--- a/python/mozversioncontrol/mozversioncontrol/__init__.py
+++ b/python/mozversioncontrol/mozversioncontrol/__init__.py
@@ -8,16 +8,19 @@ import os
 import re
 import subprocess
 import which
 
 from distutils.version import LooseVersion
 
 def get_tool_path(tool):
     """Obtain the path of `tool`."""
+    if os.path.isabs(tool) and os.path.exists(tool):
+        return tool
+
     # We use subprocess in places, which expects a Win32 executable or
     # batch script. On some versions of MozillaBuild, we have "hg.exe",
     # "hg.bat," and "hg" (a Python script). "which" will happily return the
     # Python script, which will cause subprocess to choke. Explicitly favor
     # the Windows version over the plain script.
     try:
         return which.which(tool + '.exe')
     except which.WhichError:
@@ -78,18 +81,18 @@ class Repository(object):
 
     def get_files_in_working_directory(self):
         """Obtain a list of managed files in the working directory."""
         raise NotImplementedError
 
 
 class HgRepository(Repository):
     '''An implementation of `Repository` for Mercurial repositories.'''
-    def __init__(self, path):
-        super(HgRepository, self).__init__(path, 'hg')
+    def __init__(self, path, hg='hg'):
+        super(HgRepository, self).__init__(path, tool=hg)
         self._env[b'HGPLAIN'] = b'1'
 
     def get_modified_files(self):
         # Use --no-status to print just the filename.
         return self._run('status', '--modified', '--no-status').splitlines()
 
     def get_added_files(self):
         # Use --no-status to print just the filename.
@@ -107,18 +110,18 @@ class HgRepository(Repository):
     def get_files_in_working_directory(self):
         # Can return backslashes on Windows. Normalize to forward slashes.
         return list(p.replace('\\', '/') for p in
                     self._run('files', '-0').split('\0'))
 
 
 class GitRepository(Repository):
     '''An implementation of `Repository` for Git repositories.'''
-    def __init__(self, path):
-        super(GitRepository, self).__init__(path, 'git')
+    def __init__(self, path, git='git'):
+        super(GitRepository, self).__init__(path, tool=git)
 
     def get_modified_files(self):
         return self._run('diff', '--diff-filter=M', '--name-only').splitlines()
 
     def get_added_files(self):
         return self._run('diff', '--diff-filter=A', '--name-only').splitlines()
 
     def add_remove_files(self, path):
@@ -143,25 +146,64 @@ def get_repository_object(path):
         return HgRepository(path)
     elif os.path.exists(os.path.join(path, '.git')):
         return GitRepository(path)
     else:
         raise InvalidRepoPath('Unknown VCS, or not a source checkout: %s' %
                               path)
 
 
+class MissingVCSInfo(Exception):
+    """Represents a general failure to resolve a VCS interface."""
+
+
+class MissingConfigureInfo(MissingVCSInfo):
+    """Represents error finding VCS info from configure data."""
+
+
 def get_repository_from_env():
-    """Obtain a repository object by looking at the environment."""
+    """Obtain a repository object by looking at the environment.
+
+    If inside a build environment (denoted by presence of a ``buildconfig``
+    module), VCS info is obtained from it, as found via configure. This allows
+    us to respect what was passed into configure. Otherwise, we fall back to
+    scanning the filesystem.
+    """
+    try:
+        import buildconfig
+
+        flavor = buildconfig.substs.get('VCS_CHECKOUT_TYPE')
+
+        # If in build mode, only use what configure found. That way we ensure
+        # that everything in the build system can be controlled via configure.
+        if not flavor:
+            raise MissingConfigureInfo('could not find VCS_CHECKOUT_TYPE '
+                                       'in build config; check configure '
+                                       'output and verify it could find a '
+                                       'VCS binary')
+
+        if flavor == 'hg':
+            return HgRepository(buildconfig.topsrcdir,
+                                hg=buildconfig.substs['HG'])
+        elif flavor == 'git':
+            return GitRepository(buildconfig.topsrcdir,
+                                 git=buildconfig.subst['GIT'])
+        else:
+            raise MissingVCSInfo('unknown VCS_CHECKOUT_TYPE value: %s' % flavor)
+
+    except ImportError:
+        pass
+
     def ancestors(path):
         while path:
             yield path
             path, child = os.path.split(path)
             if child == '':
                 break
 
     for path in ancestors(os.getcwd()):
         try:
             return get_repository_object(path)
         except InvalidRepoPath:
             continue
 
-    raise Exception('Could not find Mercurial or Git checkout for %s' %
-                    os.getcwd())
+    raise MissingVCSInfo('Could not find Mercurial or Git checkout for %s' %
+                         os.getcwd())