diff --git a/.flake8 b/.flake8
index d35dd58c119e..bdbdc1f67e2b 100644
--- a/.flake8
+++ b/.flake8
@@ -19,7 +19,7 @@ exclude =
     netwerk/protocol/http/make_incoming_tables.py,
     python/devtools/migrate-l10n/migrate/main.py,
     python/l10n/fluent_migrations,
-    python/mozbuild/dumbmake,
+    python/mozbuild/mozbuild/fork_interpose.py,
     security/manager/ssl/tests/unit,
     servo/components/style,
     testing/condprofile/condprof/android.py,
diff --git a/python/mozbuild/mozbuild/fork_interpose.py b/python/mozbuild/mozbuild/fork_interpose.py
new file mode 100644
index 000000000000..567c70f2f0d4
--- /dev/null
+++ b/python/mozbuild/mozbuild/fork_interpose.py
@@ -0,0 +1,42 @@
+# 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/.
+
+# This source code is indirectly used in python/mozbuild/mozbuild/util.py and
+# is not meant to be imported. This file can be entirely deleted when the
+# transition to Python 3 is complete.
+
+import imp
+import os
+import sys
+
+orig_find_module = imp.find_module
+
+def my_find_module(name, dirs):
+    if name == main_module_name:
+        path = os.path.join(dirs[0], main_file_name)
+        f = open(path)
+        return (f, path, ('', 'r', imp.PY_SOURCE))
+    return orig_find_module(name, dirs)
+
+# Don't allow writing bytecode file for the main module.
+orig_load_module = imp.load_module
+
+def my_load_module(name, file, path, description):
+    # multiprocess.forking invokes imp.load_module manually and
+    # hard-codes the name __parents_main__ as the module name.
+    if name == '__parents_main__':
+        old_bytecode = sys.dont_write_bytecode
+        sys.dont_write_bytecode = True
+        try:
+            return orig_load_module(name, file, path, description)
+        finally:
+            sys.dont_write_bytecode = old_bytecode
+
+    return orig_load_module(name, file, path, description)
+
+imp.find_module = my_find_module
+imp.load_module = my_load_module
+from multiprocessing.forking import main
+
+main()
diff --git a/python/mozbuild/mozbuild/util.py b/python/mozbuild/mozbuild/util.py
index c41ee2998c31..09c597075691 100644
--- a/python/mozbuild/mozbuild/util.py
+++ b/python/mozbuild/mozbuild/util.py
@@ -1426,7 +1426,6 @@ def patch_main():
     # not, but at least some basic mach commands appear to work without it. So
     # skip it in 3.4+ until we determine it's still needed.
     if sys.platform == "win32" and sys.version_info < (3, 4):
-        import inspect
         import os
         from multiprocessing import forking
 
@@ -1445,50 +1444,16 @@ def patch_main():
             # If main is a .py file, everything ought to work as expected.
             return
 
-        def fork_interpose():
-            import imp
-            import os
-            import sys
-
-            orig_find_module = imp.find_module
-
-            def my_find_module(name, dirs):
-                if name == main_module_name:
-                    path = os.path.join(dirs[0], main_file_name)
-                    f = open(path)
-                    return (f, path, ("", "r", imp.PY_SOURCE))
-                return orig_find_module(name, dirs)
-
-            # Don't allow writing bytecode file for the main module.
-            orig_load_module = imp.load_module
-
-            def my_load_module(name, file, path, description):
-                # multiprocess.forking invokes imp.load_module manually and
-                # hard-codes the name __parents_main__ as the module name.
-                if name == "__parents_main__":
-                    old_bytecode = sys.dont_write_bytecode
-                    sys.dont_write_bytecode = True
-                    try:
-                        return orig_load_module(name, file, path, description)
-                    finally:
-                        sys.dont_write_bytecode = old_bytecode
-
-                return orig_load_module(name, file, path, description)
-
-            imp.find_module = my_find_module
-            imp.load_module = my_load_module
-            from multiprocessing.forking import main
-
-            main()
-
         def my_get_command_line():
-            fork_code, lineno = inspect.getsourcelines(fork_interpose)
-            # Remove the first line (for 'def fork_interpose():') and the three
-            # levels of indentation (12 spaces), add our relevant globals.
+            with open(
+                os.path.join(os.path.dirname(__file__), "fork_interpose.py"), "rU"
+            ) as fork_file:
+                fork_code = fork_file.read()
+            # Add our relevant globals.
             fork_string = (
                 "main_file_name = '%s'\n" % main_file_name
                 + "main_module_name = '%s'\n" % main_module_name
-                + "".join(x[12:] for x in fork_code[1:])
+                + fork_code
             )
             cmdline = orig_command_line()
             # We don't catch errors if "-c" is not found because it's not clear
diff --git a/tools/lint/black.yml b/tools/lint/black.yml
index d4f05925bc00..79fdee13427b 100644
--- a/tools/lint/black.yml
+++ b/tools/lint/black.yml
@@ -4,6 +4,7 @@ black:
     exclude:
         - gfx/harfbuzz/src/meson.build
         - layout/style/ServoCSSPropList.mako.py
+        - python/mozbuild/mozbuild/fork_interpose.py
         - python/mozbuild/mozbuild/test/frontend/data/reader-error-syntax/moz.build
         - testing/mozharness/configs/test/test_malformed.py
         - testing/web-platform/tests
diff --git a/tools/lint/py2.yml b/tools/lint/py2.yml
index 0c576a088392..9e80ac81950c 100644
--- a/tools/lint/py2.yml
+++ b/tools/lint/py2.yml
@@ -15,6 +15,7 @@ py2:
         - netwerk
         - nsprpub
         - other-licenses
+        - python/mozbuild/mozbuild/fork_interpose.py
         - security
         - servo
         - taskcluster/docker/funsize-update-generator