bombshell-client gains a lock that prevents it from hanging if two parallel requests happen too fast

bin/bombshell-client

@@ -1,6 +1,7 @@
 #!/usr/bin/python -u
 
 import cPickle
+import contextlib
 import fcntl
 import os
 import pipes
@@ -13,6 +14,18 @@ import sys
 import threading
 
 
+@contextlib.contextmanager
+def mutexfile(filepath):
+    oldumask = os.umask(0077)
+    try:
+        f = file(filepath, "a")
+    finally:
+        os.umask(oldumask)
+    fcntl.lockf(f.fileno(), fcntl.LOCK_EX)
+    yield
+    f.close()
+
+
 debug_lock = threading.Lock()
 debug_enabled = False
 class LoggingEmu():
@@ -250,26 +263,27 @@ def main_master():
 
   saved_stderr = os.fdopen(os.dup(sys.stderr.fileno()), "a")
 
-  try:
-    p = subprocess.Popen(
-      ["qrexec-client-vm", remote_vm, "qubes.VMShell"],
-      stdin=subprocess.PIPE,
-      stdout=subprocess.PIPE,
-      close_fds=True,
-      preexec_fn=os.setpgrp,
-    )
-  except OSError, e:
-    logging.error("cannot launch qrexec-client-vm: %s", e)
-    return 127
+  with mutexfile(os.path.expanduser("~/.bombshell-lock")):
+      try:
+        p = subprocess.Popen(
+          ["qrexec-client-vm", remote_vm, "qubes.VMShell"],
+          stdin=subprocess.PIPE,
+          stdout=subprocess.PIPE,
+          close_fds=True,
+          preexec_fn=os.setpgrp,
+        )
+      except OSError, e:
+        logging.error("cannot launch qrexec-client-vm: %s", e)
+        return 127
 
-  p.stdin.write(remote_helper_text)
-  p.stdin.flush()
+      p.stdin.write(remote_helper_text)
+      p.stdin.flush()
 
-  send_command(p.stdin, remote_command)
-  confirmation, errmsg = recv_confirmation(p.stdout)
-  if confirmation != 0:
-    logging.error("remote: %s", errmsg)
-    return confirmation
+      send_command(p.stdin, remote_command)
+      confirmation, errmsg = recv_confirmation(p.stdout)
+      if confirmation != 0:
+        logging.error("remote: %s", errmsg)
+        return confirmation
 
   handled_signals = (
       signal.SIGINT,