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 f06bf650756d2c693e7c2ab5eba063a36034912e..01e3492cb837385316ed5161f5009c7f325ee53f 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);