Re-work nxt_process_check_pid_status() slightly

There has been a long standing clang-analyzer issue in
nxt_process_check_pid_status(), well ever since I introduced this
function in commit b0e2d9d ("Isolation: Switch to fork(2) & unshare(2)
on Linux."),

It is complaining that there are cases where 'status' could be returned
with an undefined or garbage value.

Now I'm convinced this can't happen

  If nxt_process_pipe_timer() returns NXT_OK
    read(2) the status value

  If the read(2) failed or if we got NXT_ERROR from
  nxt_process_pipe_timer(), NXT_OK (0) and NXT_ERROR (-1) are the only
  possible return values here, then we set status to -1

I don't see a scenario where status is either not set by the read(2) or
not set to -1.

Now I'm not a fan of initialising variables willy-nilly, however, in
this case if we initialise status to -1, then we can simply remove the

  if (ret <= 0) {

check. So it closes this (non-)issue but also provides a little code
simplification.

NOTE: There is no need to check the return from the read(2) here. We are
reading a single byte, we either get it or don't.

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

Author: ac000

src/nxt_process.c

@@ -370,18 +370,14 @@ nxt_process_pipe_timer(nxt_fd_t fd, short event)
 static nxt_int_t
 nxt_process_check_pid_status(const nxt_fd_t *gc_pipe)
 {
-    int8_t   status;
+    int8_t   status = -1;
     ssize_t  ret;
 
     close(gc_pipe[1]);
 
     ret = nxt_process_pipe_timer(gc_pipe[0], POLLIN);
     if (ret == NXT_OK) {
-        ret = read(gc_pipe[0], &status, sizeof(int8_t));
-    }
-
-    if (ret <= 0) {
-        status = -1;
+        read(gc_pipe[0], &status, sizeof(int8_t));
     }
 
     close(gc_pipe[0]);