autograde_wrapper.py improvements

- spawn a thread that calls wait(). We are pid 1 in the container
  and need to collect all zombies, otherwise autodriver might get
  confused
- Don't double fork. it's not easy to pass the full exit status
  of autodriver through a double fork, so use os.execvp instead
  of subprocess.call. This means we have to do some of the file
  descriptor munging directly.
- Don't bother creating a temporary output file. Directly pipe
  autodriver's output to the real output file

autodriver/autograde_wrapper.py

@@ -4,31 +4,61 @@
 import os
 import pwd
 import shutil
-import subprocess
+import threading
 #print("Running "+ str(sys.argv))
 
+# Wait for all processes, since we are pid 1 in the container,
+# terminate once the interesting process exits
+class WaitLoop(object):
+  def __init__(self, pid=None):
+     self.waitfor=pid
+     self.status=None
+  def __call__(self):
+      try:
+        (np, self.status)=os.wait()
+        while pid is None or np != self.waitfor:
+          (np, self.status)=os.wait()
+      except OSError:
+        if pid:
+           print("Chld process {} never exited, but no more children left".format(self.waitfor))
+           self.status=-1
+
 for f in os.listdir("mount"):
   src=os.path.join("mount", f)
   dst=os.path.join("autolab", f)
   shutil.copy(src, dst)
 
 autolabuser=pwd.getpwnam("autolab")
+(r_p, w_p)=os.pipe()
 pid=os.fork()
 if pid == 0:
+   os.close(r_p)
    os.setgroups([])
    os.setgid(autolabuser.pw_gid)
    os.setuid(autolabuser.pw_uid)
-   outfile=open("output/feedback", "w")
    args=["autodriver"]
    args.extend(sys.argv[1:])
    args.append("autolab")
-   print("Executing "+str(args), file=outfile)
-   sys.exit(subprocess.call(args, stdout=outfile, stderr=outfile, close_fds=True))
-(np, status)=os.waitpid(pid, 0)
+   if w_p != 1:
+     os.dup2(w_p, 1)
+   if w_p != 2:
+     os.dup2(w_p, 2)
+   if w_p > 2:
+     os.close(w_p)
+   os.execvp(args[0], args)
+os.close(w_p)
+waiter=WaitLoop(pid)
+thr=threading.Thread(target=waiter)
+thr.start()
+rpf=os.fdopen(r_p)
+shutil.copyfileobj(rpf, open("mount/feedback", "w"))
+#print("Copied output")
+rpf.close()
+thr.join()
 # if core, exit -1, else pass through code.
-if status & 0xff:
+if os.WIFSIGNALED(waiter.status):
    status=-1
 else:
-   status>>=8;
-shutil.copy("output/feedback", "mount/feedback")
+   status=os.WEXITSTATUS(waiter.status)
+#print("Status is {}".format(status))
 sys.exit(status)