new file mode 100644
--- /dev/null
+++ b/testing/mozharness/external_tools/purgelong.py
@@ -0,0 +1,117 @@
+'''
+Mercurial extension to enable purging long filenames on Windows
+
+It's possible to have filenaems that exceed the MAX_PATH limit (256 characters)
+on Windows, rendering them un-purgeable with the regular python API calls.
+
+To work around this limitation, we can use the DeleteFileW API
+(https://msdn.microsoft.com/en-us/library/windows/desktop/aa363915%28v=vs.85%29.aspx)
+and prefix the filename with \\?\.
+
+This extension needs to monkeypatch other modules in order to function. It
+attempts to be very conservative, and only applies the patches for the
+duration of the purge() command. The original functions are restored after
+the purge() command exits.
+'''
+from contextlib import contextmanager
+from functools import partial
+import os
+import errno
+
+import mercurial.extensions
+import mercurial.util
+
+if os.name == 'nt':
+    import ctypes
+
+    # Get a reference to the DeleteFileW function
+    # DeleteFileW accepts filenames encoded as a null terminated sequence of
+    # wide chars (UTF-16). Python's ctypes.c_wchar_p correctly encodes unicode
+    # strings to null terminated UTF-16 strings.
+    # However, we receive (byte) strings from mercurial. When these are passed
+    # to DeleteFileW via the c_wchar_p type, they are implicitly decoded via
+    # the 'mbcs' encoding on windows.
+    kernel32 = ctypes.windll.kernel32
+    DeleteFile = kernel32.DeleteFileW
+    DeleteFile.argtypes = [ctypes.c_wchar_p]
+    DeleteFile.restype = ctypes.c_bool
+
+    def unlink_long(fn):
+        normalized_path = '\\\\?\\' + os.path.normpath(fn)
+        if not DeleteFile(normalized_path):
+            raise OSError(errno.EPERM, "couldn't remove long path", fn)
+
+# Not needed on other platforms, but is handy for testing
+else:
+    def unlink_long(fn):
+        os.unlink(fn)
+
+
+def unlink_wrapper(unlink_orig, fn, ui):
+    '''Calls the original unlink function, and if that fails, calls
+    unlink_long'''
+    try:
+        ui.debug('calling unlink_orig %s\n' % fn)
+        return unlink_orig(fn)
+    except WindowsError, e:
+        # windows error 3 corresponds to ERROR_PATH_NOT_FOUND
+        # only handle this case; re-raise the exception for other kinds of
+        # failures
+        if e.winerror != 3:
+            raise
+        ui.debug('caught WindowsError ERROR_PATH_NOT_FOUND; '
+                 'calling unlink_long %s\n' % fn)
+        return unlink_long(fn)
+
+
+@contextmanager
+def wrap_unlink(ui):
+    '''Context manager that patches the required functions that are used by
+    the purge extension to remove files. When exiting the context manager
+    the original functions are restored.'''
+    version = mercurial.util.version()
+    if version >= '3.2':
+        # hg 3.2 and higher use util.unlink for purging
+        purgemod = mercurial.extensions.find('purge')
+        to_wrap = [(purgemod.util, 'unlink')]
+    else:
+        # hg 3.1 and earlier use os.remove directly
+        to_wrap = [(os, 'remove')]
+
+    # pass along the ui object to the unlink_wrapper so we can get logging out
+    # of it
+    wrapped = partial(unlink_wrapper, ui=ui)
+
+    # Wrap the original function(s) with our unlink_wrapper
+    originals = {}
+    for mod, func in to_wrap:
+        ui.debug('wrapping %s %s\n' % (mod, func))
+        originals[mod, func] = mercurial.extensions.wrapfunction(
+            mod, func, wrapped)
+
+    try:
+        yield
+    finally:
+        # Restore the originals
+        for mod, func in to_wrap:
+            ui.debug('restoring %s %s\n' % (mod, func))
+            setattr(mod, func, originals[mod, func])
+
+
+def purge_wrapper(orig, ui, *args, **kwargs):
+    '''Runs the original purge() command inside the wrap_unlink() context
+    manager.'''
+    with wrap_unlink(ui):
+        return orig(ui, *args, **kwargs)
+
+
+def extsetup(ui):
+    try:
+        purgemod = mercurial.extensions.find('purge')
+    except KeyError:
+        ui.warn('purge extension not found; '
+                'not enabling long filename support\n')
+        return
+
+    ui.note('enabling long filename support for purge\n')
+    mercurial.extensions.wrapcommand(purgemod.cmdtable, 'purge', purge_wrapper)