Prioritize history files when archiving

At the end of recovery for the post-promotion process, a new history
file is created followed by the last partial segment of the previous
timeline.  Based on the timing, the archiver would first try to archive
the last partial segment and then the history file.  This can delay the
detection of a new timeline taken, particularly depending on the time it
takes to transfer the last partial segment as it delays the moment the
history file of the new timeline gets archived.  This can cause promoted
standbys to use the same timeline as one already taken depending on the
circumstances if multiple instances look at archives at the same
location.

This commit changes the order of archiving so as history files are
archived in priority over other file types, which reduces the likelihood
of the same timeline being taken (still not reducing the window to
zero), and it makes the archiver behave more consistently with the
startup process doing its post-promotion business.

Author: David Steele
Reviewed-by: Michael Paquier, Kyotaro Horiguchi
Discussion: https://postgr.es/m/929068cf-69e1-bba2-9dc0-e05986aed471@pgmasters.net
Backpatch-through: 9.5

Author: michaelpq

src/backend/postmaster/pgarch.c

@@ -648,11 +648,12 @@ 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)
@@ -664,43 +665,62 @@ pgarch_readyXlog(char *xlog)
 	 * of calls, so....
 	 */
 	char		XLogArchiveStatusDir[MAXPGPATH];
-	char		newxlog[MAX_XFN_CHARS + 6 + 1];
 	DIR		   *rldir;
 	struct dirent *rlde;
 	bool		found = false;
+	bool		historyFound = false;
 
 	snprintf(XLogArchiveStatusDir, MAXPGPATH, XLOGDIR "/archive_status");
 	rldir = AllocateDir(XLogArchiveStatusDir);
 
 	while ((rlde = ReadDir(rldir, XLogArchiveStatusDir)) != NULL)
 	{
 		int			basenamelen = (int) strlen(rlde->d_name) - 6;
+		char		basename[MAX_XFN_CHARS + 1];
+		bool		ishistory;
 
-		if (basenamelen >= MIN_XFN_CHARS &&
-			basenamelen <= MAX_XFN_CHARS &&
-			strspn(rlde->d_name, VALID_XFN_CHARS) >= basenamelen &&
-			strcmp(rlde->d_name + basenamelen, ".ready") == 0)
+		/* Ignore entries with unexpected number of characters */
+		if (basenamelen < MIN_XFN_CHARS ||
+			basenamelen > MAX_XFN_CHARS)
+			continue;
+
+		/* Ignore entries with unexpected characters */
+		if (strspn(rlde->d_name, VALID_XFN_CHARS) < basenamelen)
+			continue;
+
+		/* Ignore anything not suffixed with .ready */
+		if (strcmp(rlde->d_name + basenamelen, ".ready") != 0)
+			continue;
+
+		/* Truncate off the .ready */
+		memcpy(basename, rlde->d_name, basenamelen);
+		basename[basenamelen] = '\0';
+
+		/* Is this a history file? */
+		ishistory = IsTLHistoryFileName(basename);
+
+		/*
+		 * Consume the file to archive.  History files have the highest
+		 * priority.  If this is the first file or the first history file
+		 * ever, copy it.  In the presence of a history file already chosen as
+		 * target, ignore all other files except history files which have been
+		 * generated for an older timeline than what is already chosen as
+		 * target to archive.
+		 */
+		if (!found || (ishistory && !historyFound))
 		{
-			if (!found)
-			{
-				strcpy(newxlog, rlde->d_name);
-				found = true;
-			}
-			else
-			{
-				if (strcmp(rlde->d_name, newxlog) < 0)
-					strcpy(newxlog, rlde->d_name);
-			}
+			strcpy(xlog, basename);
+			found = true;
+			historyFound = ishistory;
+		}
+		else if (ishistory || !historyFound)
+		{
+			if (strcmp(basename, xlog) < 0)
+				strcpy(xlog, basename);
 		}
 	}
 	FreeDir(rldir);
 
-	if (found)
-	{
-		/* truncate off the .ready */
-		newxlog[strlen(newxlog) - 6] = '\0';
-		strcpy(xlog, newxlog);
-	}
 	return found;
 }