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: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/Makefile b/Makefile
index 87fc7ff4769e6fd41e4918eb0561122c89d307a8..22c1546e2f150d15a659ce4d9bab0c4fafb2f7ba 100644 (file)
--- a/Makefile
+++ b/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
 

diff --git a/run-command.c b/run-command.c
index dccac37c4bac6945577c24d1e0cccac9ff216d80..efe9fe413887966ced2bb3cd976789ec8789ec4d 100644 (file)
--- a/run-command.c
+++ b/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 */

diff --git a/t/t0061-run-command.sh b/t/t0061-run-command.sh
new file mode 100755 (executable)
index 0000000..10b26e4
--- /dev/null
+++ b/t/t0061-run-command.sh
@@ -0,0 +1,14 @@
+#!/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

diff --git a/test-run-command.c b/test-run-command.c
new file mode 100644 (file)
index 0000000..0612bfa
--- /dev/null
+++ b/test-run-command.c
@@ -0,0 +1,35 @@
+/*
+ * 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;
+}