tree-wide: introduce new safe_fork() helper and port everything over

This adds a new safe_fork() wrapper around fork() and makes use of it
everywhere. The new wrapper does a couple of things we previously did
manually and separately in a safer, more correct and automatic way:

1. Optionally resets signal handlers/mask in the child

2. Sets a name on all processes we fork off right after forking off (and
   the patch assigns useful names for all processes we fork off now,
   following a systematic naming scheme: always enclosed in () – in order
   to indicate that these are not proper, exec()ed processes, but only
   forked off children, and if the process is long-running with only our
   own code, without execve()'ing something else, it gets am "sd-" prefix.)

3. Optionally closes all file descriptors in the child

4. Optionally sets a PR_SET_DEATHSIG to SIGTERM in the child, in a safe
   way so that the parent dying before this happens being handled
   safely.

5. Optionally reopens the logs

6. Optionally connects stdin/stdout/stderr to /dev/null

7. Debug logs about the forked off processes.

1 files changed, 9 insertions, 16 deletions

diff --git a/src/basic/exec-util.c b/src/basic/exec-util.c
index 718fbf7ee..19ac3686c 100644
--- a/src/basic/exec-util.c
+++ b/src/basic/exec-util.c
@@ -48,20 +48,19 @@ assert_cc(EAGAIN == EWOULDBLOCK);
 static int do_spawn(const char *path, char *argv[], int stdout_fd, pid_t *pid) {
 
         pid_t _pid;
+        int r;
 
         if (null_or_empty_path(path)) {
                 log_debug("%s is empty (a mask).", path);
                 return 0;
         }
 
-        _pid = fork();
-        if (_pid < 0)
-                return log_error_errno(errno, "Failed to fork: %m");
-        if (_pid == 0) {
+        r = safe_fork("(direxec)", FORK_DEATHSIG, &_pid);
+        if (r < 0)
+                return log_error_errno(r, "Failed to fork: %m");
+        if (r == 0) {
                 char *_argv[2];
 
-                assert_se(prctl(PR_SET_PDEATHSIG, SIGTERM) == 0);
-
                 if (stdout_fd >= 0) {
                         /* If the fd happens to be in the right place, go along with that */
                         if (stdout_fd != STDOUT_FILENO &&
@@ -107,11 +106,6 @@ static int do_execute(
          * If callbacks is nonnull, execution is serial. Otherwise, we default to parallel.
          */
 
-        (void) reset_all_signal_handlers();
-        (void) reset_signal_mask();
-
-        assert_se(prctl(PR_SET_PDEATHSIG, SIGTERM) == 0);
-
         r = conf_files_list_strv(&paths, NULL, NULL, CONF_FILES_EXECUTABLE, (const char* const*) directories);
         if (r < 0)
                 return r;
@@ -226,11 +220,10 @@ int execute_directories(
          * them to finish. Optionally a timeout is applied. If a file with the same name
          * exists in more than one directory, the earliest one wins. */
 
-        executor_pid = fork();
-        if (executor_pid < 0)
-                return log_error_errno(errno, "Failed to fork: %m");
-
-        if (executor_pid == 0) {
+        r = safe_fork("(sd-executor)", FORK_RESET_SIGNALS|FORK_DEATHSIG, &executor_pid);
+        if (r < 0)
+                return log_error_errno(r, "Failed to fork: %m");
+        if (r == 0) {
                 r = do_execute(dirs, timeout, callbacks, callback_args, fd, argv);
                 _exit(r < 0 ? EXIT_FAILURE : EXIT_SUCCESS);
         }