Provide a better error message for misplaced dispatch options.

Before this patch, misplacing a special must-be-first option for
dispatching to a subprogram (e.g., postgres -D . --single) would
fail with an error like

	FATAL:  --single requires a value

This patch adjusts this error to more accurately complain that the
special option wasn't listed first.  The aforementioned error
message now looks like

	FATAL:  --single must be first argument

The dispatch option parsing code has been refactored for use
wherever ParseLongOption() is called.  Beyond the obvious advantage
of avoiding code duplication, this should prevent similar problems
when new dispatch options are added.  Note that we assume that none
of the dispatch option names match another valid command-line
argument, such as the name of a configuration parameter.

Ideally, we'd remove this must-be-first requirement for these
options, but after some investigation, we decided that wasn't worth
the added complexity and behavior changes.

Author: Nathan Bossart, Greg Sabino Mullane
Reviewed-by: Greg Sabino Mullane, Peter Eisentraut, Álvaro Herrera, Tom Lane
Discussion: https://postgr.es/m/CAKAnmmJkZtZAiSryho%3DgYpbvC7H-HNjEDAh16F3SoC9LPu8rqQ%40mail.gmail.com

Author: nathan-bossart

src/backend/bootstrap/bootstrap.c

@@ -224,8 +224,21 @@ BootstrapModeMain(int argc, char *argv[], bool check_only)
 			case 'B':
 				SetConfigOption("shared_buffers", optarg, PGC_POSTMASTER, PGC_S_ARGV);
 				break;
-			case 'c':
 			case '-':
+
+				/*
+				 * Error if the user misplaced a special must-be-first option
+				 * for dispatching to a subprogram.  parse_dispatch_option()
+				 * returns DISPATCH_POSTMASTER if it doesn't find a match, so
+				 * error for anything else.
+				 */
+				if (parse_dispatch_option(optarg) != DISPATCH_POSTMASTER)
+					ereport(ERROR,
+							(errcode(ERRCODE_SYNTAX_ERROR),
+							 errmsg("--%s must be first argument", optarg)));
+
+				/* FALLTHROUGH */
+			case 'c':
 				{
 					char	   *name,
 							   *value;

src/backend/main/main.c

@@ -43,6 +43,19 @@
 const char *progname;
 static bool reached_main = false;
 
+/* names of special must-be-first options for dispatching to subprograms */
+static const char *const DispatchOptionNames[] =
+{
+	[DISPATCH_CHECK] = "check",
+	[DISPATCH_BOOT] = "boot",
+	[DISPATCH_FORKCHILD] = "forkchild",
+	[DISPATCH_DESCRIBE_CONFIG] = "describe-config",
+	[DISPATCH_SINGLE] = "single",
+	/* DISPATCH_POSTMASTER has no name */
+};
+
+StaticAssertDecl(lengthof(DispatchOptionNames) == DISPATCH_POSTMASTER,
+				 "array length mismatch");
 
 static void startup_hacks(const char *progname);
 static void init_locale(const char *categoryname, int category, const char *locale);
@@ -57,6 +70,7 @@ int
 main(int argc, char *argv[])
 {
 	bool		do_check_root = true;
+	DispatchOption dispatch_option = DISPATCH_POSTMASTER;
 
 	reached_main = true;
 
@@ -179,26 +193,72 @@ main(int argc, char *argv[])
 	 * Dispatch to one of various subprograms depending on first argument.
 	 */
 
-	if (argc > 1 && strcmp(argv[1], "--check") == 0)
-		BootstrapModeMain(argc, argv, true);
-	else if (argc > 1 && strcmp(argv[1], "--boot") == 0)
-		BootstrapModeMain(argc, argv, false);
+	if (argc > 1 && argv[1][0] == '-' && argv[1][1] == '-')
+		dispatch_option = parse_dispatch_option(&argv[1][2]);
+
+	switch (dispatch_option)
+	{
+		case DISPATCH_CHECK:
+			BootstrapModeMain(argc, argv, true);
+			break;
+		case DISPATCH_BOOT:
+			BootstrapModeMain(argc, argv, false);
+			break;
+		case DISPATCH_FORKCHILD:
 #ifdef EXEC_BACKEND
-	else if (argc > 1 && strncmp(argv[1], "--forkchild", 11) == 0)
-		SubPostmasterMain(argc, argv);
+			SubPostmasterMain(argc, argv);
+#else
+			Assert(false);		/* should never happen */
 #endif
-	else if (argc > 1 && strcmp(argv[1], "--describe-config") == 0)
-		GucInfoMain();
-	else if (argc > 1 && strcmp(argv[1], "--single") == 0)
-		PostgresSingleUserMain(argc, argv,
-							   strdup(get_user_name_or_exit(progname)));
-	else
-		PostmasterMain(argc, argv);
+			break;
+		case DISPATCH_DESCRIBE_CONFIG:
+			GucInfoMain();
+			break;
+		case DISPATCH_SINGLE:
+			PostgresSingleUserMain(argc, argv,
+								   strdup(get_user_name_or_exit(progname)));
+			break;
+		case DISPATCH_POSTMASTER:
+			PostmasterMain(argc, argv);
+			break;
+	}
+
 	/* the functions above should not return */
 	abort();
 }
 
+/*
+ * Returns the matching DispatchOption value for the given option name.  If no
+ * match is found, DISPATCH_POSTMASTER is returned.
+ */
+DispatchOption
+parse_dispatch_option(const char *name)
+{
+	for (int i = 0; i < lengthof(DispatchOptionNames); i++)
+	{
+		/*
+		 * Unlike the other dispatch options, "forkchild" takes an argument,
+		 * so we just look for the prefix for that one.  For non-EXEC_BACKEND
+		 * builds, we never want to return DISPATCH_FORKCHILD, so skip over it
+		 * in that case.
+		 */
+		if (i == DISPATCH_FORKCHILD)
+		{
+#ifdef EXEC_BACKEND
+			if (strncmp(DispatchOptionNames[DISPATCH_FORKCHILD], name,
+						strlen(DispatchOptionNames[DISPATCH_FORKCHILD])) == 0)
+				return DISPATCH_FORKCHILD;
+#endif
+			continue;
+		}
+
+		if (strcmp(DispatchOptionNames[i], name) == 0)
+			return (DispatchOption) i;
+	}
 
+	/* no match means this is a postmaster */
+	return DISPATCH_POSTMASTER;
+}
 
 /*
  * Place platform-specific startup hacks here.  This is the right

src/backend/postmaster/postmaster.c

@@ -589,8 +589,21 @@ PostmasterMain(int argc, char *argv[])
 				output_config_variable = strdup(optarg);
 				break;
 
-			case 'c':
 			case '-':
+
+				/*
+				 * Error if the user misplaced a special must-be-first option
+				 * for dispatching to a subprogram.  parse_dispatch_option()
+				 * returns DISPATCH_POSTMASTER if it doesn't find a match, so
+				 * error for anything else.
+				 */
+				if (parse_dispatch_option(optarg) != DISPATCH_POSTMASTER)
+					ereport(ERROR,
+							(errcode(ERRCODE_SYNTAX_ERROR),
+							 errmsg("--%s must be first argument", optarg)));
+
+				/* FALLTHROUGH */
+			case 'c':
 				{
 					char	   *name,
 							   *value;

src/backend/tcop/postgres.c

@@ -3947,8 +3947,21 @@ process_postgres_switches(int argc, char *argv[], GucContext ctx,
 				/* ignored for consistency with the postmaster */
 				break;
 
-			case 'c':
 			case '-':
+
+				/*
+				 * Error if the user misplaced a special must-be-first option
+				 * for dispatching to a subprogram.  parse_dispatch_option()
+				 * returns DISPATCH_POSTMASTER if it doesn't find a match, so
+				 * error for anything else.
+				 */
+				if (parse_dispatch_option(optarg) != DISPATCH_POSTMASTER)
+					ereport(ERROR,
+							(errcode(ERRCODE_SYNTAX_ERROR),
+							 errmsg("--%s must be first argument", optarg)));
+
+				/* FALLTHROUGH */
+			case 'c':
 				{
 					char	   *name,
 							   *value;

src/include/postmaster/postmaster.h

@@ -138,4 +138,21 @@ extern PMChild *FindPostmasterChildByPid(int pid);
  */
 #define MAX_BACKENDS	0x3FFFF
 
+/*
+ * These values correspond to the special must-be-first options for dispatching
+ * to various subprograms.  parse_dispatch_option() can be used to convert an
+ * option name to one of these values.
+ */
+typedef enum DispatchOption
+{
+	DISPATCH_CHECK,
+	DISPATCH_BOOT,
+	DISPATCH_FORKCHILD,
+	DISPATCH_DESCRIBE_CONFIG,
+	DISPATCH_SINGLE,
+	DISPATCH_POSTMASTER,		/* must be last */
+} DispatchOption;
+
+extern DispatchOption parse_dispatch_option(const char *name);
+
 #endif							/* _POSTMASTER_H */

src/tools/pgindent/typedefs.list

@@ -619,6 +619,7 @@ DirectoryMethodFile
 DisableTimeoutParams
 DiscardMode
 DiscardStmt
+DispatchOption
 DistanceValue
 DistinctExpr
 DoState