tests: make check-py3-compat.py actually load the specified files correctly

For most uses, this change is essentially a no-op, as this script is generally
only run by test-check-py3-compat.t, which will already put `$TESTDIR/..` in
`$PYTHONPATH`.

When running outside of tests, however, `$PYTHONPATH` is likely not set, causing
check-py3-compat.py to parse the file from the repo, but then import the
installed version, and raise any errors about the installed version, not the one
currently in the repo.

Additionally, this helps users (like me) who have a strange set up where their
home directory (and thus their hg repos) happen to be in a subdirectory of
sys.prefix (which is /usr on my system). Since the '.' entry added to sys.path
takes precedence over the absolute path of `$TESTDIR/..` in `$PYTHONPATH`, the
path to the modules that it imports (and that show up in any stack trace) are
*relative*, meaning that we don't detect them as starting with `sys.prefix`.

Sample non-test invocation, and the difference this change makes (the path for
'error at <path>:<line>' is correct now)::

Before:
```
$ python3 contrib/check-py3-compat.py mercurial/win*.py
mercurial/win32.py: error importing: <ValueError> _type_ 'v' not supported (error at check-py3-compat.py:65)
mercurial/windows.py: error importing: <ModuleNotFoundError> No module named 'msvcrt' (error at check-py3-compat.py:65)
```

After:
```
$ python3 contrib/check-py3-compat.py mercurial/win*.py
mercurial/win32.py: error importing: <ValueError> _type_ 'v' not supported (error at win32.py:11)
mercurial/windows.py: error importing: <ModuleNotFoundError> No module named 'msvcrt' (error at windows.py:12)
```

Differential Revision: https://phab.mercurial-scm.org/D8814

contrib/check-py3-compat.py

@@ -1,113 +1,122 b''
 #!/usr/bin/env python
 #
 # check-py3-compat - check Python 3 compatibility of Mercurial files
 #
 # 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, print_function
 
 import ast
 import importlib
 import os
 import sys
 import traceback
 import warnings
 
 
 def check_compat_py2(f):
     """Check Python 3 compatibility for a file with Python 2"""
     with open(f, 'rb') as fh:
         content = fh.read()
     root = ast.parse(content)
 
     # Ignore empty files.
     if not root.body:
         return
 
     futures = set()
     haveprint = False
     for node in ast.walk(root):
         if isinstance(node, ast.ImportFrom):
             if node.module == '__future__':
                 futures |= {n.name for n in node.names}
         elif isinstance(node, ast.Print):
             haveprint = True
 
     if 'absolute_import' not in futures:
         print('%s not using absolute_import' % f)
     if haveprint and 'print_function' not in futures:
         print('%s requires print_function' % f)
 
 
 def check_compat_py3(f):
     """Check Python 3 compatibility of a file with Python 3."""
     with open(f, 'rb') as fh:
         content = fh.read()
 
     try:
         ast.parse(content, filename=f)
     except SyntaxError as e:
         print('%s: invalid syntax: %s' % (f, e))
         return
 
     # Try to import the module.
     # For now we only support modules in packages because figuring out module
     # paths for things not in a package can be confusing.
     if f.startswith(
         ('hgdemandimport/', 'hgext/', 'mercurial/')
     ) and not f.endswith('__init__.py'):
         assert f.endswith('.py')
         name = f.replace('/', '.')[:-3]
         try:
             importlib.import_module(name)
         except Exception as e:
             exc_type, exc_value, tb = sys.exc_info()
             # We walk the stack and ignore frames from our custom importer,
             # import mechanisms, and stdlib modules. This kinda/sorta
             # emulates CPython behavior in import.c while also attempting
             # to pin blame on a Mercurial file.
             for frame in reversed(traceback.extract_tb(tb)):
                 if frame.name == '_call_with_frames_removed':
                     continue
                 if 'importlib' in frame.filename:
                     continue
                 if 'mercurial/__init__.py' in frame.filename:
                     continue
                 if frame.filename.startswith(sys.prefix):
                     continue
                 break
 
             if frame.filename:
                 filename = os.path.basename(frame.filename)
                 print(
                     '%s: error importing: <%s> %s (error at %s:%d)'
                     % (f, type(e).__name__, e, filename, frame.lineno)
                 )
             else:
                 print(
                     '%s: error importing module: <%s> %s (line %d)'
                     % (f, type(e).__name__, e, frame.lineno)
                 )
 
 
 if __name__ == '__main__':
     if sys.version_info[0] == 2:
         fn = check_compat_py2
     else:
+        # check_compat_py3 will import every filename we specify as long as it
+        # starts with one of a few prefixes. It does this by converting
+        # specified filenames like 'mercurial/foo.py' to 'mercurial.foo' and
+        # importing that. When running standalone (not as part of a test), this
+        # means we actually import the installed versions, not the files we just
+        # specified. When running as test-check-py3-compat.t, we technically
+        # would import the correct paths, but it's cleaner to have both cases
+        # use the same import logic.
+        sys.path.insert(0, '.')
         fn = check_compat_py3
 
     for f in sys.argv[1:]:
         with warnings.catch_warnings(record=True) as warns:
             fn(f)
 
         for w in warns:
             print(
                 warnings.formatwarning(
                     w.message, w.category, w.filename, w.lineno
                 ).rstrip()
             )
 
     sys.exit(0)