WIP: Forward parent death to descendant processes

If the uds_fd connection to the parent BEAM is broken or closed, react
by killing all children and any descendants in the same process group.

Changing from _exit to exit is significant. This patch installs an
atexit handler, which wouldn't be executed when using the _exit
variant.  It's unclear why the "immediate" variant was being used
until now, it would make sense if there were a danger of eg.  memory
corruption which would cause segfaults on freeing, but many of these
were only being used on wire protocol errors.

To start a child process which can outlive BEAM termination, give it a
new process group for example by using `setsid`:

    erl -noshell -eval 'os:cmd("setsid sleep 60")'

FIXME: Killing child processes may be a breaking change for any code
which assumed that the child process will outlive the VM.  Is this
fixing a bug, or causing one?  If the default should remain unchanged
we can add an optional flag such as `kill_group` to open_port making
this behavior switchable per child.

TODO: Needs docs and tests.

erts/emulator/sys/unix/erl_child_setup.c

@@ -174,6 +174,7 @@ static ssize_t write_all(int fd, const char *buff, size_t size) {
     return pos;
 }
 
+static void kill_all_children(void);
 static void add_os_pid_to_port_id_mapping(Eterm, pid_t);
 static Eterm get_port_id(pid_t);
 static int forker_hash_init(void);
@@ -300,7 +301,7 @@ start_new_child(int pipes[])
     if (wd && chdir(wd) < 0) {
         int err = errno;
         fprintf(stderr,"spawn: Could not cd to %s\r\n", wd);
-        _exit(err);
+        exit(err);
     }
 
     DEBUG_PRINT("Do that forking business: '%s'",cmd);
@@ -364,12 +365,12 @@ start_new_child(int pipes[])
     }
 
     DEBUG_PRINT("exec error: %d",errno);
-    _exit(errno);
+    exit(errno);
 
 child_error:
     fprintf(stderr,"erl_child_setup: failed with error %d on line %d\r\n",
             errno, errln);
-    _exit(errno);
+    exit(errno);
 }
 
 
@@ -537,6 +538,10 @@ main(int argc, char *argv[])
 
     DEBUG_PRINT("Starting forker %d", max_files);
 
+    if (atexit(kill_all_children) != 0) {
+        ABORT("Failed to install atexit handler");
+    }
+
     while (1) {
         fd_set read_fds;
         int res;
@@ -564,15 +569,15 @@ main(int argc, char *argv[])
                     tcsetattr(0,TCSANOW,&initial_tty_mode);
                 }
                 DEBUG_PRINT("erl_child_setup failed to read from uds: %d, %d", res, errno);
-                _exit(0);
+                exit(0);
             }
 
             if (res == 0) {
                 DEBUG_PRINT("uds was closed!");
                 if (isatty(0) && isatty(1)) {
                     tcsetattr(0,TCSANOW,&initial_tty_mode);
                 }
-                _exit(0);
+                exit(0);
             }
             /* Since we use unix domain sockets and send the entire data in
                one go we *should* get the entire payload at once. */
@@ -642,6 +647,20 @@ typedef struct exit_status {
 
 static Hash *forker_hash;
 
+/*
+ * Kill child processes so they don't become orphaned.
+ * TODO: Decide whether there should be an option to leave some children unkilled. */
+
+static void fun_kill_foreach(ErtsSysExitStatus *es, void *unused) {
+    if (kill(-es->os_pid, SIGKILL) != 0) {
+        DEBUG_PRINT("error killing process group %d: %d", es->os_pid, errno);
+    }
+}
+
+static void kill_all_children(void) {
+    hash_foreach(forker_hash, (HFOREACH_FUN)fun_kill_foreach, NULL);
+}
+
 static void add_os_pid_to_port_id_mapping(Eterm port_id, pid_t os_pid)
 {
     ErtsSysExitStatus es;