journal: guarantee async-signal-safety in sd_journald_sendv

signal(7) provides a list of functions which may be called from a
signal handler. Other functions, which only call those functions and
don't access global memory and are reentrant are also safe.
sd_j_sendv was mostly OK, but would call mkostemp and writev in a
fallback path, which are unsafe.

Being able to call sd_j_sendv in a async-signal-safe way is important
because it allows it be used in signal handlers.

Safety is achieved by replacing mkostemp with open(O_TMPFILE) and an
open-coded writev replacement which uses write. Unfortunately,
O_TMPFILE is only available on kernels >= 3.11. When O_TMPFILE is
unavailable, an open-coded mkostemp is used.

https://bugzilla.gnome.org/show_bug.cgi?id=722889

src/test/test-tmpfiles.c

@@ -0,0 +1,51 @@
+/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
+
+/***
+  This file is part of systemd.
+
+  Copyright 2014 Zbigniew Jędrzejewski-Szmek
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+#include "util.h"
+
+int main(int argc, char** argv) {
+        const char *p = argv[1] ?: "/tmp";
+        char *pattern = strappenda(p, "/systemd-test-XXXXXX");
+        _cleanup_close_ int fd, fd2;
+        _cleanup_free_ char *cmd, *cmd2;
+
+        fd = open_tmpfile(p, O_RDWR);
+        assert(fd >= 0);
+
+        assert_se(asprintf(&cmd, "ls -l /proc/"PID_FMT"/fd/%d", getpid(), fd) > 0);
+        system(cmd);
+
+        fd2 = mkostemp_safe(pattern, O_RDWR);
+        assert(fd >= 0);
+        assert_se(unlink(pattern) == 0);
+
+        assert_se(asprintf(&cmd2, "ls -l /proc/"PID_FMT"/fd/%d", getpid(), fd2) > 0);
+        system(cmd2);
+
+        return 0;
+}

src/test/test-util.c

@@ -28,6 +28,8 @@
 
 #include "util.h"
 #include "strv.h"
+#include "def.h"
+#include "fileio.h"
 
 static void test_streq_ptr(void) {
         assert_se(streq_ptr(NULL, NULL));
@@ -578,6 +580,29 @@ static void test_in_set(void) {
         assert_se(!IN_SET(0, 1, 2, 3, 4));
 }
 
+static void test_writev_safe(void) {
+        char name[] = "/tmp/test-writev_safe.XXXXXX";
+        _cleanup_free_ char *contents;
+        size_t size;
+        int fd, r;
+
+        struct iovec iov[3];
+        IOVEC_SET_STRING(iov[0], "abc\n");
+        IOVEC_SET_STRING(iov[1], ALPHANUMERICAL "\n");
+        IOVEC_SET_STRING(iov[2], "");
+
+        fd = mkstemp(name);
+        printf("test_writev_safe: %s", name);
+
+        r = writev_safe(fd, iov, 3);
+        assert(r == 0);
+
+        r = read_full_file(name, &contents, &size);
+        assert(r == 0);
+        printf("contents: %s", contents);
+        assert(streq(contents, "abc\n" ALPHANUMERICAL "\n"));
+}
+
 int main(int argc, char *argv[]) {
         test_streq_ptr();
         test_first_word();
@@ -615,6 +640,7 @@ int main(int argc, char *argv[]) {
         test_fstab_node_to_udev_node();
         test_get_files_in_directory();
         test_in_set();
+        test_writev_safe();
 
         return 0;
 }