--- a/build/ConfigStatus.py
+++ b/build/ConfigStatus.py
@@ -1,228 +1,32 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 # Combined with build/autoconf/config.status.m4, ConfigStatus is an almost
 # drop-in replacement for autoconf 2.13's config.status, with features
 # borrowed from autoconf > 2.5, and additional features.
 
-from __future__ import with_statement
+import os
+import sys
+
 from optparse import OptionParser
-import sys, re, os, posixpath, ntpath
-import errno
-from StringIO import StringIO
-from os.path import relpath
 
-# Standalone js doesn't have virtualenv.
-sys.path.append(os.path.join(os.path.dirname(__file__), '..', 'config'))
+from mozbuild.backend.configenvironment import ConfigEnvironment
+
 from Preprocessor import Preprocessor
 
 # Basic logging facility
 verbose = False
 def log(string):
     if verbose:
         print >>sys.stderr, string
 
 
-def ensureParentDir(file):
-    '''Ensures the directory parent to the given file exists'''
-    dir = os.path.dirname(file)
-    if dir and not os.path.exists(dir):
-        try:
-            os.makedirs(dir)
-        except OSError, error:
-            if error.errno != errno.EEXIST:
-                raise
-
-class FileAvoidWrite(StringIO):
-    '''file-like object that buffers its output and only writes it to disk
-    if the new contents are different from what the file may already contain.
-    '''
-    def __init__(self, filename):
-        self.filename = filename
-        StringIO.__init__(self)
-
-    def close(self):
-        buf = self.getvalue()
-        StringIO.close(self)
-        try:
-            file = open(self.filename, 'rU')
-        except IOError:
-            pass
-        else:
-            try:
-                 if file.read() == buf:
-                     log("%s is unchanged" % relpath(self.filename, os.curdir))
-                     return
-            except IOError:
-                pass
-            finally:
-                file.close()
-
-        log("creating %s" % relpath(self.filename, os.curdir))
-        ensureParentDir(self.filename)
-        with open(self.filename, 'w') as file:
-            file.write(buf)
-
-    def __enter__(self):
-        return self
-    def __exit__(self, type, value, traceback):
-        self.close()
-
-def shell_escape(s):
-    '''Escape some characters with a backslash, and double dollar signs.
-    '''
-    return re.sub('''([ \t`#$^&*(){}\\|;'"<>?\[\]])''', r'\\\1', str(s)).replace('$', '$$')
-
-class ConfigEnvironment(object):
-    '''A ConfigEnvironment is defined by a source directory and a build
-    directory. It preprocesses files from the source directory and stores
-    the result in the object directory.
-
-    There are two types of files: config files and config headers,
-    each treated through a different member function.
-
-    Creating a ConfigEnvironment requires a few arguments:
-      - topsrcdir and topobjdir are, respectively, the top source and
-        the top object directory.
-      - defines is a list of (name, value) tuples. In autoconf, these are
-        set with AC_DEFINE and AC_DEFINE_UNQUOTED
-      - non_global_defines are a list of names appearing in defines above
-        that are not meant to be exported in ACDEFINES and ALLDEFINES (see
-        below)
-      - substs is a list of (name, value) tuples. In autoconf, these are
-        set with AC_SUBST.
-
-    ConfigEnvironment automatically defines two additional substs variables
-    from all the defines not appearing in non_global_defines:
-      - ACDEFINES contains the defines in the form -DNAME=VALUE, for use on
-        preprocessor command lines. The order in which defines were given
-        when creating the ConfigEnvironment is preserved.
-      - ALLDEFINES contains the defines in the form #define NAME VALUE, in
-        sorted order, for use in config files, for an automatic listing of
-        defines.
-    and another additional subst variable from all the other substs:
-      - ALLSUBSTS contains the substs in the form NAME = VALUE, in sorted
-        order, for use in autoconf.mk. It includes ACDEFINES, but doesn't
-        include ALLDEFINES.
-
-    ConfigEnvironment expects a "top_srcdir" subst to be set with the top
-    source directory, in msys format on windows. It is used to derive a
-    "srcdir" subst when treating config files. It can either be an absolute
-    path or a path relative to the topobjdir.
-    '''
-
-    def __init__(self, topobjdir = '.', topsrcdir = '.',
-                 defines = [], non_global_defines = [], substs = []):
-        self.defines = dict(defines)
-        self.substs = dict(substs)
-        self.topsrcdir = topsrcdir
-        self.topobjdir = topobjdir
-        global_defines = [name for name, value in defines if not name in non_global_defines]
-        self.substs['ACDEFINES'] = ' '.join(["-D%s=%s" % (name, shell_escape(self.defines[name])) for name in global_defines])
-        self.substs['ALLSUBSTS'] = '\n'.join(sorted(["%s = %s" % (name, self.substs[name]) for name in self.substs]))
-        self.substs['ALLDEFINES'] = '\n'.join(sorted(["#define %s %s" % (name, self.defines[name]) for name in global_defines]))
-
-    def get_relative_srcdir(self, file):
-        '''Returns the relative source directory for the given file, always
-        using / as a path separator.
-        '''
-        assert(isinstance(file, basestring))
-        dir = posixpath.dirname(relpath(file, self.topobjdir).replace(os.sep, '/'))
-        if dir:
-            return dir
-        return '.'
-
-    def get_top_srcdir(self, file):
-        '''Returns a normalized top_srcdir for the given file: if
-        substs['top_srcdir'] is a relative path, it is relative to the
-        topobjdir. Adjust it to be relative to the file path.'''
-        top_srcdir = self.substs['top_srcdir']
-        if posixpath.isabs(top_srcdir) or ntpath.isabs(top_srcdir):
-            return top_srcdir
-        return posixpath.normpath(posixpath.join(self.get_depth(file), top_srcdir))
-
-    def get_file_srcdir(self, file):
-        '''Returns the srcdir for the given file, where srcdir is in msys
-        format on windows, thus derived from top_srcdir.
-        '''
-        dir = self.get_relative_srcdir(file)
-        top_srcdir = self.get_top_srcdir(file)
-        return posixpath.normpath(posixpath.join(top_srcdir, dir))
-
-    def get_depth(self, file):
-        '''Returns the DEPTH for the given file, that is, the path to the
-        object directory relative to the directory containing the given file.
-        Always uses / as a path separator.
-        '''
-        return relpath(self.topobjdir, os.path.dirname(file)).replace(os.sep, '/')
-
-    def get_input(self, file):
-        '''Returns the input file path in the source tree that can be used
-        to create the given config file or header.
-        '''
-        assert(isinstance(file, basestring))
-        return os.path.normpath(os.path.join(self.topsrcdir, "%s.in" % relpath(file, self.topobjdir)))
-
-    def create_config_file(self, path):
-        '''Creates the given config file. A config file is generated by
-        taking the corresponding source file and replacing occurences of
-        "@VAR@" by the value corresponding to "VAR" in the substs dict.
-
-        Additional substs are defined according to the file being treated:
-            "srcdir" for its the path to its source directory
-            "relativesrcdir" for its source directory relative to the top
-            "DEPTH" for the path to the top object directory
-        '''
-        input = self.get_input(path)
-        pp = Preprocessor()
-        pp.context.update(self.substs)
-        pp.context.update(top_srcdir = self.get_top_srcdir(path))
-        pp.context.update(srcdir = self.get_file_srcdir(path))
-        pp.context.update(relativesrcdir = self.get_relative_srcdir(path))
-        pp.context.update(DEPTH = self.get_depth(path))
-        pp.do_filter('attemptSubstitution')
-        pp.setMarker(None)
-        with FileAvoidWrite(path) as pp.out:
-            pp.do_include(input)
-
-    def create_config_header(self, path):
-        '''Creates the given config header. A config header is generated by
-        taking the corresponding source file and replacing some #define/#undef
-        occurences:
-            "#undef NAME" is turned into "#define NAME VALUE"
-            "#define NAME" is unchanged
-            "#define NAME ORIGINAL_VALUE" is turned into "#define NAME VALUE"
-            "#undef UNKNOWN_NAME" is turned into "/* #undef UNKNOWN_NAME */"
-            Whitespaces are preserved.
-        '''
-        with open(self.get_input(path), 'rU') as input:
-            ensureParentDir(path)
-            output = FileAvoidWrite(path)
-            r = re.compile('^\s*#\s*(?P<cmd>[a-z]+)(?:\s+(?P<name>\S+)(?:\s+(?P<value>\S+))?)?', re.U)
-            for l in input:
-                m = r.match(l)
-                if m:
-                    cmd = m.group('cmd')
-                    name = m.group('name')
-                    value = m.group('value')
-                    if name:
-                        if name in self.defines:
-                            if cmd == 'define' and value:
-                                l = l[:m.start('value')] + str(self.defines[name]) + l[m.end('value'):]
-                            elif cmd == 'undef':
-                                l = l[:m.start('cmd')] + 'define' + l[m.end('cmd'):m.end('name')] + ' ' + str(self.defines[name]) + l[m.end('name'):]
-                        elif cmd == 'undef':
-                           l = '/* ' + l[:m.end('name')] + ' */' + l[m.end('name'):]
-
-                output.write(l)
-            output.close()
-
 def config_status(topobjdir = '.', topsrcdir = '.',
                   defines = [], non_global_defines = [], substs = [],
                   files = [], headers = []):
     '''Main function, providing config.status functionality.
 
     Contrary to config.status, it doesn't use CONFIG_FILES or CONFIG_HEADERS
     variables, but like config.status from autoconf 2.6, single files may be
     generated with the --file and --header options. Several such options can
@@ -271,19 +75,18 @@ def config_status(topobjdir = '.', topsr
     parser.add_option('-n', dest='not_topobjdir', action='store_true',
                       help='do not consider current directory as top object directory')
     (options, args) = parser.parse_args()
 
     # Without -n, the current directory is meant to be the top object directory
     if not options.not_topobjdir:
         topobjdir = '.'
 
-    env = ConfigEnvironment(topobjdir = topobjdir, topsrcdir = topsrcdir,
-                            defines = defines, non_global_defines = non_global_defines,
-                            substs = substs)
+    env = ConfigEnvironment(topsrcdir, topobjdir, defines=defines,
+            non_global_defines=non_global_defines, substs=substs)
 
     if options.recheck:
         # Execute configure from the top object directory
         if not os.path.isabs(topsrcdir):
             topsrcdir = relpath(topsrcdir, topobjdir)
         os.chdir(topobjdir)
         os.execlp('sh', 'sh', '-c', ' '.join([os.path.join(topsrcdir, 'configure'), env.substs['ac_configure_args'], '--no-create', '--no-recursion']))
 

--- a/build/Makefile.in
+++ b/build/Makefile.in
@@ -98,17 +98,16 @@ endif
 # Put a useful .gdbinit in the bin directory, to be picked up automatically
 # by GDB when we debug executables there.
 # NOTE: Keep .gdbinit in the topsrcdir for people who run gdb from the topsrcdir.
 GDBINIT_FILES := $(topsrcdir)/.gdbinit
 GDBINIT_DEST = $(FINAL_TARGET)
 INSTALL_TARGETS += GDBINIT
 
 PYTHON_UNIT_TESTS := \
-  tests/unit-ConfigStatus.py \
   tests/test.py \
   $(NULL)
 
 include $(topsrcdir)/config/rules.mk
 
 # we install to _leaktest/
 TARGET_DEPTH = ..
 include $(srcdir)/automation-build.mk

--- a/js/src/build/ConfigStatus.py
+++ b/js/src/build/ConfigStatus.py
@@ -1,228 +1,32 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 # Combined with build/autoconf/config.status.m4, ConfigStatus is an almost
 # drop-in replacement for autoconf 2.13's config.status, with features
 # borrowed from autoconf > 2.5, and additional features.
 
-from __future__ import with_statement
+import os
+import sys
+
 from optparse import OptionParser
-import sys, re, os, posixpath, ntpath
-import errno
-from StringIO import StringIO
-from os.path import relpath
 
-# Standalone js doesn't have virtualenv.
-sys.path.append(os.path.join(os.path.dirname(__file__), '..', 'config'))
+from mozbuild.backend.configenvironment import ConfigEnvironment
+
 from Preprocessor import Preprocessor
 
 # Basic logging facility
 verbose = False
 def log(string):
     if verbose:
         print >>sys.stderr, string
 
 
-def ensureParentDir(file):
-    '''Ensures the directory parent to the given file exists'''
-    dir = os.path.dirname(file)
-    if dir and not os.path.exists(dir):
-        try:
-            os.makedirs(dir)
-        except OSError, error:
-            if error.errno != errno.EEXIST:
-                raise
-
-class FileAvoidWrite(StringIO):
-    '''file-like object that buffers its output and only writes it to disk
-    if the new contents are different from what the file may already contain.
-    '''
-    def __init__(self, filename):
-        self.filename = filename
-        StringIO.__init__(self)
-
-    def close(self):
-        buf = self.getvalue()
-        StringIO.close(self)
-        try:
-            file = open(self.filename, 'rU')
-        except IOError:
-            pass
-        else:
-            try:
-                 if file.read() == buf:
-                     log("%s is unchanged" % relpath(self.filename, os.curdir))
-                     return
-            except IOError:
-                pass
-            finally:
-                file.close()
-
-        log("creating %s" % relpath(self.filename, os.curdir))
-        ensureParentDir(self.filename)
-        with open(self.filename, 'w') as file:
-            file.write(buf)
-
-    def __enter__(self):
-        return self
-    def __exit__(self, type, value, traceback):
-        self.close()
-
-def shell_escape(s):
-    '''Escape some characters with a backslash, and double dollar signs.
-    '''
-    return re.sub('''([ \t`#$^&*(){}\\|;'"<>?\[\]])''', r'\\\1', str(s)).replace('$', '$$')
-
-class ConfigEnvironment(object):
-    '''A ConfigEnvironment is defined by a source directory and a build
-    directory. It preprocesses files from the source directory and stores
-    the result in the object directory.
-
-    There are two types of files: config files and config headers,
-    each treated through a different member function.
-
-    Creating a ConfigEnvironment requires a few arguments:
-      - topsrcdir and topobjdir are, respectively, the top source and
-        the top object directory.
-      - defines is a list of (name, value) tuples. In autoconf, these are
-        set with AC_DEFINE and AC_DEFINE_UNQUOTED
-      - non_global_defines are a list of names appearing in defines above
-        that are not meant to be exported in ACDEFINES and ALLDEFINES (see
-        below)
-      - substs is a list of (name, value) tuples. In autoconf, these are
-        set with AC_SUBST.
-
-    ConfigEnvironment automatically defines two additional substs variables
-    from all the defines not appearing in non_global_defines:
-      - ACDEFINES contains the defines in the form -DNAME=VALUE, for use on
-        preprocessor command lines. The order in which defines were given
-        when creating the ConfigEnvironment is preserved.
-      - ALLDEFINES contains the defines in the form #define NAME VALUE, in
-        sorted order, for use in config files, for an automatic listing of
-        defines.
-    and another additional subst variable from all the other substs:
-      - ALLSUBSTS contains the substs in the form NAME = VALUE, in sorted
-        order, for use in autoconf.mk. It includes ACDEFINES, but doesn't
-        include ALLDEFINES.
-
-    ConfigEnvironment expects a "top_srcdir" subst to be set with the top
-    source directory, in msys format on windows. It is used to derive a
-    "srcdir" subst when treating config files. It can either be an absolute
-    path or a path relative to the topobjdir.
-    '''
-
-    def __init__(self, topobjdir = '.', topsrcdir = '.',
-                 defines = [], non_global_defines = [], substs = []):
-        self.defines = dict(defines)
-        self.substs = dict(substs)
-        self.topsrcdir = topsrcdir
-        self.topobjdir = topobjdir
-        global_defines = [name for name, value in defines if not name in non_global_defines]
-        self.substs['ACDEFINES'] = ' '.join(["-D%s=%s" % (name, shell_escape(self.defines[name])) for name in global_defines])
-        self.substs['ALLSUBSTS'] = '\n'.join(sorted(["%s = %s" % (name, self.substs[name]) for name in self.substs]))
-        self.substs['ALLDEFINES'] = '\n'.join(sorted(["#define %s %s" % (name, self.defines[name]) for name in global_defines]))
-
-    def get_relative_srcdir(self, file):
-        '''Returns the relative source directory for the given file, always
-        using / as a path separator.
-        '''
-        assert(isinstance(file, basestring))
-        dir = posixpath.dirname(relpath(file, self.topobjdir).replace(os.sep, '/'))
-        if dir:
-            return dir
-        return '.'
-
-    def get_top_srcdir(self, file):
-        '''Returns a normalized top_srcdir for the given file: if
-        substs['top_srcdir'] is a relative path, it is relative to the
-        topobjdir. Adjust it to be relative to the file path.'''
-        top_srcdir = self.substs['top_srcdir']
-        if posixpath.isabs(top_srcdir) or ntpath.isabs(top_srcdir):
-            return top_srcdir
-        return posixpath.normpath(posixpath.join(self.get_depth(file), top_srcdir))
-
-    def get_file_srcdir(self, file):
-        '''Returns the srcdir for the given file, where srcdir is in msys
-        format on windows, thus derived from top_srcdir.
-        '''
-        dir = self.get_relative_srcdir(file)
-        top_srcdir = self.get_top_srcdir(file)
-        return posixpath.normpath(posixpath.join(top_srcdir, dir))
-
-    def get_depth(self, file):
-        '''Returns the DEPTH for the given file, that is, the path to the
-        object directory relative to the directory containing the given file.
-        Always uses / as a path separator.
-        '''
-        return relpath(self.topobjdir, os.path.dirname(file)).replace(os.sep, '/')
-
-    def get_input(self, file):
-        '''Returns the input file path in the source tree that can be used
-        to create the given config file or header.
-        '''
-        assert(isinstance(file, basestring))
-        return os.path.normpath(os.path.join(self.topsrcdir, "%s.in" % relpath(file, self.topobjdir)))
-
-    def create_config_file(self, path):
-        '''Creates the given config file. A config file is generated by
-        taking the corresponding source file and replacing occurences of
-        "@VAR@" by the value corresponding to "VAR" in the substs dict.
-
-        Additional substs are defined according to the file being treated:
-            "srcdir" for its the path to its source directory
-            "relativesrcdir" for its source directory relative to the top
-            "DEPTH" for the path to the top object directory
-        '''
-        input = self.get_input(path)
-        pp = Preprocessor()
-        pp.context.update(self.substs)
-        pp.context.update(top_srcdir = self.get_top_srcdir(path))
-        pp.context.update(srcdir = self.get_file_srcdir(path))
-        pp.context.update(relativesrcdir = self.get_relative_srcdir(path))
-        pp.context.update(DEPTH = self.get_depth(path))
-        pp.do_filter('attemptSubstitution')
-        pp.setMarker(None)
-        with FileAvoidWrite(path) as pp.out:
-            pp.do_include(input)
-
-    def create_config_header(self, path):
-        '''Creates the given config header. A config header is generated by
-        taking the corresponding source file and replacing some #define/#undef
-        occurences:
-            "#undef NAME" is turned into "#define NAME VALUE"
-            "#define NAME" is unchanged
-            "#define NAME ORIGINAL_VALUE" is turned into "#define NAME VALUE"
-            "#undef UNKNOWN_NAME" is turned into "/* #undef UNKNOWN_NAME */"
-            Whitespaces are preserved.
-        '''
-        with open(self.get_input(path), 'rU') as input:
-            ensureParentDir(path)
-            output = FileAvoidWrite(path)
-            r = re.compile('^\s*#\s*(?P<cmd>[a-z]+)(?:\s+(?P<name>\S+)(?:\s+(?P<value>\S+))?)?', re.U)
-            for l in input:
-                m = r.match(l)
-                if m:
-                    cmd = m.group('cmd')
-                    name = m.group('name')
-                    value = m.group('value')
-                    if name:
-                        if name in self.defines:
-                            if cmd == 'define' and value:
-                                l = l[:m.start('value')] + str(self.defines[name]) + l[m.end('value'):]
-                            elif cmd == 'undef':
-                                l = l[:m.start('cmd')] + 'define' + l[m.end('cmd'):m.end('name')] + ' ' + str(self.defines[name]) + l[m.end('name'):]
-                        elif cmd == 'undef':
-                           l = '/* ' + l[:m.end('name')] + ' */' + l[m.end('name'):]
-
-                output.write(l)
-            output.close()
-
 def config_status(topobjdir = '.', topsrcdir = '.',
                   defines = [], non_global_defines = [], substs = [],
                   files = [], headers = []):
     '''Main function, providing config.status functionality.
 
     Contrary to config.status, it doesn't use CONFIG_FILES or CONFIG_HEADERS
     variables, but like config.status from autoconf 2.6, single files may be
     generated with the --file and --header options. Several such options can
@@ -271,19 +75,18 @@ def config_status(topobjdir = '.', topsr
     parser.add_option('-n', dest='not_topobjdir', action='store_true',
                       help='do not consider current directory as top object directory')
     (options, args) = parser.parse_args()
 
     # Without -n, the current directory is meant to be the top object directory
     if not options.not_topobjdir:
         topobjdir = '.'
 
-    env = ConfigEnvironment(topobjdir = topobjdir, topsrcdir = topsrcdir,
-                            defines = defines, non_global_defines = non_global_defines,
-                            substs = substs)
+    env = ConfigEnvironment(topsrcdir, topobjdir, defines=defines,
+            non_global_defines=non_global_defines, substs=substs)
 
     if options.recheck:
         # Execute configure from the top object directory
         if not os.path.isabs(topsrcdir):
             topsrcdir = relpath(topsrcdir, topobjdir)
         os.chdir(topobjdir)
         os.execlp('sh', 'sh', '-c', ' '.join([os.path.join(topsrcdir, 'configure'), env.substs['ac_configure_args'], '--no-create', '--no-recursion']))
 

--- a/python/Makefile.in
+++ b/python/Makefile.in
@@ -6,16 +6,17 @@ DEPTH := @DEPTH@
 topsrcdir := @top_srcdir@
 srcdir := @srcdir@
 VPATH = @srcdir@
 
 include $(DEPTH)/config/autoconf.mk
 
 test_dirs := \
   mozbuild/mozbuild/test \
+  mozbuild/mozbuild/test/backend \
   mozbuild/mozbuild/test/compilation \
   mozbuild/mozbuild/test/frontend \
   mozbuild/mozpack/test \
   $(NULL)
 
 PYTHON_UNIT_TESTS := $(foreach dir,$(test_dirs),$(wildcard $(srcdir)/$(dir)/*.py))
 
 include $(topsrcdir)/config/rules.mk

new file mode 100644

new file mode 100644
--- /dev/null
+++ b/python/mozbuild/mozbuild/backend/configenvironment.py
@@ -0,0 +1,179 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+from __future__ import unicode_literals
+
+import ntpath
+import os
+import posixpath
+import re
+
+from os.path import relpath
+
+from Preprocessor import Preprocessor
+
+from ..util import (
+    ensureParentDir,
+    FileAvoidWrite,
+)
+
+
+RE_SHELL_ESCAPE = re.compile('''([ \t`#$^&*(){}\\|;'"<>?\[\]])''')
+
+
+def shell_escape(s):
+    """Escape some characters with a backslash, and double dollar signs."""
+    return RE_SHELL_ESCAPE.sub(r'\\\1', str(s)).replace('$', '$$')
+
+
+class ConfigEnvironment(object):
+    """Perform actions associated with a configured but bare objdir.
+
+    The purpose of this class is to preprocess files from the source directory
+    and output results in the object directory.
+
+    There are two types of files: config files and config headers,
+    each treated through a different member function.
+
+    Creating a ConfigEnvironment requires a few arguments:
+      - topsrcdir and topobjdir are, respectively, the top source and
+        the top object directory.
+      - defines is a list of (name, value) tuples. In autoconf, these are
+        set with AC_DEFINE and AC_DEFINE_UNQUOTED
+      - non_global_defines are a list of names appearing in defines above
+        that are not meant to be exported in ACDEFINES and ALLDEFINES (see
+        below)
+      - substs is a list of (name, value) tuples. In autoconf, these are
+        set with AC_SUBST.
+
+    ConfigEnvironment automatically defines two additional substs variables
+    from all the defines not appearing in non_global_defines:
+      - ACDEFINES contains the defines in the form -DNAME=VALUE, for use on
+        preprocessor command lines. The order in which defines were given
+        when creating the ConfigEnvironment is preserved.
+      - ALLDEFINES contains the defines in the form #define NAME VALUE, in
+        sorted order, for use in config files, for an automatic listing of
+        defines.
+    and another additional subst variable from all the other substs:
+      - ALLSUBSTS contains the substs in the form NAME = VALUE, in sorted
+        order, for use in autoconf.mk. It includes ACDEFINES, but doesn't
+        include ALLDEFINES.
+
+    ConfigEnvironment expects a "top_srcdir" subst to be set with the top
+    source directory, in msys format on windows. It is used to derive a
+    "srcdir" subst when treating config files. It can either be an absolute
+    path or a path relative to the topobjdir.
+    """
+
+    def __init__(self, topsrcdir, topobjdir, defines=[], non_global_defines=[],
+            substs=[]):
+
+        self.defines = dict(defines)
+        self.substs = dict(substs)
+        self.topsrcdir = topsrcdir
+        self.topobjdir = topobjdir
+        global_defines = [name for name, value in defines
+            if not name in non_global_defines]
+        self.substs['ACDEFINES'] = ' '.join(['-D%s=%s' % (name,
+            shell_escape(self.defines[name])) for name in global_defines])
+        self.substs['ALLSUBSTS'] = '\n'.join(sorted(['%s = %s' % (name,
+            self.substs[name]) for name in self.substs]))
+        self.substs['ALLDEFINES'] = '\n'.join(sorted(['#define %s %s' % (name,
+            self.defines[name]) for name in global_defines]))
+
+    def get_relative_srcdir(self, file):
+        '''Returns the relative source directory for the given file, always
+        using / as a path separator.
+        '''
+        assert(isinstance(file, basestring))
+        dir = posixpath.dirname(relpath(file, self.topobjdir).replace(os.sep, '/'))
+        if dir:
+            return dir
+        return '.'
+
+    def get_top_srcdir(self, file):
+        '''Returns a normalized top_srcdir for the given file: if
+        substs['top_srcdir'] is a relative path, it is relative to the
+        topobjdir. Adjust it to be relative to the file path.'''
+        top_srcdir = self.substs['top_srcdir']
+        if posixpath.isabs(top_srcdir) or ntpath.isabs(top_srcdir):
+            return top_srcdir
+        return posixpath.normpath(posixpath.join(self.get_depth(file), top_srcdir))
+
+    def get_file_srcdir(self, file):
+        '''Returns the srcdir for the given file, where srcdir is in msys
+        format on windows, thus derived from top_srcdir.
+        '''
+        dir = self.get_relative_srcdir(file)
+        top_srcdir = self.get_top_srcdir(file)
+        return posixpath.normpath(posixpath.join(top_srcdir, dir))
+
+    def get_depth(self, file):
+        '''Returns the DEPTH for the given file, that is, the path to the
+        object directory relative to the directory containing the given file.
+        Always uses / as a path separator.
+        '''
+        return relpath(self.topobjdir, os.path.dirname(file)).replace(os.sep, '/')
+
+    def get_input(self, file):
+        '''Returns the input file path in the source tree that can be used
+        to create the given config file or header.
+        '''
+        assert(isinstance(file, basestring))
+        return os.path.normpath(os.path.join(self.topsrcdir, "%s.in" % relpath(file, self.topobjdir)))
+
+    def create_config_file(self, path):
+        '''Creates the given config file. A config file is generated by
+        taking the corresponding source file and replacing occurences of
+        "@VAR@" by the value corresponding to "VAR" in the substs dict.
+
+        Additional substs are defined according to the file being treated:
+            "srcdir" for its the path to its source directory
+            "relativesrcdir" for its source directory relative to the top
+            "DEPTH" for the path to the top object directory
+        '''
+        input = self.get_input(path)
+        pp = Preprocessor()
+        pp.context.update(self.substs)
+        pp.context.update(top_srcdir = self.get_top_srcdir(path))
+        pp.context.update(srcdir = self.get_file_srcdir(path))
+        pp.context.update(relativesrcdir = self.get_relative_srcdir(path))
+        pp.context.update(DEPTH = self.get_depth(path))
+        pp.do_filter('attemptSubstitution')
+        pp.setMarker(None)
+        with FileAvoidWrite(path) as pp.out:
+            pp.do_include(input)
+
+    def create_config_header(self, path):
+        '''Creates the given config header. A config header is generated by
+        taking the corresponding source file and replacing some #define/#undef
+        occurences:
+            "#undef NAME" is turned into "#define NAME VALUE"
+            "#define NAME" is unchanged
+            "#define NAME ORIGINAL_VALUE" is turned into "#define NAME VALUE"
+            "#undef UNKNOWN_NAME" is turned into "/* #undef UNKNOWN_NAME */"
+            Whitespaces are preserved.
+        '''
+        with open(self.get_input(path), 'rU') as input:
+            ensureParentDir(path)
+            output = FileAvoidWrite(path)
+            r = re.compile('^\s*#\s*(?P<cmd>[a-z]+)(?:\s+(?P<name>\S+)(?:\s+(?P<value>\S+))?)?', re.U)
+            for l in input:
+                m = r.match(l)
+                if m:
+                    cmd = m.group('cmd')
+                    name = m.group('name')
+                    value = m.group('value')
+                    if name:
+                        if name in self.defines:
+                            if cmd == 'define' and value:
+                                l = l[:m.start('value')] + str(self.defines[name]) + l[m.end('value'):]
+                            elif cmd == 'undef':
+                                l = l[:m.start('cmd')] + 'define' + l[m.end('cmd'):m.end('name')] + ' ' + str(self.defines[name]) + l[m.end('name'):]
+                        elif cmd == 'undef':
+                           l = '/* ' + l[:m.end('name')] + ' */' + l[m.end('name'):]
+
+                output.write(l)
+            output.close()
+

new file mode 100644

rename from build/tests/unit-ConfigStatus.py
rename to python/mozbuild/mozbuild/test/backend/test_configenvironment.py
--- a/build/tests/unit-ConfigStatus.py
+++ b/python/mozbuild/mozbuild/test/backend/test_configenvironment.py
@@ -1,68 +1,36 @@
-from __future__ import with_statement
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
 import os, posixpath
 from StringIO import StringIO
 import unittest
 from mozunit import main, MockedOpen
-import ConfigStatus
-from ConfigStatus import FileAvoidWrite
+
+import mozbuild.backend.configenvironment as ConfigStatus
 
 class ConfigEnvironment(ConfigStatus.ConfigEnvironment):
-    def __init__(self, **args):
-        ConfigStatus.ConfigEnvironment.__init__(self, **args)
+    def __init__(self, *args, **kwargs):
+        ConfigStatus.ConfigEnvironment.__init__(self, *args, **kwargs)
         # Be helpful to unit tests
         if not 'top_srcdir' in self.substs:
             if os.path.isabs(self.topsrcdir):
                 self.substs['top_srcdir'] = self.topsrcdir.replace(os.sep, '/')
             else:
-                self.substs['top_srcdir'] = ConfigStatus.relpath(self.topsrcdir, self.topobjdir).replace(os.sep, '/')
-
-class TestFileAvoidWrite(unittest.TestCase):
-    def test_file_avoid_write(self):
-        '''Test the FileAvoidWrite class
-        '''
-        with MockedOpen({'file': 'content'}):
-            # Overwriting an existing file replaces its content
-            with FileAvoidWrite('file') as file:
-                file.write('bazqux')
-            self.assertEqual(open('file', 'r').read(), 'bazqux')
-
-            # Creating a new file (obviously) stores its content
-            with FileAvoidWrite('file2') as file:
-                file.write('content')
-            self.assertEqual(open('file2').read(), 'content')
-
-        class MyMockedOpen(MockedOpen):
-            '''MockedOpen extension to raise an exception if something
-            attempts to write in an opened file.
-            '''
-            def __call__(self, name, mode):
-                if 'w' in mode:
-                    raise Exception, 'Unexpected open with write mode'
-                return MockedOpen.__call__(self, name, mode)
-
-        with MyMockedOpen({'file': 'content'}):
-            # Validate that MyMockedOpen works as intended
-            file = FileAvoidWrite('file')
-            file.write('foobar')
-            self.assertRaises(Exception, file.close)
-
-            # Check that no write actually happens when writing the
-            # same content as what already is in the file
-            with FileAvoidWrite('file') as file:
-                file.write('content')
+                self.substs['top_srcdir'] = os.path.relpath(self.topsrcdir, self.topobjdir).replace(os.sep, '/')
 
 
 class TestEnvironment(unittest.TestCase):
     def test_auto_substs(self):
         '''Test the automatically set values of ACDEFINES, ALLDEFINES
         and ALLSUBSTS.
         '''
-        env = ConfigEnvironment(
+        env = ConfigEnvironment('.', '.',
                   defines = [ ('foo', 'bar'), ('baz', 'qux 42'),
                               ('abc', 'def'), ('extra', 'foobar') ],
                   non_global_defines = ['extra', 'ignore'],
                   substs = [ ('FOO', 'bar'), ('ABC', 'def'),
                              ('bar', 'baz qux'), ('zzz', '"abc def"') ])
         # non_global_defines should be filtered out in ACDEFINES and
         # ALLDEFINES.
         # Original order of the defines need to be respected in ACDEFINES
@@ -81,17 +49,17 @@ zzz = "abc def"''')
 
     def test_config_file(self):
         '''Test the creation of config files.
         '''
         with MockedOpen({'file.in': '''#ifdef foo
 @foo@
 @bar@
 '''}):
-            env = ConfigEnvironment(substs = [ ('foo', 'bar baz') ])
+            env = ConfigEnvironment('.', '.', substs = [ ('foo', 'bar baz') ])
             env.create_config_file('file')
             self.assertEqual(open('file', 'r').read(), '''#ifdef foo
 bar baz
 @bar@
 ''')
 
     def test_config_header(self):
         '''Test the creation of config headers.
@@ -108,17 +76,17 @@ bar baz
 # undef baz
 
 #ifdef foo
 #   undef   foo
 #  define foo    42
   #     define   foo   42   
 #endif
 '''}):
-            env = ConfigEnvironment(defines = [ ('foo', 'baz qux'), ('baz', 1) ])
+            env = ConfigEnvironment('.', '.', defines = [ ('foo', 'baz qux'), ('baz', 1) ])
             env.create_config_header('file')
             self.assertEqual(open('file','r').read(), '''
 /* Comment */
 #define foo
 #define foo baz qux
 #define foo baz qux
 #define bar
 #define bar 42

--- a/python/mozbuild/mozbuild/test/test_util.py
+++ b/python/mozbuild/mozbuild/test/test_util.py
@@ -1,21 +1,27 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
-# License, v. 2.0. If a copy of the MPL was not distributed with this file,
-# You can obtain one at http://mozilla.org/MPL/2.0/.
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 from __future__ import unicode_literals
 
 import hashlib
 import unittest
 
 from mozfile.mozfile import NamedTemporaryFile
-from mozunit import main
+from mozunit import (
+    main,
+    MockedOpen,
+)
 
-from mozbuild.util import hash_file
+from mozbuild.util import (
+    FileAvoidWrite,
+    hash_file,
+)
 
 
 class TestHashing(unittest.TestCase):
     def test_hash_file_known_hash(self):
         """Ensure a known hash value is recreated."""
         data = b'The quick brown fox jumps over the lazy cog'
         expected = 'de9f2c7fd25e1b3afad3e85a0bd17d9b100db4b3'
 
@@ -39,10 +45,44 @@ class TestHashing(unittest.TestCase):
         temp.write(data)
         temp.flush()
 
         actual = hash_file(temp.name)
 
         self.assertEqual(actual, expected)
 
 
+class TestFileAvoidWrite(unittest.TestCase):
+    def test_file_avoid_write(self):
+        with MockedOpen({'file': 'content'}):
+            # Overwriting an existing file replaces its content
+            with FileAvoidWrite('file') as file:
+                file.write('bazqux')
+            self.assertEqual(open('file', 'r').read(), 'bazqux')
+
+            # Creating a new file (obviously) stores its content
+            with FileAvoidWrite('file2') as file:
+                file.write('content')
+            self.assertEqual(open('file2').read(), 'content')
+
+        class MyMockedOpen(MockedOpen):
+            '''MockedOpen extension to raise an exception if something
+            attempts to write in an opened file.
+            '''
+            def __call__(self, name, mode):
+                if 'w' in mode:
+                    raise Exception, 'Unexpected open with write mode'
+                return MockedOpen.__call__(self, name, mode)
+
+        with MyMockedOpen({'file': 'content'}):
+            # Validate that MyMockedOpen works as intended
+            file = FileAvoidWrite('file')
+            file.write('foobar')
+            self.assertRaises(Exception, file.close)
+
+            # Check that no write actually happens when writing the
+            # same content as what already is in the file
+            with FileAvoidWrite('file') as file:
+                file.write('content')
+
+
 if __name__ == '__main__':
     main()

--- a/python/mozbuild/mozbuild/util.py
+++ b/python/mozbuild/mozbuild/util.py
@@ -3,17 +3,21 @@
 # You can obtain one at http://mozilla.org/MPL/2.0/.
 
 # This file contains miscellaneous utility functions that don't belong anywhere
 # in particular.
 
 from __future__ import unicode_literals
 
 import copy
+import errno
 import hashlib
+import os
+
+from StringIO import StringIO
 
 
 def hash_file(path):
     """Hashes a file specified by the path given and returns the hex digest."""
 
     # If the hashing function changes, this may invalidate lots of cached data.
     # Don't change it lightly.
     h = hashlib.sha1()
@@ -79,8 +83,57 @@ class DefaultOnReadDict(dict):
         return dict.__getitem__(self, k)
 
 
 class ReadOnlyDefaultDict(DefaultOnReadDict, ReadOnlyDict):
     """A read-only dictionary that supports default values on retrieval."""
     def __init__(self, d, defaults=None, global_default=undefined):
         DefaultOnReadDict.__init__(self, d, defaults, global_default)
 
+
+def ensureParentDir(path):
+    """Ensures the directory parent to the given file exists."""
+    d = os.path.dirname(path)
+    if d and not os.path.exists(path):
+        try:
+            os.makedirs(d)
+        except OSError, error:
+            if error.errno != errno.EEXIST:
+                raise
+
+
+class FileAvoidWrite(StringIO):
+    """File-like object that buffers output and only writes if content changed.
+
+    We create an instance from an existing filename. New content is written to
+    it. When we close the file object, if the content in the in-memory buffer
+    differs from what is on disk, then we write out the new content. Otherwise,
+    the original file is untouched.
+    """
+    def __init__(self, filename):
+        StringIO.__init__(self)
+        self.filename = filename
+
+    def close(self):
+        buf = self.getvalue()
+        StringIO.close(self)
+        try:
+            existing = open(self.filename, 'rU')
+        except IOError:
+            pass
+        else:
+            try:
+                if existing.read() == buf:
+                    return
+            except IOError:
+                pass
+            finally:
+                existing.close()
+
+        ensureParentDir(self.filename)
+        with open(self.filename, 'w') as file:
+            file.write(buf)
+
+    def __enter__(self):
+        return self
+    def __exit__(self, type, value, traceback):
+        self.close()
+