Change inv_truncate() to not repeat its systable_getnext_ordered() scan.

In the case where the initial call of systable_getnext_ordered() returned
NULL, this function would nonetheless call it again.  That's undefined
behavior that only by chance failed to not give visibly incorrect results.
Put an if-test around the final loop to prevent that, and in passing
improve some comments.  No back-patch since there's no actual failure.

Per report from YAMAMOTO Takashi.

diff --git a/src/backend/storage/large_object/inv_api.c b/src/backend/storage/large_object/inv_api.c
index f06bf65..01e3492 100644 (file)
--- a/src/backend/storage/large_object/inv_api.c
+++ b/src/backend/storage/large_object/inv_api.c
@@ -762,6 +762,9 @@ inv_truncate(LargeObjectDesc *obj_desc, int len)
 
        indstate = CatalogOpenIndexes(lo_heap_r);
 
+       /*
+        * Set up to find all pages with desired loid and pageno >= target
+        */
        ScanKeyInit(&skey[0],
                                Anum_pg_largeobject_loid,
                                BTEqualStrategyNumber, F_OIDEQ,
@@ -841,10 +844,14 @@ inv_truncate(LargeObjectDesc *obj_desc, int len)
        {
                /*
                 * If the first page we found was after the truncation point, we're in
-                * a hole that we'll fill, but we need to delete the later page.
+                * a hole that we'll fill, but we need to delete the later page because
+                * the loop below won't visit it again.
                 */
-               if (olddata != NULL && olddata->pageno > pageno)
+               if (olddata != NULL)
+               {
+                       Assert(olddata->pageno > pageno);
                        simple_heap_delete(lo_heap_r, &oldtuple->t_self);
+               }
 
                /*
                 * Write a brand new page.
@@ -873,11 +880,15 @@ inv_truncate(LargeObjectDesc *obj_desc, int len)
        }
 
        /*
-        * Delete any pages after the truncation point
+        * Delete any pages after the truncation point.  If the initial search
+        * didn't find a page, then of course there's nothing more to do.
         */
-       while ((oldtuple = systable_getnext_ordered(sd, ForwardScanDirection)) != NULL)
+       if (olddata != NULL)
        {
-               simple_heap_delete(lo_heap_r, &oldtuple->t_self);
+               while ((oldtuple = systable_getnext_ordered(sd, ForwardScanDirection)) != NULL)
+               {
+                       simple_heap_delete(lo_heap_r, &oldtuple->t_self);
+               }
        }
 
        systable_endscan_ordered(sd);