Remove exclusive backup mode

Exclusive-mode backups have been deprecated since 9.6 (when
non-exclusive backups were introduced) due to the issues
they can cause should the system crash while one is running and
generally because non-exclusive provides a much better interface.
Further, exclusive backup mode wasn't really being tested (nor was most
of the related code- like being able to log in just to stop an exclusive
backup and the bits of the state machine related to that) and having to
possibly deal with an exclusive backup and the backup_label file
existing during pg_basebackup, pg_rewind, etc, added other complexities
that we are better off without.

This patch removes the exclusive backup mode, the various special cases
for dealing with it, and greatly simplifies the online backup code and
documentation.

Authors: David Steele, Nathan Bossart
Reviewed-by: Chapman Flack
Discussion: https://postgr.es/m/ac7339ca-3718-3c93-929f-99e725d1172c@pgmasters.net
https://postgr.es/m/CAHg+QDfiM+WU61tF6=nPZocMZvHDzCK47Kneyb0ZRULYzV5sKQ@mail.gmail.com

1 files changed, 15 insertions, 5 deletions

diff --git a/src/backend/access/transam/xlogrecovery.c b/src/backend/access/transam/xlogrecovery.c
index 8d2395dae25..1b7bae387a0 100644
--- a/src/backend/access/transam/xlogrecovery.c
+++ b/src/backend/access/transam/xlogrecovery.c
@@ -1183,9 +1183,14 @@ read_backup_label(XLogRecPtr *checkPointLoc, TimeLineID *backupLabelTLI,
 	*backupLabelTLI = tli_from_walseg;
 
 	/*
-	 * BACKUP METHOD and BACKUP FROM lines are new in 9.2. We can't restore
-	 * from an older backup anyway, but since the information on it is not
-	 * strictly required, don't error out if it's missing for some reason.
+	 * BACKUP METHOD lets us know if this was a typical backup ("streamed",
+	 * which could mean either pg_basebackup or the pg_backup_start/stop
+	 * method was used) or if this label came from somewhere else (the only
+	 * other option today being from pg_rewind).  If this was a streamed
+	 * backup then we know that we need to play through until we get to the
+	 * end of the WAL which was generated during the backup (at which point
+	 * we will have reached consistency and backupEndRequired will be reset
+	 * to be false).
 	 */
 	if (fscanf(lfp, "BACKUP METHOD: %19s\n", backuptype) == 1)
 	{
@@ -1193,6 +1198,11 @@ read_backup_label(XLogRecPtr *checkPointLoc, TimeLineID *backupLabelTLI,
 			*backupEndRequired = true;
 	}
 
+	/*
+	 * BACKUP FROM lets us know if this was from a primary or a standby.  If
+	 * it was from a standby, we'll double-check that the control file state
+	 * matches that of a standby.
+	 */
 	if (fscanf(lfp, "BACKUP FROM: %19s\n", backupfrom) == 1)
 	{
 		if (strcmp(backupfrom, "standby") == 0)
@@ -1970,7 +1980,7 @@ xlogrecovery_redo(XLogReaderState *record, TimeLineID replayTLI)
 		{
 			/*
 			 * We have reached the end of base backup, the point where
-			 * pg_stop_backup() was done.  The data on disk is now consistent
+			 * pg_backup_stop() was done.  The data on disk is now consistent
 			 * (assuming we have also reached minRecoveryPoint).  Set
 			 * backupEndPoint to the current LSN, so that the next call to
 			 * CheckRecoveryConsistency() will notice it and do the
@@ -2033,7 +2043,7 @@ CheckRecoveryConsistency(void)
 
 	/*
 	 * Have we passed our safe starting point? Note that minRecoveryPoint is
-	 * known to be incorrectly set if ControlFile->backupEndRequired, until
+	 * known to be incorrectly set if recovering from a backup, until
 	 * the XLOG_BACKUP_END arrives to advise us of the correct
 	 * minRecoveryPoint. All we know prior to that is that we're not
 	 * consistent yet.