Fix dsa.c with different resource owners.

The comments in dsa.c suggested that areas were owned by resource
owners, but it was not in fact tracked explicitly. The DSM attachments
held by the dsa were owned by resource owners, but not the area
itself.  That led to confusion if you used one resource owner to
attach or create the area, but then switched to a different resource
owner before allocating or even just accessing the allocations in the
area with dsa_get_address(). The additional DSM segments associated
with the area would get owned by a different resource owner than the
initial segment.  To fix, add an explicit 'resowner' field to
dsa_area.  It replaces the 'mapping_pinned' flag; resowner == NULL now
indicates that the mapping is pinned.

This is arguably a bug fix, but I'm not backpatching because it
doesn't seem to be a live bug in the back branches. In 'master', it is
a bug because commit b8bff07 made ResourceOwners more strict so
that you are no longer allowed to remember new resources in a
ResourceOwner after you have started to release it. Merely accessing a
dsa pointer might need to attach a new DSM segment, and before this
commit it was temporarily remembered in the current owner for a very
brief period even if the DSA was pinned. And that could happen in
AtEOXact_PgStat(), which is called after the owner is already released.

Reported-by: Alexander Lakhin
Reviewed-by: Alexander Lakhin, Thomas Munro, Andres Freund
Discussion: https://www.postgresql.org/message-id/11b70743-c5f3-3910-8e5b-dd6c115ff829%40gmail.com

Author: hlinnaka

src/backend/utils/mmgr/dsa.c

@@ -59,6 +59,7 @@
 #include "utils/dsa.h"
 #include "utils/freepage.h"
 #include "utils/memutils.h"
+#include "utils/resowner.h"
 
 /*
  * The size of the initial DSM segment that backs a dsa_area created by
@@ -368,8 +369,13 @@ struct dsa_area
 	/* Pointer to the control object in shared memory. */
 	dsa_area_control *control;
 
-	/* Has the mapping been pinned? */
-	bool		mapping_pinned;
+	/*
+	 * All the mappings are owned by this.  The dsa_area itself is not
+	 * directly tracked by the ResourceOwner, but the effect is the same. NULL
+	 * if the attachment has session lifespan, i.e if dsa_pin_mapping() has
+	 * been called.
+	 */
+	ResourceOwner resowner;
 
 	/*
 	 * This backend's array of segment maps, ordered by segment index
@@ -645,12 +651,14 @@ dsa_pin_mapping(dsa_area *area)
 {
 	int			i;
 
-	Assert(!area->mapping_pinned);
-	area->mapping_pinned = true;
+	if (area->resowner != NULL)
+	{
+		area->resowner = NULL;
 
-	for (i = 0; i <= area->high_segment_index; ++i)
-		if (area->segment_maps[i].segment != NULL)
-			dsm_pin_mapping(area->segment_maps[i].segment);
+		for (i = 0; i <= area->high_segment_index; ++i)
+			if (area->segment_maps[i].segment != NULL)
+				dsm_pin_mapping(area->segment_maps[i].segment);
+	}
 }
 
 /*
@@ -1264,7 +1272,7 @@ create_internal(void *place, size_t size,
 	 */
 	area = palloc(sizeof(dsa_area));
 	area->control = control;
-	area->mapping_pinned = false;
+	area->resowner = CurrentResourceOwner;
 	memset(area->segment_maps, 0, sizeof(dsa_segment_map) * DSA_MAX_SEGMENTS);
 	area->high_segment_index = 0;
 	area->freed_segment_counter = 0;
@@ -1320,7 +1328,7 @@ attach_internal(void *place, dsm_segment *segment, dsa_handle handle)
 	/* Build the backend-local area object. */
 	area = palloc(sizeof(dsa_area));
 	area->control = control;
-	area->mapping_pinned = false;
+	area->resowner = CurrentResourceOwner;
 	memset(&area->segment_maps[0], 0,
 		   sizeof(dsa_segment_map) * DSA_MAX_SEGMENTS);
 	area->high_segment_index = 0;
@@ -1743,6 +1751,7 @@ get_segment_by_index(dsa_area *area, dsa_segment_index index)
 		dsm_handle	handle;
 		dsm_segment *segment;
 		dsa_segment_map *segment_map;
+		ResourceOwner oldowner;
 
 		/*
 		 * If we are reached by dsa_free or dsa_get_address, there must be at
@@ -1761,11 +1770,12 @@ get_segment_by_index(dsa_area *area, dsa_segment_index index)
 			elog(ERROR,
 				 "dsa_area could not attach to a segment that has been freed");
 
+		oldowner = CurrentResourceOwner;
+		CurrentResourceOwner = area->resowner;
 		segment = dsm_attach(handle);
+		CurrentResourceOwner = oldowner;
 		if (segment == NULL)
 			elog(ERROR, "dsa_area could not attach to segment");
-		if (area->mapping_pinned)
-			dsm_pin_mapping(segment);
 		segment_map = &area->segment_maps[index];
 		segment_map->segment = segment;
 		segment_map->mapped_address = dsm_segment_address(segment);
@@ -2067,6 +2077,7 @@ make_new_segment(dsa_area *area, size_t requested_pages)
 	size_t		usable_pages;
 	dsa_segment_map *segment_map;
 	dsm_segment *segment;
+	ResourceOwner oldowner;
 
 	Assert(LWLockHeldByMe(DSA_AREA_LOCK(area)));
 
@@ -2151,12 +2162,13 @@ make_new_segment(dsa_area *area, size_t requested_pages)
 	}
 
 	/* Create the segment. */
+	oldowner = CurrentResourceOwner;
+	CurrentResourceOwner = area->resowner;
 	segment = dsm_create(total_size, 0);
+	CurrentResourceOwner = oldowner;
 	if (segment == NULL)
 		return NULL;
 	dsm_pin_segment(segment);
-	if (area->mapping_pinned)
-		dsm_pin_mapping(segment);
 
 	/* Store the handle in shared memory to be found by index. */
 	area->control->segment_handles[new_index] =