Teach the "git" command to handle some commands internally

This is another patch in the "prepare to do more in C" series, where the
git wrapper command is taught about the notion of handling some
functionality internally.

Right now, the only internal commands are "version" and "help", but the
point being that we can now easily extend it to handle some of the trivial
scripts internally. Things like "git log" and "git diff" wouldn't need
separate external scripts any more.

This also implies that to support the old "git-log" and "git-diff" syntax,
the "git" wrapper now automatically looks at the name it was executed as,
and if it is "git-xxxx", it will assume that it is to internally do what
"git xxxx" would do.

In other words, you can (once you implement an internal command) soft- or
hard-link that command to the "git" wrapper command, and it will do the
right thing, whether you use the "git xxxx" or the "git-xxxx" format.

There's one other change: the search order for external programs is
modified slightly, so that the first entry remains GIT_EXEC_DIR, but the
second entry is the same directory as the git wrapper itself was executed
out of - if we can figure it out from argv[0], of course.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git.c

@@ -230,62 +230,141 @@ static void show_man_page(char *git_cmd)
 	execlp("man", "man", page, NULL);
 }
 
+static int cmd_version(int argc, char **argv, char **envp)
+{
+	printf("git version %s\n", GIT_VERSION);
+	return 0;
+}
+
+static int cmd_help(int argc, char **argv, char **envp)
+{
+	char *help_cmd = argv[1];
+	if (!help_cmd)
+		cmd_usage(git_exec_path(), NULL);
+	show_man_page(help_cmd);
+	return 0;
+}
+
+#define ARRAY_SIZE(x) (sizeof(x)/sizeof(x[0]))
+
+static void handle_internal_command(int argc, char **argv, char **envp)
+{
+	const char *cmd = argv[0];
+	static struct cmd_struct {
+		const char *cmd;
+		int (*fn)(int, char **, char **);
+	} commands[] = {
+		{ "version", cmd_version },
+		{ "help", cmd_help },
+	};
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(commands); i++) {
+		struct cmd_struct *p = commands+i;
+		if (strcmp(p->cmd, cmd))
+			continue;
+		exit(p->fn(argc, argv, envp));
+	}
+}
+
 int main(int argc, char **argv, char **envp)
 {
+	char *cmd = argv[0];
+	char *slash = strrchr(cmd, '/');
 	char git_command[PATH_MAX + 1];
-	char wd[PATH_MAX + 1];
-	int i, show_help = 0;
-	const char *exec_path;
+	const char *exec_path = NULL;
+
+	/*
+	 * Take the basename of argv[0] as the command
+	 * name, and the dirname as the default exec_path
+	 * if it's an absolute path and we don't have
+	 * anything better.
+	 */
+	if (slash) {
+		*slash++ = 0;
+		if (*cmd == '/')
+			exec_path = cmd;
+		cmd = slash;
+	}
 
-	getcwd(wd, PATH_MAX);
+	/*
+	 * "git-xxxx" is the same as "git xxxx", but we obviously:
+	 *
+	 *  - cannot take flags in between the "git" and the "xxxx".
+	 *  - cannot execute it externally (since it would just do
+	 *    the same thing over again)
+	 *
+	 * So we just directly call the internal command handler, and
+	 * die if that one cannot handle it.
+	 */
+	if (!strncmp(cmd, "git-", 4)) {
+		cmd += 4;
+		argv[0] = cmd;
+		handle_internal_command(argc, argv, envp);
+		die("cannot handle %s internally", cmd);
+	}
 
-	for (i = 1; i < argc; i++) {
-		char *arg = argv[i];
+	/* Default command: "help" */
+	cmd = "help";
 
-		if (!strcmp(arg, "help")) {
-			show_help = 1;
-			continue;
-		}
+	/* Look for flags.. */
+	while (argc > 1) {
+		cmd = *++argv;
+		argc--;
 
-		if (strncmp(arg, "--", 2))
+		if (strncmp(cmd, "--", 2))
 			break;
 
-		arg += 2;
+		cmd += 2;
+
+		/*
+		 * For legacy reasons, the "version" and "help"
+		 * commands can be written with "--" prepended
+		 * to make them look like flags.
+		 */
+		if (!strcmp(cmd, "help"))
+			break;
+		if (!strcmp(cmd, "version"))
+			break;
 
-		if (!strncmp(arg, "exec-path", 9)) {
-			arg += 9;
-			if (*arg == '=') {
-				exec_path = arg + 1;
-				git_set_exec_path(exec_path);
-			} else {
-				puts(git_exec_path());
-				exit(0);
+		/*
+		 * Check remaining flags (which by now must be
+		 * "--exec-path", but maybe we will accept
+		 * other arguments some day)
+		 */
+		if (!strncmp(cmd, "exec-path", 9)) {
+			cmd += 9;
+			if (*cmd == '=') {
+				git_set_exec_path(cmd + 1);
+				continue;
 			}
-		}
-		else if (!strcmp(arg, "version")) {
-			printf("git version %s\n", GIT_VERSION);
+			puts(git_exec_path());
 			exit(0);
 		}
-		else if (!strcmp(arg, "help"))
-			show_help = 1;
-		else if (!show_help)
-			cmd_usage(NULL, NULL);
-	}
-
-	if (i >= argc || show_help) {
-		if (i >= argc)
-			cmd_usage(git_exec_path(), NULL);
-
-		show_man_page(argv[i]);
+		cmd_usage(NULL, NULL);
 	}
-
+	argv[0] = cmd;
+
+	/*
+	 * We search for git commands in the following order:
+	 *  - git_exec_path()
+	 *  - the path of the "git" command if we could find it
+	 *    in $0
+	 *  - the regular PATH.
+	 */
+	if (exec_path)
+		prepend_to_path(exec_path, strlen(exec_path));
 	exec_path = git_exec_path();
 	prepend_to_path(exec_path, strlen(exec_path));
 
-	execv_git_cmd(argv + i);
+	/* See if it's an internal command */
+	handle_internal_command(argc, argv, envp);
+
+	/* .. then try the external ones */
+	execv_git_cmd(argv);
 
 	if (errno == ENOENT)
-		cmd_usage(exec_path, "'%s' is not a git-command", argv[i]);
+		cmd_usage(exec_path, "'%s' is not a git-command", cmd);
 
 	fprintf(stderr, "Failed to run command '%s': %s\n",
 		git_command, strerror(errno));