Fix deadlock with parallel SBY procs each with parallel tasks

sbysrc/sby_jobserver.py

@@ -80,7 +80,16 @@ def process_jobserver_environment():
 
 def jobserver_helper(jobserver_read_fd, jobserver_write_fd, request_fd, response_fd):
  """Helper process to handle blocking jobserver pipes."""
+ def handle_sigusr1(*args):
+ # Since Python doesn't allow user code to handle EINTR anymore, we replace the
+ # jobserver fd with an fd at EOF to interrupt a blocking read in a way that
+ # cannot lose any read data
+ r, w = os.pipe()
+ os.close(w)
+ os.dup2(r, jobserver_read_fd)
+ os.close(r)
  signal.signal(signal.SIGINT, signal.SIG_IGN)
+ signal.signal(signal.SIGUSR1, handle_sigusr1)
  pending = 0
  while True:
  try:
@@ -110,6 +119,8 @@ def jobserver_helper(jobserver_read_fd, jobserver_write_fd, request_fd, response
  except BlockingIOError:
  select.select([jobserver_read_fd], [], [])
  continue
+ if not token:
+ break
 
  pending -= 1
 
@@ -240,6 +251,10 @@ def atexit_blocking(self):
  # Closing the request pipe singals the helper that we want to exit
  os.close(self.request_write_fd)
 
+ # Additionally we send a signal to interrupt a blocking read within the
+ # helper
+ self.helper_process.send_signal(signal.SIGUSR1)
+
  # The helper might have been in the process of sending us some tokens, which
  # we still need to return
  while True: