Bug 1272851 - Use objdir for temporary directory for mach xpcshell-test; r=ted

When running xpcshell tests on my i7-6700K, the Windows Defender
MsMpEng.exe process regularly consumes up to 50% CPU - 2 whole cores.
This is because it is canning the thousands of files we create in the
temporary directory. (We create a profile for every xpcshell.exe process.)

This commit changes the behavior of `mach xpcshell-test` to use a temporary
directory under the topobjdir. We encourage people to disable file indexing
and A/V scanning in the source and object directories because this can add
overhead. So by putting temporary files in the object directory, we should
minimize the chances of file scanning slowing down test execution.

On my machine (which has Windows Defender disabled in my source and objdirs),
`mach xpcshell-test` execution time drops from ~13:40 to ~7:30. Seriously.

I'm told we have Windows Defender disabled in automation, so this hack
shouldn't need to be investigated there. i.e. this is a pure local development
win.



MozReview-Commit-ID: BMcSAZ16Yei

--HG--
extra : rebase_source : 5490be35b59ff675639180b928ee77dcc70de485

testing/xpcshell/mach_commands.py

@@ -7,6 +7,7 @@
 from __future__ import absolute_import, unicode_literals, print_function
 
 import argparse
+import errno
 import os
 import shutil
 import sys
@@ -122,6 +123,18 @@ class XPCShellRunner(MozbuildObject):
         if kwargs["failure_manifest"] is None:
             kwargs["failure_manifest"] = os.path.join(self.statedir, 'xpcshell.failures.ini')
 
+        # Use the object directory for the temp directory to minimize the chance
+        # of file scanning. The overhead from e.g. search indexers and anti-virus
+        # scanners like Windows Defender can add tons of overhead to test execution.
+        # We encourage people to disable these things in the object directory.
+        temp_dir = os.path.join(self.topobjdir, 'temp')
+        try:
+            os.mkdir(temp_dir)
+        except OSError as e:
+            if e.errno != errno.EEXIST:
+                raise
+        kwargs['tempDir'] = temp_dir
+
         # Python through 2.7.2 has issues with unicode in some of the
         # arguments. Work around that.
         filtered_args = {}