setup: create a module for the modulepolicy

Instead of rewriting __init__ to define the modulepolicy,
write out a __modulepolicy__.py file like __version__.py

This should work for both system-wide installation and in-place build. Therefore
we can avoid relying on two separate modulepolicy rules, '@MODULELOADPOLICY@'
and 'mercurial/modulepolicy'.

.hgignore

@@ -40,6 +40,7 b' doc/*.[0-9].{x,ht}ml'
 MANIFEST
 MANIFEST.in
 patches
+mercurial/__modulepolicy__.py
 mercurial/__version__.py
 mercurial/hgpythonlib.h
 mercurial.egg-info

Makefile

@@ -63,6 +63,7 b' clean:'
 		\( -name '*.py[cdo]' -o -name '*.so' \) -exec rm -f '{}' ';'
 	rm -f $(addprefix mercurial/,$(notdir $(wildcard mercurial/pure/[a-z]*.py)))
 	rm -f MANIFEST MANIFEST.in hgext/__index__.py tests/*.err
+	rm -f mercurial/__modulepolicy__.py
 	if test -d .hg; then rm -f mercurial/__version__.py; fi
 	rm -rf build mercurial/locale
 	$(MAKE) -C doc clean

mercurial/__init__.py

@@ -19,11 +19,14 b' import zipimport'
 #    c - require C extensions
 #    allow - allow pure Python implementation when C loading fails
 #    py - only load pure Python modules
-modulepolicy = '@MODULELOADPOLICY@'
-
+#
 # By default, require the C extensions for performance reasons.
-if modulepolicy == '@' 'MODULELOADPOLICY' '@':
-    modulepolicy = 'c'
+modulepolicy = 'c'
+try:
+    from . import __modulepolicy__
+    modulepolicy = __modulepolicy__.modulepolicy
+except ImportError:
+    pass
 
 # PyPy doesn't load C extensions.
 #

setup.py

@@ -314,21 +314,16 b' class hgbuildpy(build_py):'
                 raise SystemExit('Python headers are required to build '
                                  'Mercurial but weren\'t found in %s' % h)
 
-    def copy_file(self, *args, **kwargs):
-        dst, copied = build_py.copy_file(self, *args, **kwargs)
+    def run(self):
+        if self.distribution.pure:
+            modulepolicy = 'py'
+        else:
+            modulepolicy = 'c'
+        with open("mercurial/__modulepolicy__.py", "w") as f:
+            f.write('# this file is autogenerated by setup.py\n')
+            f.write('modulepolicy = "%s"\n' % modulepolicy)
 
-        if copied and dst.endswith('__init__.py'):
-            if self.distribution.pure:
-                modulepolicy = 'py'
-            else:
-                modulepolicy = 'c'
-            content = open(dst, 'rb').read()
-            content = content.replace(b'@MODULELOADPOLICY@',
-                                      modulepolicy.encode(libdir_escape))
-            with open(dst, 'wb') as fh:
-                fh.write(content)
-
-        return dst, copied
+        build_py.run(self)
 
 class buildhgextindex(Command):
     description = 'generate prebuilt index of hgext (for frozen package)'