issue #406: clean up FDs on failure explicitly

The previous approach was crap since it left e.g. socketpair instances
lying around for GC with their underlying FD already closed, coupled
with FD number reuse, led to random madness when GC finally runs.

mitogen/parent.py

@@ -211,7 +211,7 @@ def create_socketpair():
     return parentfp, childfp
 
 
-def detach_popen(close_on_error=None, **kwargs):
+def detach_popen(**kwargs):
     """
     Use :class:`subprocess.Popen` to construct a child process, then hack the
     Popen so that it forgets the child it created, allowing it to survive a
@@ -232,13 +232,7 @@ def detach_popen(close_on_error=None, **kwargs):
     # handling, without tying the surrounding code into managing a Popen
     # object, which isn't possible for at least :mod:`mitogen.fork`. This
     # should be replaced by a swappable helper class in a future version.
-    try:
     proc = subprocess.Popen(**kwargs)
-    except Exception:
-        for fd in close_on_error or ():
-            os.close(fd)
-        raise
-
     proc._child_created = False
     return proc.pid
 
@@ -279,15 +273,20 @@ def create_child(args, merge_stdio=False, stderr_pipe=False, preexec_fn=None):
         mitogen.core.set_cloexec(stderr_w)
         extra = {'stderr': stderr_w}
 
+    try:
         pid = detach_popen(
             args=args,
             stdin=childfp,
             stdout=childfp,
             close_fds=True,
             preexec_fn=preexec_fn,
-        close_on_error=[parentfp.fileno(), childfp.fileno()],
             **extra
         )
+    except Exception:
+        childfp.close()
+        parentfp.close()
+        raise
+
     if stderr_pipe:
         os.close(stderr_w)
     childfp.close()
@@ -347,15 +346,19 @@ def tty_create_child(args):
     disable_echo(master_fd)
     disable_echo(slave_fd)
 
+    try:
         pid = detach_popen(
             args=args,
             stdin=slave_fd,
             stdout=slave_fd,
             stderr=slave_fd,
             preexec_fn=_acquire_controlling_tty,
-        close_on_error=[master_fd, slave_fd],
             close_fds=True,
         )
+    except Exception:
+        os.close(master_fd)
+        os.close(slave_fd)
+        raise
 
     os.close(slave_fd)
     LOG.debug('tty_create_child() child %d fd %d, parent %d, cmd: %s',
@@ -383,6 +386,7 @@ def hybrid_tty_create_child(args):
     disable_echo(master_fd)
     disable_echo(slave_fd)
 
+    try:
         pid = detach_popen(
             args=args,
             stdin=childfp,
@@ -390,8 +394,13 @@ def hybrid_tty_create_child(args):
             stderr=slave_fd,
             preexec_fn=_acquire_controlling_tty,
             close_fds=True,
-        close_on_error=[master_fd, slave_fd, parentfp.fileno(), childfp.fileno()],
         )
+    except Exception:
+        os.close(master_fd)
+        os.close(slave_fd)
+        parentfp.close()
+        childfp.close()
+        raise
 
     os.close(slave_fd)
     childfp.close()