Cap test threads on Windows to avoid open file limit

[amccarth]

Summary

Python uses stdio from the C runtime for file descriptors and pipes. On Windows, by default, the CRT has a limit of 512 open file descriptors. https://msdn.microsoft.com/en-us/library/6e3b887c.aspx

The parent dotest process ends up with several FDs for each process running a test in parallel. At about 37-38 logical cores we started hitting this limit regularly.

This patch works around the problem by capping the threads to 32 on Windows.

[amccarth]

Hold off. I just got a failure even at only 32 subprocesses.

[amccarth]

Using the multiprocessing-pool mode seems more reliable than capping num_threads. I'm doing it only when the number of threads is high because of the loss of Ctrl+C support.

[tfiala]

LGTM, Adrian!

lldb/trunk/test/dosep.py

def default_test_runner_name(num_threads):
 
     @return the test runner name that should be used by default when
     no test runner was explicitly called out on the command line.
     """
     if num_threads == 1:
         # Use the serial runner.
         test_runner_name = "serial"
     elif os.name == "nt":
-        # Currently the multiprocessing test runner with ctrl-c
-        # support isn't running correctly on nt.  Use the pool
-        # support without ctrl-c.
+        # On Windows, Python uses CRT with a low limit on the number of open
+        # files.  If you have a lot of cores, the threading-pool runner will
+        # often fail because it exceeds that limit.
+        if num_threads > 32:
+            test_runner_name = "multiprocessing-pool"
+        else:
             test_runner_name = "threading-pool"
     elif is_darwin_version_lower_than(
             distutils.version.StrictVersion("10.10.0")):
         # OS X versions before 10.10 appear to have an issue using
         # the threading test runner.  Fall back to multiprocessing.
         # Supports Ctrl-C.
         test_runner_name = "multiprocessing"
     else:
         # For everyone else, use the ctrl-c-enabled threading support.