Fix systable_recheck_tuple() for MVCC scan snapshots.

Since this function assumed non-MVCC snapshots, it broke when commit
568d4138c646cd7cd8a837ac244ef2caf27c6bb8 switched its one caller from
SnapshotNow scans to MVCC-snapshot scans.

Reviewed by Robert Haas, Tom Lane and Andres Freund.

diff --git a/src/backend/access/index/genam.c b/src/backend/access/index/genam.c
index 2bfe78a92a3c0c809763687b3a2079a6d62fb857..60a2b4e43898e69b3a112f2bd6ac75b2c92bc4be 100644 (file)
--- a/src/backend/access/index/genam.c
+++ b/src/backend/access/index/genam.c
@@ -361,6 +361,10 @@ systable_getnext(SysScanDesc sysscan)
 /*
  * systable_recheck_tuple --- recheck visibility of most-recently-fetched tuple
  *
+ * In particular, determine if this tuple would be visible to a catalog scan
+ * that started now.  We don't handle the case of a non-MVCC scan snapshot,
+ * because no caller needs that yet.
+ *
  * This is useful to test whether an object was deleted while we waited to
  * acquire lock on it.
  *
@@ -370,30 +374,38 @@ systable_getnext(SysScanDesc sysscan)
 bool
 systable_recheck_tuple(SysScanDesc sysscan, HeapTuple tup)
 {
+   Snapshot    freshsnap;
    bool        result;
 
+   /*
+    * Trust that LockBuffer() and HeapTupleSatisfiesMVCC() do not themselves
+    * acquire snapshots, so we need not register the snapshot.  Those
+    * facilities are too low-level to have any business scanning tables.
+    */
+   freshsnap = GetCatalogSnapshot(RelationGetRelid(sysscan->heap_rel));
+
    if (sysscan->irel)
    {
        IndexScanDesc scan = sysscan->iscan;
 
+       Assert(IsMVCCSnapshot(scan->xs_snapshot));
        Assert(tup == &scan->xs_ctup);
        Assert(BufferIsValid(scan->xs_cbuf));
        /* must hold a buffer lock to call HeapTupleSatisfiesVisibility */
        LockBuffer(scan->xs_cbuf, BUFFER_LOCK_SHARE);
-       result = HeapTupleSatisfiesVisibility(tup, scan->xs_snapshot,
-                                             scan->xs_cbuf);
+       result = HeapTupleSatisfiesVisibility(tup, freshsnap, scan->xs_cbuf);
        LockBuffer(scan->xs_cbuf, BUFFER_LOCK_UNLOCK);
    }
    else
    {
        HeapScanDesc scan = sysscan->scan;
 
+       Assert(IsMVCCSnapshot(scan->rs_snapshot));
        Assert(tup == &scan->rs_ctup);
        Assert(BufferIsValid(scan->rs_cbuf));
        /* must hold a buffer lock to call HeapTupleSatisfiesVisibility */
        LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);
-       result = HeapTupleSatisfiesVisibility(tup, scan->rs_snapshot,
-                                             scan->rs_cbuf);
+       result = HeapTupleSatisfiesVisibility(tup, freshsnap, scan->rs_cbuf);
        LockBuffer(scan->rs_cbuf, BUFFER_LOCK_UNLOCK);
    }
    return result;