start_command: detect execvp failures early

Previously, failures during execvp could be detected only by
finish_command. However, in some situations it is beneficial for the
parent process to know earlier that the child process will not run.

The idea to use a pipe to signal failures to the parent process and
the test case were lifted from patches by Ilari Liusvaara.
Signed-off-by: NJohannes Sixt <j6t@kdbg.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -1785,6 +1785,7 @@ TEST_PROGRAMS += test-genrandom$X
 TEST_PROGRAMS += test-match-trees$X
 TEST_PROGRAMS += test-parse-options$X
 TEST_PROGRAMS += test-path-utils$X
+TEST_PROGRAMS += test-run-command$X
 TEST_PROGRAMS += test-sha1$X
 TEST_PROGRAMS += test-sigchain$X
 

run-command.c

@@ -17,6 +17,12 @@ static inline void dup_devnull(int to)
 
 #ifndef WIN32
 static int child_err = 2;
+static int child_notifier = -1;
+
+static void notify_parent(void)
+{
+	write(child_notifier, "", 1);
+}
 
 static NORETURN void die_child(const char *err, va_list params)
 {
@@ -142,6 +148,11 @@ int start_command(struct child_process *cmd)
 	trace_argv_printf(cmd->argv, "trace: run_command:");
 
 #ifndef WIN32
+{
+	int notify_pipe[2];
+	if (pipe(notify_pipe))
+		notify_pipe[0] = notify_pipe[1] = -1;
+
 	fflush(NULL);
 	cmd->pid = fork();
 	if (!cmd->pid) {
@@ -156,6 +167,11 @@ int start_command(struct child_process *cmd)
 		}
 		set_die_routine(die_child);
 
+		close(notify_pipe[0]);
+		set_cloexec(notify_pipe[1]);
+		child_notifier = notify_pipe[1];
+		atexit(notify_parent);
+
 		if (cmd->no_stdin)
 			dup_devnull(0);
 		else if (need_in) {
@@ -196,8 +212,16 @@ int start_command(struct child_process *cmd)
 					unsetenv(*cmd->env);
 			}
 		}
-		if (cmd->preexec_cb)
+		if (cmd->preexec_cb) {
+			/*
+			 * We cannot predict what the pre-exec callback does.
+			 * Forgo parent notification.
+			 */
+			close(child_notifier);
+			child_notifier = -1;
+
 			cmd->preexec_cb();
+		}
 		if (cmd->git_cmd) {
 			execv_git_cmd(cmd->argv);
 		} else {
@@ -215,6 +239,27 @@ int start_command(struct child_process *cmd)
 	if (cmd->pid < 0)
 		error("cannot fork() for %s: %s", cmd->argv[0],
 			strerror(failed_errno = errno));
+
+	/*
+	 * Wait for child's execvp. If the execvp succeeds (or if fork()
+	 * failed), EOF is seen immediately by the parent. Otherwise, the
+	 * child process sends a single byte.
+	 * Note that use of this infrastructure is completely advisory,
+	 * therefore, we keep error checks minimal.
+	 */
+	close(notify_pipe[1]);
+	if (read(notify_pipe[0], &notify_pipe[1], 1) == 1) {
+		/*
+		 * At this point we know that fork() succeeded, but execvp()
+		 * failed. Errors have been reported to our stderr.
+		 */
+		wait_or_whine(cmd->pid, cmd->argv[0],
+			      cmd->silent_exec_failure);
+		failed_errno = errno;
+		cmd->pid = -1;
+	}
+	close(notify_pipe[0]);
+}
 #else
 {
 	int s0 = -1, s1 = -1, s2 = -1;	/* backups of stdin, stdout, stderr */

t/t0061-run-command.sh

+#!/bin/sh
+#
+# Copyright (c) 2009 Ilari Liusvaara
+#
+
+test_description='Test run command'
+
+. ./test-lib.sh
+
+test_expect_success 'start_command reports ENOENT' '
+	test-run-command start-command-ENOENT ./does-not-exist
+'
+
+test_done

test-run-command.c

+/*
+ * test-run-command.c: test run command API.
+ *
+ * (C) 2009 Ilari Liusvaara <ilari.liusvaara@elisanet.fi>
+ *
+ * This code is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#include "git-compat-util.h"
+#include "run-command.h"
+#include <string.h>
+#include <errno.h>
+
+int main(int argc, char **argv)
+{
+	struct child_process proc;
+
+	memset(&proc, 0, sizeof(proc));
+
+	if (argc < 3)
+		return 1;
+	proc.argv = (const char **)argv+2;
+
+	if (!strcmp(argv[1], "start-command-ENOENT")) {
+		if (start_command(&proc) < 0 && errno == ENOENT)
+			return 0;
+		fprintf(stderr, "FAIL %s\n", argv[1]);
+		return 1;
+	}
+
+	fprintf(stderr, "check usage\n");
+	return 1;
+}