Index: lldb/trunk/test/lldbtest.py
===================================================================
--- lldb/trunk/test/lldbtest.py
+++ lldb/trunk/test/lldbtest.py
@@ -33,6 +33,7 @@
 
 import abc
 import glob
+import lock
 import os, sys, traceback
 import os.path
 import re
@@ -47,6 +48,14 @@
 import lldbutil
 from _pyio import __metaclass__
 
+# dosep.py starts lots and lots of dotest instances
+# This option helps you find if two (or more) dotest instances are using the same
+# directory at the same time
+# Enable it to cause test failures and stderr messages if dotest instances try to run in
+# the same directory simultaneously
+# it is disabled by default because it litters the test directories with ".dirlock" files
+debug_confirm_directory_exclusivity = False
+
 # See also dotest.parseOptionsAndInitTestdirs(), where the environment variables
 # LLDB_COMMAND_TRACE and LLDB_DO_CLEANUP are set from '-t' and '-r dir' options.
 
@@ -936,7 +945,6 @@
         Python unittest framework class setup fixture.
         Do current directory manipulation.
         """
-
         # Fail fast if 'mydir' attribute is not overridden.
         if not cls.mydir or len(cls.mydir) == 0:
             raise Exception("Subclasses must override the 'mydir' attribute.")
@@ -947,10 +955,26 @@
         # Change current working directory if ${LLDB_TEST} is defined.
         # See also dotest.py which sets up ${LLDB_TEST}.
         if ("LLDB_TEST" in os.environ):
+            full_dir = os.path.join(os.environ["LLDB_TEST"], cls.mydir)
             if traceAlways:
-                print >> sys.stderr, "Change dir to:", os.path.join(os.environ["LLDB_TEST"], cls.mydir)
+                print >> sys.stderr, "Change dir to:", full_dir
             os.chdir(os.path.join(os.environ["LLDB_TEST"], cls.mydir))
 
+        if debug_confirm_directory_exclusivity:
+            cls.dir_lock = lock.Lock(os.path.join(full_dir, ".dirlock"))
+            try:
+                cls.dir_lock.try_acquire()
+                # write the class that owns the lock into the lock file
+                cls.dir_lock.handle.write(cls.__name__)
+            except IOError as ioerror:
+                # nothing else should have this directory lock
+                # wait here until we get a lock
+                cls.dir_lock.acquire()
+                # read the previous owner from the lock file
+                lock_id = cls.dir_lock.handle.read()
+                print >> sys.stderr, "LOCK ERROR: {} wants to lock '{}' but it is already locked by '{}'".format(cls.__name__, full_dir, lock_id)
+                raise ioerror
+
         # Set platform context.
         if platformIsDarwin():
             cls.platformContext = _PlatformContext('DYLD_LIBRARY_PATH', 'lib', 'dylib')
@@ -982,6 +1006,10 @@
                     exc_type, exc_value, exc_tb = sys.exc_info()
                     traceback.print_exception(exc_type, exc_value, exc_tb)
 
+        if debug_confirm_directory_exclusivity:
+            cls.dir_lock.release()
+            del cls.dir_lock
+
         # Restore old working directory.
         if traceAlways:
             print >> sys.stderr, "Restore dir to:", cls.oldcwd
Index: lldb/trunk/test/lock.py
===================================================================
--- lldb/trunk/test/lock.py
+++ lldb/trunk/test/lock.py
@@ -10,12 +10,16 @@
     def __init__(self, filename):
         self.filename = filename
         # This will create it if it does not exist already
-        self.handle = open(filename, 'w')
+        unbuffered = 0
+        self.handle = open(filename, 'a+', unbuffered)
 
-    # Bitwise OR fcntl.LOCK_NB if you need a non-blocking lock
     def acquire(self):
         fcntl.flock(self.handle, fcntl.LOCK_EX)
 
+    # will throw IOError if unavailable
+    def try_acquire(self):
+        fcntl.flock(self.handle, fcntl.LOCK_NB | fcntl.LOCK_EX)
+
     def release(self):
         fcntl.flock(self.handle, fcntl.LOCK_UN)