Add postmaster -C option to query configuration parameters, and have

pg_ctl use that to query the data directory for config-only installs.
This fixes awkward or impossible pg_ctl operation for config-only
installs.

doc/src/sgml/ref/postgres-ref.sgml

@@ -140,6 +140,20 @@ PostgreSQL documentation
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><option>-C <replaceable>name</replaceable></option></term>
+      <listitem>
+       <para>
+        Returns the value of a named run-time parameter, and exits.
+        (See the <option>-c</> option above for details.)  This can
+        be used on a running server, and returns values from
+        <filename>postgresql.conf</>, modified by any parameters
+        supplied in this invocation.  It does not reflect parameters
+        supplied when the cluster was started.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><option>-d <replaceable>debug-level</replaceable></option></term>
       <listitem>

src/backend/main/main.c

@@ -283,6 +283,7 @@ help(const char *progname)
 #endif
 	printf(_("  -B NBUFFERS     number of shared buffers\n"));
 	printf(_("  -c NAME=VALUE   set run-time parameter\n"));
+	printf(_("  -C NAME         return run-time parameter\n"));
 	printf(_("  -d 1-5          debugging level\n"));
 	printf(_("  -D DATADIR      database directory\n"));
 	printf(_("  -e              use European date input format (DMY)\n"));

src/backend/postmaster/postmaster.c

@@ -203,6 +203,8 @@ bool		enable_bonjour = false;
 char	   *bonjour_name;
 bool		restart_after_crash = true;
 
+char 		*output_config_variable = NULL;
+
 /* PIDs of special child processes; 0 when not running */
 static pid_t StartupPID = 0,
 			BgWriterPID = 0,
@@ -537,7 +539,7 @@ PostmasterMain(int argc, char *argv[])
 	 * tcop/postgres.c (the option sets should not conflict) and with the
 	 * common help() function in main/main.c.
 	 */
-	while ((opt = getopt(argc, argv, "A:B:bc:D:d:EeFf:h:ijk:lN:nOo:Pp:r:S:sTt:W:-:")) != -1)
+	while ((opt = getopt(argc, argv, "A:B:bc:C:D:d:EeFf:h:ijk:lN:nOo:Pp:r:S:sTt:W:-:")) != -1)
 	{
 		switch (opt)
 		{
@@ -554,6 +556,10 @@ PostmasterMain(int argc, char *argv[])
 				IsBinaryUpgrade = true;
 				break;
 
+			case 'C':
+				output_config_variable = optarg;
+				break;
+
 			case 'D':
 				userDoption = optarg;
 				break;
@@ -728,6 +734,13 @@ PostmasterMain(int argc, char *argv[])
 	if (!SelectConfigFiles(userDoption, progname))
 		ExitPostmaster(2);
 
+	if (output_config_variable != NULL)
+	{
+		/* permission is handled because the user is reading inside the data dir */
+		puts(GetConfigOption(output_config_variable, false, false));
+		ExitPostmaster(0);
+	}
+	
 	/* Verify that DataDir looks reasonable */
 	checkDataDir();
 

src/backend/tcop/postgres.c

@@ -3170,7 +3170,7 @@ process_postgres_switches(int argc, char *argv[], GucContext ctx)
 	 * postmaster/postmaster.c (the option sets should not conflict) and with
 	 * the common help() function in main/main.c.
 	 */
-	while ((flag = getopt(argc, argv, "A:B:bc:D:d:EeFf:h:ijk:lN:nOo:Pp:r:S:sTt:v:W:-:")) != -1)
+	while ((flag = getopt(argc, argv, "A:B:bc:C:D:d:EeFf:h:ijk:lN:nOo:Pp:r:S:sTt:v:W:-:")) != -1)
 	{
 		switch (flag)
 		{
@@ -3187,6 +3187,10 @@ process_postgres_switches(int argc, char *argv[], GucContext ctx)
 				IsBinaryUpgrade = true;
 				break;
 
+			case 'C':
+				/* ignored for consistency with the postmaster */
+				break;
+
 			case 'D':
 				if (secure)
 					userDoption = strdup(optarg);
@@ -3272,7 +3276,7 @@ process_postgres_switches(int argc, char *argv[], GucContext ctx)
 				break;
 
 			case 'T':
-				/* ignored for consistency with postmaster */
+				/* ignored for consistency with the postmaster */
 				break;
 
 			case 't':

src/bin/pg_ctl/pg_ctl.c

@@ -81,6 +81,7 @@ static ShutdownMode shutdown_mode = SMART_MODE;
 static int	sig = SIGTERM;		/* default */
 static CtlCommand ctl_command = NO_COMMAND;
 static char *pg_data = NULL;
+static char *pg_config = NULL;
 static char *pgdata_opt = NULL;
 static char *post_opts = NULL;
 static const char *progname;
@@ -131,6 +132,7 @@ static void do_status(void);
 static void do_promote(void);
 static void do_kill(pgpid_t pid);
 static void print_msg(const char *msg);
+static void adjust_data_dir(void);
 
 #if defined(WIN32) || defined(__CYGWIN__)
 static bool pgwin32_IsInstalled(SC_HANDLE);
@@ -1265,10 +1267,10 @@ pgwin32_CommandLine(bool registration)
 		strcat(cmdLine, "\"");
 	}
 
-	if (pg_data)
+	if (pg_config)
 	{
 		strcat(cmdLine, " -D \"");
-		strcat(cmdLine, pg_data);
+		strcat(cmdLine, pg_config);
 		strcat(cmdLine, "\"");
 	}
 
@@ -1886,6 +1888,59 @@ set_starttype(char *starttypeopt)
 }
 #endif
 
+/*
+ * adjust_data_dir
+ *
+ * If a configuration-only directory was specified, find the real data dir.
+ */
+void
+adjust_data_dir(void)
+{
+	char		cmd[MAXPGPATH], filename[MAXPGPATH], *my_exec_path;
+	FILE	   *fd;
+
+	/* If there is no postgresql.conf, it can't be a config-only dir */
+	snprintf(filename, sizeof(filename), "%s/postgresql.conf", pg_config);
+	if ((fd = fopen(filename, "r")) == NULL)
+		return;
+	fclose(fd);
+
+	/* If PG_VERSION exists, it can't be a config-only dir */
+	snprintf(filename, sizeof(filename), "%s/PG_VERSION", pg_config);
+	if ((fd = fopen(filename, "r")) != NULL)
+	{
+		fclose(fd);
+		return;
+	}
+
+	/* Must be a configuration directory, so find the data directory */
+
+	/* we use a private my_exec_path to avoid interfering with later uses */
+	if (exec_path == NULL)
+		my_exec_path = find_other_exec_or_die(argv0, "postgres", PG_BACKEND_VERSIONSTR);
+	else
+		my_exec_path = xstrdup(exec_path);
+
+	snprintf(cmd, MAXPGPATH, SYSTEMQUOTE "\"%s\" %s%s -C data_directory" SYSTEMQUOTE,
+			 my_exec_path, pgdata_opt ? pgdata_opt : "", post_opts ?
+			 post_opts : "");
+
+	fd = popen(cmd, "r");
+	if (fd == NULL || fgets(filename, sizeof(filename), fd) == NULL)
+	{
+		write_stderr(_("%s: cannot find the data directory using %s\n"), progname, my_exec_path);
+		exit(1);
+	}
+	pclose(fd);
+	free(my_exec_path);
+
+	if (strlen(filename) > 0 && filename[strlen(filename) - 1] == '\n')
+		filename[strlen(filename) - 1] = '\0';
+	free(pg_data);
+	pg_data = xstrdup(filename);
+	canonicalize_path(pg_data);
+}
+
 
 int
 main(int argc, char **argv)
@@ -2120,14 +2175,17 @@ main(int argc, char **argv)
 	}
 
 	/* Note we put any -D switch into the env var above */
-	pg_data = getenv("PGDATA");
-	if (pg_data)
+	pg_config = getenv("PGDATA");
+	if (pg_config)
 	{
-		pg_data = xstrdup(pg_data);
-		canonicalize_path(pg_data);
+		pg_config = xstrdup(pg_config);
+		canonicalize_path(pg_config);
+		pg_data = xstrdup(pg_config);
 	}
 
-	if (pg_data == NULL &&
+	adjust_data_dir();
+	
+	if (pg_config == NULL &&
 		ctl_command != KILL_COMMAND && ctl_command != UNREGISTER_COMMAND)
 	{
 		write_stderr(_("%s: no database directory specified and environment variable PGDATA unset\n"),