Avoid calling PageGetSpecialPointer() on an all-zeros page.

That was otherwise harmless, but tripped the new assertion in
PageGetSpecialPointer().

Reported by Amit Langote. Backpatch to 9.5, where the assertion was added.

diff --git a/src/backend/access/nbtree/nbtree.c b/src/backend/access/nbtree/nbtree.c
index 9431ab5d042811a45fd53df2918c34abac3249f1..cf4a6dc7c478075286f00e88903336cafdeca760 100644 (file)
--- a/src/backend/access/nbtree/nbtree.c
+++ b/src/backend/access/nbtree/nbtree.c
@@ -881,7 +881,7 @@ btvacuumpage(BTVacState *vstate, BlockNumber blkno, BlockNumber orig_blkno)
        BlockNumber recurse_to;
        Buffer          buf;
        Page            page;
-       BTPageOpaque opaque;
+       BTPageOpaque opaque = NULL;
 
 restart:
        delete_now = false;
@@ -900,9 +900,11 @@ restart:
                                                         info->strategy);
        LockBuffer(buf, BT_READ);
        page = BufferGetPage(buf);
-       opaque = (BTPageOpaque) PageGetSpecialPointer(page);
        if (!PageIsNew(page))
+       {
                _bt_checkpage(rel, buf);
+               opaque = (BTPageOpaque) PageGetSpecialPointer(page);
+       }
 
        /*
         * If we are recursing, the only case we want to do anything with is a