;
 	bool		first_time = true;
 
-	callerContext = CurrentMemoryContext;
+	/*
+	 * Execute per-tablefunc actions in appropriate context.
+	 *
+	 * The FunctionCallInfo needs to live across all the calls to a
+	 * ValuePerCall function, so it can't be allocated in the per-tuple
+	 * context. Similarly, the function arguments need to be evaluated in a
+	 * context that is longer lived than the per-tuple context: The argument
+	 * values would otherwise disappear when we reset that context in the
+	 * inner loop.  As the caller's CurrentMemoryContext is typically a
+	 * query-lifespan context, 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);
+	callerContext = MemoryContextSwitchTo(argContext);
 
 	funcrettype = exprType((Node *) setexpr->expr);
 
@@ -163,21 +176,9 @@ ExecMakeTableFunctionResult(SetExprState *setexpr,
 								 list_length(setexpr->args),
 								 setexpr->fcinfo->fncollation,
 								 NULL, (Node *) &rsinfo);
-
-		/*
-		 * Evaluate the function's argument list.
-		 *
-		 * 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);
+		/* evaluate the function's argument list */
+		Assert(CurrentMemoryContext == argContext);
 		ExecEvalFuncArgs(fcinfo, setexpr->args, econtext);
-		MemoryContextSwitchTo(oldcontext);
 
 		/*
 		 * If function is strict, and there are any NULL arguments, skip
@@ -217,7 +218,7 @@ ExecMakeTableFunctionResult(SetExprState *setexpr,
 		CHECK_FOR_INTERRUPTS();
 
 		/*
-		 * reset per-tuple memory context before each call of the function or
+		 * Reset per-tuple memory context before each call of the function or
 		 * expression. This cleans up any local memory the function may leak
 		 * when called.
 		 */
@@ -257,7 +258,9 @@ ExecMakeTableFunctionResult(SetExprState *setexpr,
 			 */
 			if (first_time)
 			{
-				oldcontext = MemoryContextSwitchTo(econtext->ecxt_per_query_memory);
+				MemoryContext oldcontext =
+					MemoryContextSwitchTo(econtext->ecxt_per_query_memory);
+
 				tupstore = tuplestore_begin_heap(randomAccess, false, work_mem);
 				rsinfo.setResult = tupstore;
 				if (!returnsTuple)
@@ -285,13 +288,15 @@ ExecMakeTableFunctionResult(SetExprState *setexpr,
 
 					if (tupdesc == NULL)
 					{
+						MemoryContext oldcontext =
+							MemoryContextSwitchTo(econtext->ecxt_per_query_memory);
+
 						/*
 						 * This is the first non-NULL result from the
 						 * function.  Use the type info embedded in the
 						 * rowtype Datum to look up the needed tupdesc.  Make
 						 * a copy for the query.
 						 */
-						oldcontext = MemoryContextSwitchTo(econtext->ecxt_per_query_memory);
 						tupdesc = lookup_rowtype_tupdesc_copy(HeapTupleHeaderGetTypeId(td),
 															  HeapTupleHeaderGetTypMod(td));
 						rsinfo.setDesc = tupdesc;
@@ -378,15 +383,18 @@ no_function_result:
 	 */
 	if (rsinfo.setResult == NULL)
 	{
-		MemoryContextSwitchTo(econtext->ecxt_per_query_memory);
+		MemoryContext oldcontext =
+			MemoryContextSwitchTo(econtext->ecxt_per_query_memory);
+
 		tupstore = tuplestore_begin_heap(randomAccess, false, work_mem);
 		rsinfo.setResult = tupstore;
+		MemoryContextSwitchTo(oldcontext);
+
 		if (!returnsSet)
 		{
 			int			natts = expectedDesc->natts;
 			bool	   *nullflags;
 
-			MemoryContextSwitchTo(econtext->ecxt_per_tuple_memory);
 			nullflags = (bool *) palloc(natts * sizeof(bool));
 			memset(nullflags, true, natts * sizeof(bool));
 			tuplestore_putvalues(tupstore, expectedDesc, NULL, nullflags);