Avoid crash in "postgres -C guc" for a GUC with a null string value.

tglsfdc · tglsfdc · commit 29987b2e1f8c · 2016-06-16T12:17:20.000-04:00
Emit "(null)" instead, which was the behavior all along on platforms
that don't crash, eg OS X.  Per report from Jehan-Guillaume de Rorthais.
Back-patch to 9.2 where -C option was introduced.

Michael Paquier

Report: &lt;20160615204036.2d35d86a@firost&gt;
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
@@ -847,10 +847,14 @@ PostmasterMain(int argc, char *argv[])
 	if (output_config_variable != NULL)
 	{
 		/*
-		 * permission is handled because the user is reading inside the data
-		 * dir
+		 * "-C guc" was specified, so print GUC's value and exit.  No extra
+		 * permission check is needed because the user is reading inside the
+		 * data dir.
 		 */
-		puts(GetConfigOption(output_config_variable, false, false));
+		const char *config_val = GetConfigOption(output_config_variable,
+												 false, false);
+
+		puts(config_val ? config_val : "(null)");
 		ExitPostmaster(0);
 	}
 
