Bug 1158898 - Cache config.guess output; r=mshal

mach dispatch makes separate, independent calls to construct build
system state. Part of this resolution is determining the object
directory. For environments without an object directory defined, we must
execute config.guess to determine the object directory. This redundant
execution of config.guess can result in significant execution overhead.

Before this patch, `mach help` with no mozconfig took ~1.5s on my OS X
machine. After this patch, it goes down to ~0.750s. On Windows, the
difference is even more pronounced, with execution time dropping from
8.5s to 0.930s.

python/mozbuild/mozbuild/base.py

@@ -28,6 +28,9 @@ from .mozconfig import (
 from .virtualenv import VirtualenvManager
 
 
+_config_guess_output = []
+
+
 def ancestors(path):
     """Emit the parent directories of a path."""
     while path:
@@ -353,6 +356,11 @@ class MozbuildObject(ProcessExecutionMixin):
         if config_guess:
             return config_guess
 
+        # config.guess results should be constant for process lifetime. Cache
+        # it.
+        if _config_guess_output:
+            return _config_guess_output[0]
+
         p = os.path.join(topsrcdir, 'build', 'autoconf', 'config.guess')
 
         # This is a little kludgy. We need access to the normalize_command
@@ -362,7 +370,9 @@ class MozbuildObject(ProcessExecutionMixin):
         o = MozbuildObject(topsrcdir, None, None, None)
         args = o._normalize_command([p], True)
 
-        return subprocess.check_output(args, cwd=topsrcdir).strip()
+        _config_guess_output.append(
+                subprocess.check_output(args, cwd=topsrcdir).strip())
+        return _config_guess_output[0]
 
     def notify(self, msg):
         """Show a desktop notification with the supplied message