autotest: use a lock file

this ensures we don't run two copies of the tests at once

Tools/autotest/autotest.py

@@ -243,6 +243,12 @@ def run_tests(steps):
 
     return passed
 
+
+lck = util.lock_file(util.reltopdir('../buildlogs/autotest.lck'))
+if lck is None:
+    print("autotest is locked - exiting")
+    sys.exit(0)
+
 try:
     if not run_tests(steps):
         sys.exit(1)

Tools/autotest/util.py

@@ -160,3 +160,13 @@ def loadfile(fname):
     r = f.read()
     f.close()
     return r
+
+def lock_file(fname):
+    '''lock a file'''
+    import fcntl
+    f = open(fname, mode='w')
+    try:
+        fcntl.lockf(f, fcntl.LOCK_EX | fcntl.LOCK_NB)
+    except Exception:
+        return None
+    return f