Exec git programs without using PATH.

The git suite may not be in PATH (and thus programs such as
git-send-pack could not exec git-rev-list).  Thus there is a need for
logic that will locate these programs.  Modifying PATH is not
desirable as it result in behavior differing from the user's
intentions, as we may end up prepending "/usr/bin" to PATH.

- git C programs will use exec*_git_cmd() APIs to exec sub-commands.
- exec*_git_cmd() will execute a git program by searching for it in
  the following directories:
	1. --exec-path (as used by "git")
	2. The GIT_EXEC_PATH environment variable.
	3. $(gitexecdir) as set in Makefile (default value $(bindir)).
- git wrapper will modify PATH as before to enable shell scripts to
  invoke "git-foo" commands.

Ideally, shell scripts should use the git wrapper to become independent
of PATH, and then modifying PATH will not be necessary.

[jc: with minor updates after a brief review.]

Signed-off-by: Michal Ostrowski <mostrows@watson.ibm.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

1 files changed, 117 insertions, 0 deletions

diff --git a/exec_cmd.c b/exec_cmd.c
new file mode 100644
index 0000000000..55af33bb7e
--- /dev/null
+++ b/exec_cmd.c
@@ -0,0 +1,117 @@
+#include "cache.h"
+#include "exec_cmd.h"
+#define MAX_ARGS	32
+
+extern char **environ;
+static const char *builtin_exec_path = GIT_EXEC_PATH;
+static const char *current_exec_path = NULL;
+
+void git_set_exec_path(const char *exec_path)
+{
+	current_exec_path = exec_path;
+}
+
+
+/* Returns the highest-priority, location to look for git programs. */
+const char *git_exec_path()
+{
+	const char *env;
+
+	if (current_exec_path)
+		return current_exec_path;
+
+	env = getenv("GIT_EXEC_PATH");
+	if (env) {
+		return env;
+	}
+
+	return builtin_exec_path;
+}
+
+
+int execv_git_cmd(char **argv)
+{
+	char git_command[PATH_MAX + 1];
+	char *tmp;
+	int len, err, i;
+	const char *paths[] = { current_exec_path,
+				getenv("GIT_EXEC_PATH"),
+				builtin_exec_path };
+
+	for (i = 0; i < sizeof(paths)/sizeof(paths[0]); ++i) {
+		const char *exec_dir = paths[i];
+		if (!exec_dir) continue;
+
+		if (*exec_dir != '/') {
+			if (!getcwd(git_command, sizeof(git_command))) {
+				fprintf(stderr, "git: cannot determine "
+					"current directory\n");
+				exit(1);
+			}
+			len = strlen(git_command);
+
+			/* Trivial cleanup */
+			while (!strncmp(exec_dir, "./", 2)) {
+				exec_dir += 2;
+				while (*exec_dir == '/')
+					exec_dir++;
+			}
+			snprintf(git_command + len, sizeof(git_command) - len,
+				 "/%s", exec_dir);
+		} else {
+			strcpy(git_command, exec_dir);
+		}
+
+		len = strlen(git_command);
+		len += snprintf(git_command + len, sizeof(git_command) - len,
+				"/git-%s", argv[0]);
+
+		if (sizeof(git_command) <= len) {
+			fprintf(stderr,
+				"git: command name given is too long.\n");
+			break;
+		}
+
+		/* argv[0] must be the git command, but the argv array
+		 * belongs to the caller, and my be reused in
+		 * subsequent loop iterations. Save argv[0] and
+		 * restore it on error.
+		 */
+
+		tmp = argv[0];
+		argv[0] = git_command;
+
+		/* execve() can only ever return if it fails */
+		execve(git_command, argv, environ);
+
+		err = errno;
+
+		argv[0] = tmp;
+	}
+	return -1;
+
+}
+
+
+int execl_git_cmd(char *cmd,...)
+{
+	int argc;
+	char *argv[MAX_ARGS + 1];
+	char *arg;
+	va_list param;
+
+	va_start(param, cmd);
+	argv[0] = cmd;
+	argc = 1;
+	while (argc < MAX_ARGS) {
+		arg = argv[argc++] = va_arg(param, char *);
+		if (!arg)
+			break;
+	}
+	va_end(param);
+	if (MAX_ARGS <= argc)
+		return error("too many args to run %s", cmd);
+
+	argv[argc] = NULL;
+	return execv_git_cmd(argv);
+}