Fix yet another crash in page split during GiST index creation.

Commit a7ee7c8513 fixed a bug in GiST page split during index creation,
where we failed to re-find the position of a downlink after the page
containing it was split. However, that fix was incomplete; the other call
to gistinserttuples() in the same function needs to also clear
'downlinkoffnum'.

Fixes bug #16134 reported by Alexander Lakhin, for real this time. The
previous fix was enough to fix the crash with the reproducer script for
bug #16162, but the original script for #16134 was still crashing.

Backpatch to v12, like the previous incomplete fix.

Discussion: https://www.postgresql.org/message-id/d869f537-abe4-d2ea-0510-38cd053f5152%40gmail.com

diff --git a/src/backend/access/gist/gist.c b/src/backend/access/gist/gist.c
index b87facb2fa97e39d88b782aad5e3ae591dc56d65..a259c80616d8085e366c79405a5f19f2188ebf53 100644 (file)
--- a/src/backend/access/gist/gist.c
+++ b/src/backend/access/gist/gist.c
@@ -1325,10 +1325,9 @@ gistfinishsplit(GISTInsertState *state, GISTInsertStack *stack,
         * downlink for the original page as one operation.
         */
        LockBuffer(stack->parent->buffer, GIST_EXCLUSIVE);
-       gistFindCorrectParent(state->r, stack);
 
        /*
-        * insert downlinks for the siblings from right to left, until there are
+        * Insert downlinks for the siblings from right to left, until there are
         * only two siblings left.
         */
        for (int pos = list_length(splitinfo) - 1; pos > 1; pos--)
@@ -1336,17 +1335,17 @@ gistfinishsplit(GISTInsertState *state, GISTInsertStack *stack,
                right = (GISTPageSplitInfo *) list_nth(splitinfo, pos);
                left = (GISTPageSplitInfo *) list_nth(splitinfo, pos - 1);
 
+               gistFindCorrectParent(state->r, stack);
                if (gistinserttuples(state, stack->parent, giststate,
                                                         &right->downlink, 1,
                                                         InvalidOffsetNumber,
                                                         left->buf, right->buf, false, false))
                {
                        /*
-                        * If the parent page was split, need to relocate the original
-                        * parent pointer.
+                        * If the parent page was split, the existing downlink might
+                        * have moved.
                         */
                        stack->downlinkoffnum = InvalidOffsetNumber;
-                       gistFindCorrectParent(state->r, stack);
                }
                /* gistinserttuples() released the lock on right->buf. */
        }
@@ -1361,13 +1360,21 @@ gistfinishsplit(GISTInsertState *state, GISTInsertStack *stack,
         */
        tuples[0] = left->downlink;
        tuples[1] = right->downlink;
-       gistinserttuples(state, stack->parent, giststate,
-                                        tuples, 2,
-                                        stack->downlinkoffnum,
-                                        left->buf, right->buf,
-                                        true,          /* Unlock parent */
-                                        unlockbuf      /* Unlock stack->buffer if caller wants that */
-               );
+       gistFindCorrectParent(state->r, stack);
+       if (gistinserttuples(state, stack->parent, giststate,
+                                                tuples, 2,
+                                                stack->downlinkoffnum,
+                                                left->buf, right->buf,
+                                                true,          /* Unlock parent */
+                                                unlockbuf      /* Unlock stack->buffer if caller wants that */
+                       ))
+       {
+               /*
+                * If the parent page was split, the downlink might have moved.
+                */
+               stack->downlinkoffnum = InvalidOffsetNumber;
+       }
+
        Assert(left->buf == stack->buffer);
 
        /*