policy: add helper to import cext/pure module

These functions are sysstr API since __import__() and getattr() hate byte
strings on Python 3.

There's a minor BC, which is ImportError will be raised if invalid
HGMODULEPOLICY is specified. I think this is more desired behavior.

We're planning to add strict checking for C API compatibility. This patch
includes the stub for it.

mercurial/policy.py

@@ -1,51 +1,96 b''
 # policy.py - module policy logic for Mercurial.
 #
 # Copyright 2015 Gregory Szorc <[e-mail]>
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 from __future__ import absolute_import
 
 import os
 import sys
 
 # Rules for how modules can be loaded. Values are:
 #
 #    c - require C extensions
 #    allow - allow pure Python implementation when C loading fails
 #    cffi - required cffi versions (implemented within pure module)
 #    cffi-allow - allow pure Python implementation if cffi version is missing
 #    py - only load pure Python modules
 #
 # By default, fall back to the pure modules so the in-place build can
 # run without recompiling the C extensions. This will be overridden by
 # __modulepolicy__ generated by setup.py.
 policy = b'allow'
 policynoc = (b'cffi', b'cffi-allow', b'py')
 policynocffi = (b'c', b'py')
+_packageprefs = {
+    # policy: (versioned package, pure package)
+    b'c': (r'cext', None),
+    b'allow': (r'cext', r'pure'),
+    b'cffi': (None, r'pure'),  # TODO: (r'cffi', None)
+    b'cffi-allow': (None, r'pure'),  # TODO: (r'cffi', r'pure')
+    b'py': (None, r'pure'),
+}
 
 try:
     from . import __modulepolicy__
     policy = __modulepolicy__.modulepolicy
 except ImportError:
     pass
 
 # PyPy doesn't load C extensions.
 #
 # The canonical way to do this is to test platform.python_implementation().
 # But we don't import platform and don't bloat for it here.
 if r'__pypy__' in sys.builtin_module_names:
     policy = b'cffi'
 
 # Our C extensions aren't yet compatible with Python 3. So use pure Python
 # on Python 3 for now.
 if sys.version_info[0] >= 3:
     policy = b'py'
 
 # Environment variable can always force settings.
 if sys.version_info[0] >= 3:
     if r'HGMODULEPOLICY' in os.environ:
         policy = os.environ[r'HGMODULEPOLICY'].encode(r'utf-8')
 else:
     policy = os.environ.get(r'HGMODULEPOLICY', policy)
+
+def _importfrom(pkgname, modname):
+    # from .<pkgname> import <modname> (where . is looked through this module)
+    fakelocals = {}
+    pkg = __import__(pkgname, globals(), fakelocals, [modname], level=1)
+    try:
+        fakelocals[modname] = mod = getattr(pkg, modname)
+    except AttributeError:
+        raise ImportError(r'cannot import name %s' % modname)
+    # force import; fakelocals[modname] may be replaced with the real module
+    getattr(mod, r'__doc__', None)
+    return fakelocals[modname]
+
+def _checkmod(pkgname, modname, mod):
+    expected = 1  # TODO: maybe defined in table?
+    actual = getattr(mod, r'version', None)
+    if actual != expected:
+        raise ImportError(r'cannot import module %s.%s '
+                          r'(expected version: %d, actual: %r)'
+                          % (pkgname, modname, expected, actual))
+
+def importmod(modname):
+    """Import module according to policy and check API version"""
+    try:
+        verpkg, purepkg = _packageprefs[policy]
+    except KeyError:
+        raise ImportError(r'invalid HGMODULEPOLICY %r' % policy)
+    assert verpkg or purepkg
+    if verpkg:
+        try:
+            mod = _importfrom(verpkg, modname)
+            _checkmod(verpkg, modname, mod)
+            return mod
+        except ImportError:
+            if not purepkg:
+                raise
+    return _importfrom(purepkg, modname)