From d5fd60b4e329fe99b2f6565355014040dcdcee26 Mon Sep 17 00:00:00 2001
From: David Steele <david@pgmasters.net>
Date: Thu, 13 Dec 2018 12:58:27 -0500
Subject: [PATCH] Change pgarch_readyXlog() to return .history files first.

The alphabetical ordering of pgarch_readyXlog() means that on promotion
000000010000000100000001.partial will be archived before 00000002.history.

This appears harmless, but the .history files are what other potential primaries
use to decide what timeline they should pick.  The additional latency of
compressing/transferring the much larger partial file means that archiving of
the .history file is delayed and greatly increases the chance that another
primary will promote to the same timeline.

Teach pgarch_readyXlog() to return .history files first (and in order) to reduce
the window where this can happen.  This won't prevent all conflicts, but it is a
simple change and should greatly reduce real-world occurrences.
---
 src/backend/postmaster/pgarch.c | 30 +++++++++++++++++++++++-------
 1 file changed, 23 insertions(+), 7 deletions(-)

diff --git a/src/backend/postmaster/pgarch.c b/src/backend/postmaster/pgarch.c
index 0dcf609f19..95898080d7 100644
--- a/src/backend/postmaster/pgarch.c
+++ b/src/backend/postmaster/pgarch.c
@@ -704,11 +704,11 @@ pgarch_archiveXlog(char *xlog)
  * 2) because the oldest ones will sooner become candidates for
  * recycling at time of checkpoint
  *
- * NOTE: the "oldest" comparison will presently consider all segments of
- * a timeline with a smaller ID to be older than all segments of a timeline
- * with a larger ID; the net result being that past timelines are given
- * higher priority for archiving.  This seems okay, or at least not
- * obviously worth changing.
+ * NOTE: the "oldest" comparison will consider any .history file to be older
+ * than any other file except another .history file.  Segments on a timeline
+ * with a smaller ID will be older than all segments on a timeline with a larger
+ * ID; the net result being that past timelines are given higher priority for
+ * archiving.  This seems okay, or at least not obviously worth changing.
  */
 static bool
 pgarch_readyXlog(char *xlog)
@@ -724,6 +724,7 @@ pgarch_readyXlog(char *xlog)
 	DIR		   *rldir;
 	struct dirent *rlde;
 	bool		found = false;
+	bool		historyFound = false;
 
 	snprintf(XLogArchiveStatusDir, MAXPGPATH, XLOGDIR "/archive_status");
 	rldir = AllocateDir(XLogArchiveStatusDir);
@@ -737,12 +738,27 @@ pgarch_readyXlog(char *xlog)
 			strspn(rlde->d_name, VALID_XFN_CHARS) >= basenamelen &&
 			strcmp(rlde->d_name + basenamelen, ".ready") == 0)
 		{
-			if (!found)
+			/* Is this a history file? */
+			bool history = basenamelen >= sizeof(".history") &&
+				strcmp(rlde->d_name + (basenamelen - sizeof(".history") + 1),
+					   ".history.ready") == 0;
+
+			/*
+			 * If this is the first file or the first history file, copy it
+			 */
+			if (!found || (history && !historyFound))
 			{
 				strcpy(newxlog, rlde->d_name);
 				found = true;
+				historyFound = history;
 			}
-			else
+			/*
+			 * Else compare and see if this file is alpabetically less than
+			 * what we already have.  If so, copy it.  History files always get
+			 * this comparison but other files only do when no history file has
+			 * been found yet.
+			 */
+			else if (history || (!history && !historyFound))
 			{
 				if (strcmp(rlde->d_name, newxlog) < 0)
 					strcpy(newxlog, rlde->d_name);
-- 
2.17.2 (Apple Git-113)