Add pg_rewind --no-sync

This is an option consistent with what pg_dump and pg_basebackup provide
which is useful for leveraging the I/O effort when testing things, not
to be used in a production environment.

Author: Michael Paquier
Reviewed-by: Heikki Linnakangas
Discussion: https://postgr.es/m/20180325122607.GB3707@paquier.xyz

doc/src/sgml/ref/pg_rewind.sgml

@@ -171,6 +171,22 @@ PostgreSQL documentation
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><option>-N</option></term>
+      <term><option>--no-sync</option></term>
+      <listitem>
+       <para>
+        By default, <command>pg_rewind</command> will wait for all files
+        to be written safely to disk.  This option causes
+        <command>pg_rewind</command> to return without waiting, which is
+        faster, but means that a subsequent operating system crash can leave
+        the synchronized data folder corrupt.  Generally, this option is
+        useful for testing but should not be used when creating a production
+        installation.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><option>-P</option></term>
       <term><option>--progress</option></term>

src/bin/pg_rewind/RewindTest.pm

@@ -231,7 +231,8 @@ sub run_pg_rewind
 				'pg_rewind',
 				"--debug",
 				"--source-pgdata=$standby_pgdata",
-				"--target-pgdata=$master_pgdata"
+				"--target-pgdata=$master_pgdata",
+				"--no-sync"
 			],
 			'pg_rewind local');
 	}
@@ -243,7 +244,8 @@ sub run_pg_rewind
 			[
 				'pg_rewind',       "--debug",
 				"--source-server", $standby_connstr,
-				"--target-pgdata=$master_pgdata"
+				"--target-pgdata=$master_pgdata",
+				"--no-sync"
 			],
 			'pg_rewind remote');
 	}

src/bin/pg_rewind/pg_rewind.c

@@ -56,6 +56,7 @@ char	   *connstr_source = NULL;
 bool		debug = false;
 bool		showprogress = false;
 bool		dry_run = false;
+bool		do_sync = true;
 
 /* Target history */
 TimeLineHistoryEntry *targetHistory;
@@ -71,6 +72,8 @@ usage(const char *progname)
 	printf(_("      --source-pgdata=DIRECTORY  source data directory to synchronize with\n"));
 	printf(_("      --source-server=CONNSTR    source server to synchronize with\n"));
 	printf(_("  -n, --dry-run                  stop before modifying anything\n"));
+	printf(_("  -N, --no-sync                  do not wait for changes to be written\n"));
+	printf(_("                                 safely to disk\n"));
 	printf(_("  -P, --progress                 write progress messages\n"));
 	printf(_("      --debug                    write a lot of debug messages\n"));
 	printf(_("  -V, --version                  output version information, then exit\n"));
@@ -89,6 +92,7 @@ main(int argc, char **argv)
 		{"source-server", required_argument, NULL, 2},
 		{"version", no_argument, NULL, 'V'},
 		{"dry-run", no_argument, NULL, 'n'},
+		{"no-sync", no_argument, NULL, 'N'},
 		{"progress", no_argument, NULL, 'P'},
 		{"debug", no_argument, NULL, 3},
 		{NULL, 0, NULL, 0}
@@ -125,7 +129,7 @@ main(int argc, char **argv)
 		}
 	}
 
-	while ((c = getopt_long(argc, argv, "D:nP", long_options, &option_index)) != -1)
+	while ((c = getopt_long(argc, argv, "D:nNP", long_options, &option_index)) != -1)
 	{
 		switch (c)
 		{
@@ -141,6 +145,10 @@ main(int argc, char **argv)
 				dry_run = true;
 				break;
 
+			case 'N':
+				do_sync = false;
+				break;
+
 			case 3:
 				debug = true;
 				break;
@@ -709,7 +717,7 @@ updateControlFile(ControlFileData *ControlFile)
 static void
 syncTargetDirectory(const char *argv0)
 {
-	if (dry_run)
+	if (!do_sync || dry_run)
 		return;
 
 	fsync_pgdata(datadir_target, progname, PG_VERSION_NUM);