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 #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:
https://github.com/nodejs/node/issues/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()

1 files changed, 6 insertions, 3 deletions

diff --git a/src/nvim/event/libuv_process.c b/src/nvim/event/libuv_process.c
index c101cb1bb9..0c2bf397e5 100644
--- a/src/nvim/event/libuv_process.c
+++ b/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;