WT-6141 Disable checkpoint deletion during backup (#5616)

* WT-6141 Disable checkpoint deletion during backup

Keep all WiredTiger checkpoints while a hot backup is active.

Also disables a pair of tests that checked for the old behavior.

Co-authored-by: Keith A. Smith <keith.smith@mongodb>

src/txn/txn_ckpt.c

@@ -1259,12 +1259,19 @@ __checkpoint_lock_dirty_tree_int(WT_SESSION_IMPL *session, bool is_checkpoint, b
             continue;
         is_wt_ckpt = WT_PREFIX_MATCH(ckpt->name, WT_CHECKPOINT);
 
+/*
+ * If there is a hot backup, don't delete any WiredTiger checkpoint that could possibly have been
+ * created before the backup started. Fail if trying to delete any other named checkpoint.
+ */
+#ifdef DISABLED_CODE
+        if (conn->hot_backup_start != 0 && ckpt->sec <= conn->hot_backup_start) {
+#else
         /*
-         * If there is a hot backup, don't delete any WiredTiger checkpoint that could possibly have
-         * been created before the backup started. Fail if trying to delete any other named
-         * checkpoint.
+         * N.B. Despite the comment above, dropping checkpoints during backup can corrupt the
+         * backup. For now we retain all WiredTiger checkpoints.
          */
-        if (conn->hot_backup_start != 0 && ckpt->sec <= conn->hot_backup_start) {
+        if (conn->hot_backup_start != 0) {
+#endif
             if (is_wt_ckpt) {
                 F_CLR(ckpt, WT_CKPT_DELETE);
                 continue;

test/suite/test_backup01.py

@@ -192,12 +192,15 @@ def test_checkpoint_delete(self):
         # Confirm that a named checkpoint created after a backup cursor can be dropped.
         # Need to pause a couple seconds; checkpoints that are assigned the same timestamp as
         # the backup will be pinned, even if they occur after the backup starts.
+
         time.sleep(2)
-        self.session.checkpoint("name=four")
-        self.session.checkpoint("drop=(four)")
-        self.assertRaises(wiredtiger.WiredTigerError,
-            lambda: self.session.open_cursor(
-            self.objs[0][0], None, "checkpoint=four"))
+        # N.B. This test is temporarily disabled because deleting checkpoints during backup
+        # can corrupt the backup.
+        #self.session.checkpoint("name=four")
+        #self.session.checkpoint("drop=(four)")
+        #self.assertRaises(wiredtiger.WiredTigerError,
+        #    lambda: self.session.open_cursor(
+        #    self.objs[0][0], None, "checkpoint=four"))
 
         # Confirm that after closing the backup cursor the original named checkpoint can
         # be deleted.

test/suite/test_checkpoint05.py

@@ -76,7 +76,10 @@ def test_checkpoints_during_backup(self):
         # is generous.  But if WT isn't deleting checkpoints there would
         # be about 30x more checkpoints here.
         final_count = self.count_checkpoints()
-        self.assertTrue (final_count < initial_count * 3)
+
+        # N.B. This test is temporarily disabled because deleting checkpoints during backup
+        # can corrupt the backup.
+        # self.assertTrue (final_count < initial_count * 3)
 
         self.session.close()