pg_rewind: fsync target data directory.

Previously pg_rewind did not fsync any files. That's problematic, given
that the target directory is modified. If the database was started
afterwards, 2ce439f33 luckily already caused the data directory to be
synced to disk at postmaster startup; reducing the scope of the problem.

To fix, use initdb -S, at the end of the pg_rewind run. It doesn't seem
worthwhile to duplicate the code into pg_rewind, and initdb -S is
already used that way by pg_upgrade.

Reported-By: Andres Freund
Author: Michael Paquier, somewhat edited by me
Discussion: 20160310034352.iuqgvpmg5qmnxtkz@alap3.anarazel.de
    CAB7nPqSytVG1o4S3S2pA1O=692ekurJ+fckW2PywEG3sNw54Ow@mail.gmail.com
Backpatch: 9.5, where pg_rewind was introduced

src/bin/pg_rewind/file_ops.c

@@ -79,7 +79,6 @@ close_target_file(void)
 				 dstpath, strerror(errno));
 
 	dstfd = -1;
-	/* fsync? */
 }
 
 void

src/bin/pg_rewind/pg_rewind.c

@@ -36,6 +36,7 @@ static void createBackupLabel(XLogRecPtr startpoint, TimeLineID starttli,
 static void digestControlFile(ControlFileData *ControlFile, char *source,
 				  size_t size);
 static void updateControlFile(ControlFileData *ControlFile);
+static void syncTargetDirectory(const char *argv0);
 static void sanityChecks(void);
 static void findCommonAncestorTimeline(XLogRecPtr *recptr, int *tliIndex);
 
@@ -349,6 +350,9 @@ main(int argc, char **argv)
 	ControlFile_new.state = DB_IN_ARCHIVE_RECOVERY;
 	updateControlFile(&ControlFile_new);
 
+	pg_log(PG_PROGRESS, "syncing target data directory\n");
+	syncTargetDirectory(argv[0]);
+
 	printf(_("Done!\n"));
 
 	return 0;
@@ -650,3 +654,56 @@ updateControlFile(ControlFileData *ControlFile)
 
 	close_target_file();
 }
+
+/*
+ * Sync target data directory to ensure that modifications are safely on disk.
+ *
+ * We do this once, for the whole data directory, for performance reasons.  At
+ * the end of pg_rewind's run, the kernel is likely to already have flushed
+ * most dirty buffers to disk. Additionally initdb -S uses a two-pass approach
+ * (only initiating writeback in the first pass), which often reduces the
+ * overall amount of IO noticeably.
+ */
+static void
+syncTargetDirectory(const char *argv0)
+{
+	int		ret;
+#define MAXCMDLEN (2 * MAXPGPATH)
+	char	exec_path[MAXPGPATH];
+	char	cmd[MAXCMDLEN];
+
+	/* locate initdb binary */
+	if ((ret = find_other_exec(argv0, "initdb",
+							   "initdb (PostgreSQL) " PG_VERSION "\n",
+							   exec_path)) < 0)
+	{
+		char        full_path[MAXPGPATH];
+
+		if (find_my_exec(argv0, full_path) < 0)
+			strlcpy(full_path, progname, sizeof(full_path));
+
+		if (ret == -1)
+			pg_fatal("The program \"initdb\" is needed by %s but was \n"
+					 "not found in the same directory as \"%s\".\n"
+					 "Check your installation.\n", progname, full_path);
+		else
+			pg_fatal("The program \"initdb\" was found by \"%s\"\n"
+					 "but was not the same version as %s.\n"
+					 "Check your installation.\n", full_path, progname);
+	}
+
+	/* only skip processing after ensuring presence of initdb */
+	if (dry_run)
+		return;
+
+	/* finally run initdb -S */
+	if (debug)
+		snprintf(cmd, MAXCMDLEN, "\"%s\" -D \"%s\" -S",
+				 exec_path, datadir_target);
+	else
+		snprintf(cmd, MAXCMDLEN, "\"%s\" -D \"%s\" -S > \"%s\"",
+				 exec_path, datadir_target, DEVNULL);
+
+	if (system(cmd) != 0)
+		pg_fatal("sync of target directory failed\n");
+}