WIP: Forward parent death to descendant processes (unix)

If the forker's connection to the parent BEAM is broken or closed, react
by killing all spawned children.  When a spawned port is closed, kill
the associated OS process.

A concise demonstration of the problem being solved is to run the
following command with and without the patch, then kill the BEAM.
Without the patch, the "sleep" process will continue:

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

TODO:
* Needs a decision made between killing the process or process group.
* Separate patch for win32

erts/emulator/sys/unix/erl_child_setup.c

@@ -183,6 +183,8 @@ static ssize_t write_all(int fd, const char *buff, size_t size) {
     return pos;
 }
 
+static void kill_child(pid_t os_pid);
+static void kill_all_children(void);
 static int forker_hash_init(void);
 
 static int max_files = -1;
@@ -571,6 +573,7 @@ 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);
+                kill_all_children();
                 _exit(0);
             }
 
@@ -579,6 +582,7 @@ main(int argc, char *argv[])
                 if (isatty(0) && isatty(1)) {
                     tcsetattr(0,TCSANOW,&initial_tty_mode);
                 }
+                kill_all_children();
                 _exit(0);
             }
             /* Since we use unix domain sockets and send the entire data in
@@ -621,6 +625,7 @@ main(int argc, char *argv[])
                 est.os_pid = proto.u.stop.os_pid;
                 es = hash_remove(forker_hash, &est);
                 if (es) {
+                    kill_child(es->os_pid);
                     free(es);
                 }
             } else {
@@ -660,6 +665,23 @@ main(int argc, char *argv[])
     return 1;
 }
 
+/* Kill child process groups on VM termination so they don't become orphaned. */
+
+static void kill_child(pid_t os_pid) {
+    if (os_pid > 0 && kill(os_pid, SIGTERM) != 0) {
+        DEBUG_PRINT("error killing process %d: %d", os_pid, errno);
+    }
+}
+
+static void fun_kill_foreach(ErtsSysExitStatus *es, void *unused) {
+    kill_child(es->os_pid);
+}
+
+static void kill_all_children(void) {
+    DEBUG_PRINT("cleaning up by killing all %d child processes", forker_hash->nobjs);
+    hash_foreach(forker_hash, (HFOREACH_FUN)fun_kill_foreach, NULL);
+}
+
 static int fcmp(void *a, void *b)
 {
     ErtsSysExitStatus *sa = a;

erts/preloaded/src/erlang.erl

@@ -7475,6 +7475,11 @@ reported to the owning process using signals of the form
 
 The maximum number of ports that can be open at the same time can be configured
 by passing command-line flag [`+Q`](erl_cmd.md#max_ports) to [erl](erl_cmd.md).
+
+When a port is closed or the VM shuts down, spawned executables are sent a
+`SIGTERM` on unix. The child may still outlive the VM if it traps the signal.
+Note that any processes started under a shell using `spawn` will not terminate
+unless they respond to stdin or stdout being closed.
 """.
 -doc #{ category => ports }.
 -spec open_port(PortName, PortSettings) -> port() when