[PATCH] Added hook in git-receive-pack

Just before updating a ref,

    $GIT_DIR/hooks/update refname old-sha1 new-sha1

is called if executable.  The hook can decline the ref to be
updated by exiting with a non-zero status, or allow it to be
updated by exiting with a zero status.  The mechanism also
allows e.g sending of a mail with pushed commits on the remote
repository.

Documentation update with an example hook is included.

jc: The credits of the basic idea and initial implementation go
to Josef, but I ended up rewriting major parts of his patch, so
bugs are all mine.  Also I changed the semantics for the hook
from his original version (which were post-update hook) so that
the hook can optionally decline to update the ref, and also can
be used to implement the overall cleanups.  The latter was
primarily to implement a suggestion from Linus that calling
update-server-info should be made optional.

Signed-off-by: Junio C Hamano <junkio@cox.net>

run-command.c

@@ -0,0 +1,60 @@
+#include "cache.h"
+#include "run-command.h"
+#include <sys/wait.h>
+
+static int run_external_command(int argc, const char **argv)
+{
+	pid_t pid = fork();
+
+	if (pid < 0)
+		return -ERR_RUN_COMMAND_FORK;
+	if (!pid) {
+		execvp(argv[0], (char *const*) argv);
+		return -ERR_RUN_COMMAND_EXEC;
+	}
+	for (;;) {
+		int status, code;
+		int retval = waitpid(pid, &status, 0);
+
+		if (retval < 0) {
+			if (errno == EINTR)
+				continue;
+			error("waitpid failed (%s)", strerror(retval));
+			return -ERR_RUN_COMMAND_WAITPID;
+		}
+		if (retval != pid)
+			return -ERR_RUN_COMMAND_WAITPID_WRONG_PID;
+		if (WIFSIGNALED(status))
+			return -ERR_RUN_COMMAND_WAITPID_SIGNAL;
+
+		if (!WIFEXITED(status))
+			return -ERR_RUN_COMMAND_WAITPID_NOEXIT;
+		code = WEXITSTATUS(status);
+		if (code)
+			return -code;
+		return 0;
+	}
+}
+
+int run_command(const char *cmd, ...)
+{
+	int argc;
+	const char *argv[MAX_RUN_COMMAND_ARGS];
+	const char *arg;
+	va_list param;
+
+	fprintf(stderr, "run-command %s (%d)\n", cmd, ERR_RUN_COMMAND_EXEC);
+
+	va_start(param, cmd);
+	argv[0] = cmd;
+	argc = 1;
+	while (argc < MAX_RUN_COMMAND_ARGS) {
+		arg = argv[argc++] = va_arg(param, char *);
+		if (!arg)
+			break;
+	}
+	va_end(param);
+	if (MAX_RUN_COMMAND_ARGS <= argc)
+		return error("too many args to run %s", cmd);
+	return run_external_command(argc, argv);
+}