Fix aboriginal mistake in plpython's set-returning-function support.

We must stay in the function's SPI context until done calling the iterator
that returns the set result.  Otherwise, any attempt to invoke SPI features
in the python code called by the iterator will malfunction.  Diagnosis and
patch by Jan Urbanski, per bug report from Jean-Baptiste Quenot.

Back-patch to 8.2; there was no support for SRFs in previous versions of
plpython.

Author: tglsfdc

src/pl/plpython/expected/plpython_setof.out

@@ -31,6 +31,14 @@ class producer:
 		return self.icontent
 return producer(count, content)
 $$ LANGUAGE plpythonu;
+CREATE FUNCTION test_setof_spi_in_iterator() RETURNS SETOF text AS
+$$
+    for s in ('Hello', 'Brave', 'New', 'World'):
+        plpy.execute('select 1')
+        yield s
+        plpy.execute('select 2')
+$$
+LANGUAGE plpythonu;
 -- Test set returning functions
 SELECT test_setof_as_list(0, 'list');
  test_setof_as_list 
@@ -107,3 +115,12 @@ SELECT test_setof_as_iterator(2, null);
 
 (2 rows)
 
+SELECT test_setof_spi_in_iterator();
+ test_setof_spi_in_iterator 
+----------------------------
+ Hello
+ Brave
+ New
+ World
+(4 rows)
+

src/pl/plpython/plpython.c

@@ -985,7 +985,10 @@ PLy_function_handler(FunctionCallInfo fcinfo, PLyProcedure *proc)
 	{
 		if (!proc->is_setof || proc->setof == NULL)
 		{
-			/* Simple type returning function or first time for SETOF function */
+			/*
+			 * Simple type returning function or first time for SETOF function:
+			 * actually execute the function.
+			 */
 			plargs = PLy_function_build_args(fcinfo, proc);
 			plrv = PLy_procedure_call(proc, "args", plargs);
 			if (!proc->is_setof)
@@ -1000,14 +1003,10 @@ PLy_function_handler(FunctionCallInfo fcinfo, PLyProcedure *proc)
 		}
 
 		/*
-		 * Disconnect from SPI manager and then create the return values datum
-		 * (if the input function does a palloc for it this must not be
-		 * allocated in the SPI memory context because SPI_finish would free
-		 * it).
+		 * If it returns a set, call the iterator to get the next return item.
+		 * We stay in the SPI context while doing this, because PyIter_Next()
+		 * calls back into Python code which might contain SPI calls.
 		 */
-		if (SPI_finish() != SPI_OK_FINISH)
-			elog(ERROR, "SPI_finish failed");
-
 		if (proc->is_setof)
 		{
 			bool		has_error = false;
@@ -1064,11 +1063,24 @@ PLy_function_handler(FunctionCallInfo fcinfo, PLyProcedure *proc)
 							(errcode(ERRCODE_DATA_EXCEPTION),
 						  errmsg("error fetching next item from iterator")));
 
+				/* Disconnect from the SPI manager before returning */
+				if (SPI_finish() != SPI_OK_FINISH)
+					elog(ERROR, "SPI_finish failed");
+
 				fcinfo->isnull = true;
 				return (Datum) NULL;
 			}
 		}
 
+		/*
+		 * Disconnect from SPI manager and then create the return values datum
+		 * (if the input function does a palloc for it this must not be
+		 * allocated in the SPI memory context because SPI_finish would free
+		 * it).
+		 */
+		if (SPI_finish() != SPI_OK_FINISH)
+			elog(ERROR, "SPI_finish failed");
+
 		plerrcontext.callback = plpython_return_error_callback;
 		plerrcontext.previous = error_context_stack;
 		error_context_stack = &plerrcontext;

src/pl/plpython/sql/plpython_setof.sql

@@ -35,6 +35,15 @@ class producer:
 return producer(count, content)
 $$ LANGUAGE plpythonu;
 
+CREATE FUNCTION test_setof_spi_in_iterator() RETURNS SETOF text AS
+$$
+    for s in ('Hello', 'Brave', 'New', 'World'):
+        plpy.execute('select 1')
+        yield s
+        plpy.execute('select 2')
+$$
+LANGUAGE plpythonu;
+
 
 -- Test set returning functions
 SELECT test_setof_as_list(0, 'list');
@@ -51,3 +60,5 @@ SELECT test_setof_as_iterator(0, 'list');
 SELECT test_setof_as_iterator(1, 'list');
 SELECT test_setof_as_iterator(2, 'list');
 SELECT test_setof_as_iterator(2, null);
+
+SELECT test_setof_spi_in_iterator();