gc: use temporary file for editing crontab

While cron is specified by POSIX, there are a wide variety of
implementations in use.  "git maintenance" assumes that the
"crontab" command can be fed from its standard input the new
contents and the syntax to do so is not to have any filename
argument, as POSIX describes.  However, on FreeBSD, the cron
implementation requires a file name argument: if the user wants to
edit standard input, they must specify "-".

Unfortunately, POSIX systems do not have to interpret "-" on the
command line of crontab as a request to read from the standard
input.  Blindly adding "-" on the command line would not work as a
general solution.

Since POSIX tells us that cron must accept a file name argument, let's
solve this problem by specifying a temporary file instead.  This will
ensure that we work with the vast majority of implementations.

Note that because delete_tempfile closes the file for us, we should not
call fclose here on the handle, since doing so will introduce a double
free.

Reported-by: Renato Botelho <garga@FreeBSD.org>
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 25 insertions, 18 deletions

diff --git a/builtin/gc.c b/builtin/gc.c
index 021e9256ae..b04f1c5522 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -2059,6 +2059,7 @@ static int crontab_update_schedule(int run_maintenance, int fd)
 	struct child_process crontab_edit = CHILD_PROCESS_INIT;
 	FILE *cron_list, *cron_in;
 	struct strbuf line = STRBUF_INIT;
+	struct tempfile *tmpedit = NULL;
 
 	get_schedule_cmd(&cmd, NULL);
 	strvec_split(&crontab_list.args, cmd);
@@ -2073,6 +2074,17 @@ static int crontab_update_schedule(int run_maintenance, int fd)
 	/* Ignore exit code, as an empty crontab will return error. */
 	finish_command(&crontab_list);
 
+	tmpedit = mks_tempfile_t(".git_cron_edit_tmpXXXXXX");
+	if (!tmpedit) {
+		result = error(_("failed to create crontab temporary file"));
+		goto out;
+	}
+	cron_in = fdopen_tempfile(tmpedit, "w");
+	if (!cron_in) {
+		result = error(_("failed to open temporary file"));
+		goto out;
+	}
+
 	/*
 	 * Read from the .lock file, filtering out the old
 	 * schedule while appending the new schedule.
@@ -2080,19 +2092,6 @@ static int crontab_update_schedule(int run_maintenance, int fd)
 	cron_list = fdopen(fd, "r");
 	rewind(cron_list);
 
-	strvec_split(&crontab_edit.args, cmd);
-	crontab_edit.in = -1;
-	crontab_edit.git_cmd = 0;
-
-	if (start_command(&crontab_edit))
-		return error(_("failed to run 'crontab'; your system might not support 'cron'"));
-
-	cron_in = fdopen(crontab_edit.in, "w");
-	if (!cron_in) {
-		result = error(_("failed to open stdin of 'crontab'"));
-		goto done_editing;
-	}
-
 	while (!strbuf_getline_lf(&line, cron_list)) {
 		if (!in_old_region && !strcmp(line.buf, BEGIN_LINE))
 			in_old_region = 1;
@@ -2126,14 +2125,22 @@ static int crontab_update_schedule(int run_maintenance, int fd)
 	}
 
 	fflush(cron_in);
-	fclose(cron_in);
-	close(crontab_edit.in);
 
-done_editing:
+	strvec_split(&crontab_edit.args, cmd);
+	strvec_push(&crontab_edit.args, get_tempfile_path(tmpedit));
+	crontab_edit.git_cmd = 0;
+
+	if (start_command(&crontab_edit)) {
+		result = error(_("failed to run 'crontab'; your system might not support 'cron'"));
+		goto out;
+	}
+
 	if (finish_command(&crontab_edit))
 		result = error(_("'crontab' died"));
 	else
 		fclose(cron_list);
+out:
+	delete_tempfile(&tmpedit);
 	return result;
 }
 
diff --git a/t/helper/test-crontab.c b/t/helper/test-crontab.c
index e7c0137a47..2942543046 100644
--- a/t/helper/test-crontab.c
+++ b/t/helper/test-crontab.c
@@ -17,8 +17,8 @@ int cmd__crontab(int argc, const char **argv)
 		if (!from)
 			return 0;
 		to = stdout;
-	} else if (argc == 2) {
-		from = stdin;
+	} else if (argc == 3) {
+		from = fopen(argv[2], "r");
 		to = fopen(argv[1], "w");
 	} else
 		return error("unknown arguments");