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, 8 insertions, 23 deletions

diff --git a/src/activate/activate.c b/src/activate/activate.c
index 83807efdcf..67067a8f32 100644
--- a/src/activate/activate.c
+++ b/src/activate/activate.c
@@ -267,38 +267,23 @@ static int exec_process(const char* name, char **argv, char **env, int start_fd,
 
 static int fork_and_exec_process(const char* child, char** argv, char **env, int fd) {
         _cleanup_free_ char *joined = NULL;
-        pid_t parent_pid, child_pid;
+        pid_t child_pid;
+        int r;
 
         joined = strv_join(argv, " ");
         if (!joined)
                 return log_oom();
 
-        parent_pid = getpid_cached();
-
-        child_pid = fork();
-        if (child_pid < 0)
-                return log_error_errno(errno, "Failed to fork: %m");
-
-        /* In the child */
-        if (child_pid == 0) {
-
-                (void) reset_all_signal_handlers();
-                (void) reset_signal_mask();
-
-                /* Make sure the child goes away when the parent dies */
-                if (prctl(PR_SET_PDEATHSIG, SIGTERM) < 0)
-                        _exit(EXIT_FAILURE);
-
-                /* Check whether our parent died before we were able
-                 * to set the death signal */
-                if (getppid() != parent_pid)
-                        _exit(EXIT_SUCCESS);
-
+        r = safe_fork("(activate)", FORK_RESET_SIGNALS|FORK_DEATHSIG, &child_pid);
+        if (r < 0)
+                return log_error_errno(r, "Failed to fork: %m");
+        if (r == 0) {
+                /* In the child */
                 exec_process(child, argv, env, fd, 1);
                 _exit(EXIT_FAILURE);
         }
 
-        log_info("Spawned %s (%s) as PID %d", child, joined, child_pid);
+        log_info("Spawned %s (%s) as PID " PID_FMT ".", child, joined, child_pid);
         return 0;
 }