jobs: child proc must have a separate process-group

UV_PROCESS_DETACHED compels libuv:uv__process_child_init() to call
setsid() in the child just after fork().  That ensures the process and
its descendants are grouped in a separate session (and process group).

The following jobstart() call correctly groups `sh` and `sleep` in a new
session (and process-group), where `sh` is the "session leader" (and
process-group leader):

    :call jobstart(['sh','-c','sleep 60'])

     SESN  PGRP   PID  PPID  Command
    30383 30383 30383  3620  │  ├─ -bash
    30383 31432 31432 30383  │  │  └─ nvim -u NORC
    30383 31432 31433 30383  │  │     ├─ nvim -u NORC
     8105  8105  8105 31432  │  │     └─ sh -c sleep 60
     8105  8105  8106  8105  │  │        └─ sleep 60

closes neovim#6530
ref: https://stackoverflow.com/q/1046933
ref: https://unix.stackexchange.com/a/404065

Helped-by: Marco Hinz <mh.codebro+github@gmail.com>

Discussion
------------------------------------------------------------------------

On my linux box before this patch, the termclose_spec.lua:'kills job
trapping SIGTERM' test indirectly causes cmake/busted to wait for 60s.
That's because the test spawns a `sleep 60` descendant process which
hangs around even after nvim exits: nvim killed the parent PID, but not
PGID (process-group), so the grandchild "reparented" to init (PID 1).

Session contains processes (and process-groups) which are logically part
of the same "login session". Process-group is a set of
logically/informally-related processes within a session; for example,
shells assign a process group to each "job". Session IDs and PGIDs both
have type pid_t (like PIDs).

These OS-level mechanisms are, as usual, legacy accidents whose purpose
is upheld by convention and folklore.  We can use session-level grouping
(setsid), or we could use process-group-level grouping (setpgid).

Vim uses setsid() if available, otherwise setpgid(0,0).

Windows
------------------------------------------------------------------------

UV_PROCESS_DETACHED on win32 sets CREATE_NEW_PROCESS_GROUP flag.
But uv_kill() does not kill the process-group:
nodejs/node#3617

Ideas:
- Set UV_PROCESS_DETACHED (CREATE_NEW_PROCESS_GROUP), then call
  GenerateConsoleCtrlEvent(CTRL_BREAK_EVENT, pid)
   - Maybe won't work because MSDN says "Only processes that share the
     same console as the calling process receive the signal."
     https://docs.microsoft.com/en-us/windows/console/generateconsolectrlevent
     But CREATE_NEW_PROCESS_GROUP creates a new console ...
     ref https://stackoverflow.com/q/1453520
- Group processes within a "job". libuv does that *globally* for
  non-detached processes: uv__init_global_job_handle.
- Iterate through CreateToolhelp32Snapshot().
   - https://stackoverflow.com/q/1173342
   - Vim does this, see terminate_all()

src/nvim/event/libuv_process.c

@@ -26,15 +26,18 @@ int libuv_process_spawn(LibuvProcess *uvproc)
   uvproc->uvopts.file = proc->argv[0];
   uvproc->uvopts.args = proc->argv;
   uvproc->uvopts.flags = UV_PROCESS_WINDOWS_HIDE;
-  if (proc->detach) {
-      uvproc->uvopts.flags |= UV_PROCESS_DETACHED;
-  }
 #ifdef WIN32
   // libuv collapses the argv to a CommandLineToArgvW()-style string. cmd.exe
   // expects a different syntax (must be prepared by the caller before now).
   if (os_shell_is_cmdexe(proc->argv[0])) {
     uvproc->uvopts.flags |= UV_PROCESS_WINDOWS_VERBATIM_ARGUMENTS;
   }
+  if (proc->detach) {
+    uvproc->uvopts.flags |= UV_PROCESS_DETACHED;
+  }
+#else
+  // Always setsid() on unix-likes. #8107
+  uvproc->uvopts.flags |= UV_PROCESS_DETACHED;
 #endif
   uvproc->uvopts.exit_cb = exit_cb;
   uvproc->uvopts.cwd = proc->cwd;

src/nvim/event/process.c

@@ -201,6 +201,23 @@ int process_wait(Process *proc, int ms, MultiQueue *events)
   return proc->status;
 }
 
+/// Kills a process and its descendants.
+static void os_proc_tree_kill(int pid, int sig) {
+  assert(sig == SIGTERM || sig == SIGKILL);
+  int pgid = getpgid(pid);
+  if (pgid > 0) {  // Ignore error. Never kill self (pid=0).
+    if (pgid == pid) {
+      ILOG("sending %s to process group: -%d",
+           sig == SIGTERM ? "SIGTERM" : "SIGKILL",
+           pgid);
+      uv_kill(-pgid, sig);
+    } else {
+      // Should never happen, because process_spawn() did setsid() in the child.
+      ELOG("pgid %d != pid %d", pgid, pid);
+    }
+  }
+}
+
 /// Ask a process to terminate and eventually kill if it doesn't respond
 void process_stop(Process *proc) FUNC_ATTR_NONNULL_ALL
 {
@@ -215,8 +232,7 @@ void process_stop(Process *proc) FUNC_ATTR_NONNULL_ALL
       // stdout/stderr, they will be closed when it exits(possibly due to being
       // terminated after a timeout)
       stream_may_close(&proc->in);
-      ILOG("Sending SIGTERM to pid %d", proc->pid);
-      uv_kill(proc->pid, SIGTERM);
+      os_proc_tree_kill(proc->pid, SIGTERM);
       break;
     case kProcessTypePty:
       // close all streams for pty processes to send SIGHUP to the process
@@ -231,7 +247,7 @@ void process_stop(Process *proc) FUNC_ATTR_NONNULL_ALL
   if (!loop->children_stop_requests++) {
     // When there's at least one stop request pending, start a timer that
     // will periodically check if a signal should be send to the job.
-    ILOG("Starting job kill timer");
+    ILOG("starting job kill timer");
     uv_timer_start(&loop->children_kill_timer, children_kill_cb,
                    KILL_TIMEOUT_MS, KILL_TIMEOUT_MS);
   }
@@ -253,11 +269,9 @@ static void children_kill_cb(uv_timer_t *handle)
 
     if (elapsed >= KILL_TIMEOUT_MS) {
       int sig = proc->type == kProcessTypePty && elapsed < KILL_TIMEOUT_MS * 2
-                    ? SIGTERM
-                    : SIGKILL;
-      ILOG("Sending %s to pid %d", sig == SIGTERM ? "SIGTERM" : "SIGKILL",
-           proc->pid);
-      uv_kill(proc->pid, sig);
+                ? SIGTERM
+                : SIGKILL;
+      os_proc_tree_kill(proc->pid, sig);
     }
   }
 }

src/nvim/os/pty_process_unix.c

@@ -145,8 +145,12 @@ void pty_process_teardown(Loop *loop)
   uv_signal_stop(&loop->children_watcher);
 }
 
-static void init_child(PtyProcess *ptyproc) FUNC_ATTR_NONNULL_ALL
+static void init_child(PtyProcess *ptyproc)
+  FUNC_ATTR_NONNULL_ALL
 {
+  // New session/process-group. #6530
+  setsid();
+
   unsetenv("COLUMNS");
   unsetenv("LINES");
   unsetenv("TERMCAP");