shell: new no-interactive-login command to print a custom message

If I disable git-shell's interactive mode by removing the
~/git-shell-commands directory, attempts to ssh in to the service
produce a message intended for the administrator:

	$ ssh git@myserver
	fatal: Interactive git shell is not enabled.
	hint: ~/git-shell-commands should exist and have read and execute access.
	$

That is helpful for the new admin who is wondering "What? Why isn't
the git-shell I just set up working?", but once the site setup is
complete, it would be better to give the user a friendly hint that she
is on the right track, like GitHub does.

	Hi <username>! You've successfully authenticated, but
	GitHub does not provide shell access.

An appropriate greeting might even include more complex dynamic
information, like gitolite's list of repositories the user has access
to.  Add support for a ~/git-shell-commands/no-interactive-login
command that generates an arbitrary greeting.  When the user tries to
log in:

 * If the file ~/git-shell-commands/no-interactive-login exists,
   run no-interactive-login to let the server say what it likes,
   then hang up.

 * Otherwise, if ~/git-shell-commands/ is present, start an
   interactive read-eval-print loop.

 * Otherwise, print the usual configuration hint and hang up.

Reported-by: Ethan Reesor <firelizzard@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Improved-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-shell.txt

@@ -59,6 +59,26 @@ users to list repositories they have access to, create, delete, or
 rename repositories, or change repository descriptions and
 permissions.
 
+If a `no-interactive-login` command exists, then it is run and the
+interactive shell is aborted.
+
+EXAMPLE
+-------
+
+To disable interactive logins, displaying a greeting instead:
++
+----------------
+$ chsh -s /usr/bin/git-shell
+$ mkdir $HOME/git-shell-commands
+$ cat >$HOME/git-shell-commands/no-interactive-login <<\EOF
+#!/bin/sh
+printf '%s\n' "Hi $USER! You've successfully authenticated, but I do not"
+printf '%s\n' "provide interactive shell access."
+exit 128
+EOF
+$ chmod +x $HOME/git-shell-commands/no-interactive-login
+----------------
+
 SEE ALSO
 --------
 ssh(1),

shell.c

@@ -6,6 +6,7 @@
 
 #define COMMAND_DIR "git-shell-commands"
 #define HELP_COMMAND COMMAND_DIR "/help"
+#define NOLOGIN_COMMAND COMMAND_DIR "/no-interactive-login"
 
 static int do_generic_cmd(const char *me, char *arg)
 {
@@ -65,6 +66,18 @@ static void run_shell(void)
 {
 	int done = 0;
 	static const char *help_argv[] = { HELP_COMMAND, NULL };
+
+	if (!access(NOLOGIN_COMMAND, F_OK)) {
+		/* Interactive login disabled. */
+		const char *argv[] = { NOLOGIN_COMMAND, NULL };
+		int status;
+
+		status = run_command_v_opt(argv, 0);
+		if (status < 0)
+			exit(127);
+		exit(status);
+	}
+
 	/* Print help if enabled */
 	run_command_v_opt(help_argv, RUN_SILENT_EXEC_FAILURE);