Add option --config-file to pg_rewind

This option is useful to do a rewind with the server configuration file
(aka postgresql.conf) located outside the data directory, which is
something that some Linux distributions and some HA tools like to rely
on.  As a result, this can simplify the logic around a rewind by
avoiding the copy of such files before running pg_rewind.

This option affects pg_rewind when it internally starts the target
cluster with some "postgres" commands, adding -c config_file=FILE to the
command strings generated, when:
- retrieving a restore_command using a "postgres -C" command for
-c/--restore-target-wal.
- forcing crash recovery once to get the cluster into a clean shutdown
state.

Author: Gunnar "Nick" Bluth
Reviewed-by: Michael Banck, Alexander Kukushkin, Michael Paquier,
Alexander Alekseev
Discussion: https://postgr.es/m/7c59265d-ac50-b0aa-ca1e-65e8bd27642a@pro-open.de

doc/src/sgml/ref/pg_rewind.sgml

@@ -241,6 +241,21 @@ PostgreSQL documentation
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><option>--config-file=<replaceable class="parameter">filename</replaceable></option></term>
+      <listitem>
+       <para>
+        Use the specified main server configuration file for the target
+        cluster. This affects <application>pg_rewind</application> when
+        it uses internally the <application>postgres</application> command
+        for the rewind operation on this cluster (when retrieving
+        <varname>restore_command</varname> with the option
+        <option>-c/--restore-target-wal</option> and when forcing a
+        completion of crash recovery).
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><option>--debug</option></term>
       <listitem>

src/bin/pg_rewind/pg_rewind.c

@@ -61,6 +61,7 @@ char	   *datadir_target = NULL;
 char	   *datadir_source = NULL;
 char	   *connstr_source = NULL;
 char	   *restore_command = NULL;
+char	   *config_file = NULL;
 
 static bool debug = false;
 bool		showprogress = false;
@@ -87,6 +88,8 @@ usage(const char *progname)
 	printf(_("Options:\n"));
 	printf(_("  -c, --restore-target-wal       use restore_command in target configuration to\n"
 			 "                                 retrieve WAL files from archives\n"));
+	printf(_("      --config-file=FILENAME     use specified main server configuration\n"));
+	printf(_("                                 file when running target cluster\n"));
 	printf(_("  -D, --target-pgdata=DIRECTORY  existing data directory to modify\n"));
 	printf(_("      --source-pgdata=DIRECTORY  source data directory to synchronize with\n"));
 	printf(_("      --source-server=CONNSTR    source server to synchronize with\n"));
@@ -115,6 +118,7 @@ main(int argc, char **argv)
 		{"source-pgdata", required_argument, NULL, 1},
 		{"source-server", required_argument, NULL, 2},
 		{"no-ensure-shutdown", no_argument, NULL, 4},
+		{"config-file", required_argument, NULL, 5},
 		{"version", no_argument, NULL, 'V'},
 		{"restore-target-wal", no_argument, NULL, 'c'},
 		{"dry-run", no_argument, NULL, 'n'},
@@ -205,6 +209,10 @@ main(int argc, char **argv)
 			case 4:
 				no_ensure_shutdown = true;
 				break;
+
+			case 5:
+				config_file = pg_strdup(optarg);
+				break;
 		}
 	}
 
@@ -1058,6 +1066,13 @@ getRestoreCommand(const char *argv0)
 	appendPQExpBufferStr(postgres_cmd, " -D ");
 	appendShellString(postgres_cmd, datadir_target);
 
+	/* add custom configuration file only if requested */
+	if (config_file != NULL)
+	{
+		appendPQExpBufferStr(postgres_cmd, " -c config_file=");
+		appendShellString(postgres_cmd, config_file);
+	}
+
 	/* add -C switch, for restore_command */
 	appendPQExpBufferStr(postgres_cmd, " -C restore_command");
 
@@ -1136,6 +1151,13 @@ ensureCleanShutdown(const char *argv0)
 	appendPQExpBufferStr(postgres_cmd, " --single -F -D ");
 	appendShellString(postgres_cmd, datadir_target);
 
+	/* add custom configuration file only if requested */
+	if (config_file != NULL)
+	{
+		appendPQExpBufferStr(postgres_cmd, " -c config_file=");
+		appendShellString(postgres_cmd, config_file);
+	}
+
 	/* finish with the database name, and a properly quoted redirection */
 	appendPQExpBufferStr(postgres_cmd, " template1 < ");
 	appendShellString(postgres_cmd, DEVNULL);

src/bin/pg_rewind/t/RewindTest.pm

@@ -263,7 +263,9 @@ sub run_pg_rewind
 				"--debug",
 				"--source-pgdata=$standby_pgdata",
 				"--target-pgdata=$primary_pgdata",
-				"--no-sync"
+				"--no-sync",
+				"--config-file",
+				"$tmp_folder/primary-postgresql.conf.tmp"
 			],
 			'pg_rewind local');
 	}
@@ -276,7 +278,8 @@ sub run_pg_rewind
 				'pg_rewind',                       "--debug",
 				"--source-server",                 $standby_connstr,
 				"--target-pgdata=$primary_pgdata", "--no-sync",
-				"--write-recovery-conf"
+				"--write-recovery-conf",           "--config-file",
+				"$tmp_folder/primary-postgresql.conf.tmp"
 			],
 			'pg_rewind remote');
 
@@ -323,7 +326,8 @@ sub run_pg_rewind
 
 		# Note the use of --no-ensure-shutdown here.  WAL files are
 		# gone in this mode and the primary has been stopped
-		# gracefully already.
+		# gracefully already.  --config-file reuses the original
+		# postgresql.conf as restore_command has been enabled above.
 		command_ok(
 			[
 				'pg_rewind',
@@ -332,7 +336,9 @@ sub run_pg_rewind
 				"--target-pgdata=$primary_pgdata",
 				"--no-sync",
 				"--no-ensure-shutdown",
-				"--restore-target-wal"
+				"--restore-target-wal",
+				"--config-file",
+				"$primary_pgdata/postgresql.conf"
 			],
 			'pg_rewind archive');
 	}