Fix bug in cancellation of non-exclusive backup to avoid assertion failure.

Previously an assertion failure occurred when pg_stop_backup() for
non-exclusive backup was aborted while it's waiting for WAL files to
be archived. This assertion failure happened in do_pg_abort_backup()
which was called when a non-exclusive backup was canceled.
do_pg_abort_backup() assumes that there is at least one non-exclusive
backup running when it's called. But pg_stop_backup() can be canceled
even after it marks the end of non-exclusive backup (e.g.,
during waiting for WAL archiving). This broke the assumption that
do_pg_abort_backup() relies on, and which caused an assertion failure.

This commit changes do_pg_abort_backup() so that it does nothing
when non-exclusive backup has been already marked as completed.
That is, the asssumption is also changed, and do_pg_abort_backup()
now can handle even the case where it's called when there is
no running backup.

Backpatch to 9.6 where SQL-callable non-exclusive backup was added.

Author: Masahiko Sawada and Michael Paquier
Reviewed-By: Robert Haas and Fujii Masao
Discussion: https://www.postgresql.org/message-id/CAD21AoD2L1Fu2c==gnVASMyFAAaq3y-AQ2uEVj-zTCGFFjvmDg@mail.gmail.com

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 0791404263e56b498052aef3cc87c5c9b6e1d927..3e9a12dacdd0d6fe84747f56ac12c2a0218bcbf3 100644 (file)
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -10628,13 +10628,20 @@ do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
    /*
     * Mark that start phase has correctly finished for an exclusive backup.
     * Session-level locks are updated as well to reflect that state.
+    *
+    * Note that CHECK_FOR_INTERRUPTS() must not occur while updating
+    * backup counters and session-level lock. Otherwise they can be
+    * updated inconsistently, and which might cause do_pg_abort_backup()
+    * to fail.
     */
    if (exclusive)
    {
        WALInsertLockAcquireExclusive();
        XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_IN_PROGRESS;
-       WALInsertLockRelease();
+
+       /* Set session-level lock */
        sessionBackupState = SESSION_BACKUP_EXCLUSIVE;
+       WALInsertLockRelease();
    }
    else
        sessionBackupState = SESSION_BACKUP_NON_EXCLUSIVE;
@@ -10838,7 +10845,11 @@ do_pg_stop_backup(char *labelfile, bool waitforarchive, TimeLineID *stoptli_p)
    }
 
    /*
-    * OK to update backup counters and forcePageWrites
+    * OK to update backup counters, forcePageWrites and session-level lock.
+    *
+    * Note that CHECK_FOR_INTERRUPTS() must not occur while updating them.
+    * Otherwise they can be updated inconsistently, and which might cause
+    * do_pg_abort_backup() to fail.
     */
    WALInsertLockAcquireExclusive();
    if (exclusive)
@@ -10862,11 +10873,20 @@ do_pg_stop_backup(char *labelfile, bool waitforarchive, TimeLineID *stoptli_p)
    {
        XLogCtl->Insert.forcePageWrites = false;
    }
-   WALInsertLockRelease();
 
-   /* Clean up session-level lock */
+   /*
+    * Clean up session-level lock.
+    *
+    * You might think that WALInsertLockRelease() can be called
+    * before cleaning up session-level lock because session-level
+    * lock doesn't need to be protected with WAL insertion lock.
+    * But since CHECK_FOR_INTERRUPTS() can occur in it,
+    * session-level lock must be cleaned up before it.
+    */
    sessionBackupState = SESSION_BACKUP_NONE;
 
+   WALInsertLockRelease();
+
    /*
     * Read and parse the START WAL LOCATION line (this code is pretty crude,
     * but we are not expecting any variability in the file format).
@@ -11104,8 +11124,16 @@ do_pg_stop_backup(char *labelfile, bool waitforarchive, TimeLineID *stoptli_p)
 void
 do_pg_abort_backup(void)
 {
+   /*
+    * Quick exit if session is not keeping around a non-exclusive backup
+    * already started.
+    */
+   if (sessionBackupState == SESSION_BACKUP_NONE)
+       return;
+
    WALInsertLockAcquireExclusive();
    Assert(XLogCtl->Insert.nonExclusiveBackups > 0);
+   Assert(sessionBackupState == SESSION_BACKUP_NON_EXCLUSIVE);
    XLogCtl->Insert.nonExclusiveBackups--;
 
    if (XLogCtl->Insert.exclusiveBackupState == EXCLUSIVE_BACKUP_NONE &&

diff --git a/src/backend/replication/basebackup.c b/src/backend/replication/basebackup.c
index cd7d391b2ffd59089e8a557d754796c437d7295a..05ca95bac2a653e0a2ba3be0a4a839a95d8cb9ee 100644 (file)
--- a/src/backend/replication/basebackup.c
+++ b/src/backend/replication/basebackup.c
@@ -215,7 +215,7 @@ perform_base_backup(basebackup_options *opt)
     * Once do_pg_start_backup has been called, ensure that any failure causes
     * us to abort the backup so we don't "leak" a backup counter. For this
     * reason, *all* functionality between do_pg_start_backup() and
-    * do_pg_stop_backup() should be inside the error cleanup block!
+    * the end of do_pg_stop_backup() should be inside the error cleanup block!
     */
 
    PG_ENSURE_ERROR_CLEANUP(base_backup_cleanup, (Datum) 0);
@@ -324,10 +324,11 @@ perform_base_backup(basebackup_options *opt)
            else
                pq_putemptymessage('c');    /* CopyDone */
        }
+
+       endptr = do_pg_stop_backup(labelfile->data, !opt->nowait, &endtli);
    }
    PG_END_ENSURE_ERROR_CLEANUP(base_backup_cleanup, (Datum) 0);
 
-   endptr = do_pg_stop_backup(labelfile->data, !opt->nowait, &endtli);
 
    if (opt->includewal)
    {