tests: add test for Python 3 compatibility

tests: add test for Python 3 compatibility

Python 3 is inevitable. There have been incremental movements towards
converting the code base to be Python 3 compatible. Unfortunately, we
don't have any tests that look for Python 3 compatibility. This patch
changes that.

We introduce a check-py3-compat.py script whose role is to verify
Python 3 compatibility of the files passed in. We add a test that
calls this script with all .py files from the source checkout.

The script currently only verifies that absolute_import and
print_function are used. These are the low hanging fruits for Python
compatbility. Over time, we can include more checks, including
verifying we're able to load each Python file with Python 3. You
have to start somewhere.

Accepting this patch means that all new .py files must have
absolute_import and print_function (if "print" is used) to avoid
a new warning about Python 3 incompatibility. We've already
converted several files to use absolute_import and print_function
is in the same boat, so I don't think this is such a radical
proposition.

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/contrib/check-py3-compat.py	Sun Dec 06 22:39:12 2015 -0800
@@ -0,0 +1,39 @@
+#!/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 sys
+
+def check_compat(f):
+    """Check Python 3 compatibility for a file."""
+    with open(f, 'rb') as fh:
+        content = fh.read()
+
+    root = ast.parse(content)
+    futures = set()
+    haveprint = False
+    for node in ast.walk(root):
+        if isinstance(node, ast.ImportFrom):
+            if node.module == '__future__':
+                futures |= set(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)
+
+if __name__ == '__main__':
+    for f in sys.argv[1:]:
+        check_compat(f)
+
+    sys.exit(0)

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-check-py3-compat.t	Sun Dec 06 22:39:12 2015 -0800
@@ -0,0 +1,237 @@
+#require test-repo
+
+  $ cd "$TESTDIR"/..
+
+  $ hg files 'set:(**.py)' | xargs python contrib/check-py3-compat.py
+  contrib/casesmash.py not using absolute_import
+  contrib/check-code.py not using absolute_import
+  contrib/check-code.py requires print_function
+  contrib/check-config.py not using absolute_import
+  contrib/check-config.py requires print_function
+  contrib/debugcmdserver.py not using absolute_import
+  contrib/debugcmdserver.py requires print_function
+  contrib/debugshell.py not using absolute_import
+  contrib/fixpax.py not using absolute_import
+  contrib/fixpax.py requires print_function
+  contrib/hgclient.py not using absolute_import
+  contrib/hgclient.py requires print_function
+  contrib/hgfixes/__init__.py not using absolute_import
+  contrib/hgfixes/fix_bytes.py not using absolute_import
+  contrib/hgfixes/fix_bytesmod.py not using absolute_import
+  contrib/hgfixes/fix_leftover_imports.py not using absolute_import
+  contrib/import-checker.py not using absolute_import
+  contrib/import-checker.py requires print_function
+  contrib/memory.py not using absolute_import
+  contrib/perf.py not using absolute_import
+  contrib/python-hook-examples.py not using absolute_import
+  contrib/revsetbenchmarks.py not using absolute_import
+  contrib/revsetbenchmarks.py requires print_function
+  contrib/showstack.py not using absolute_import
+  contrib/synthrepo.py not using absolute_import
+  contrib/win32/hgwebdir_wsgi.py not using absolute_import
+  doc/check-seclevel.py not using absolute_import
+  doc/gendoc.py not using absolute_import
+  doc/hgmanpage.py not using absolute_import
+  hgext/__init__.py not using absolute_import
+  hgext/acl.py not using absolute_import
+  hgext/blackbox.py not using absolute_import
+  hgext/bugzilla.py not using absolute_import
+  hgext/censor.py not using absolute_import
+  hgext/children.py not using absolute_import
+  hgext/churn.py not using absolute_import
+  hgext/clonebundles.py not using absolute_import
+  hgext/color.py not using absolute_import
+  hgext/convert/__init__.py not using absolute_import
+  hgext/convert/bzr.py not using absolute_import
+  hgext/convert/common.py not using absolute_import
+  hgext/convert/convcmd.py not using absolute_import
+  hgext/convert/cvs.py not using absolute_import
+  hgext/convert/cvsps.py not using absolute_import
+  hgext/convert/darcs.py not using absolute_import
+  hgext/convert/filemap.py not using absolute_import
+  hgext/convert/git.py not using absolute_import
+  hgext/convert/gnuarch.py not using absolute_import
+  hgext/convert/hg.py not using absolute_import
+  hgext/convert/monotone.py not using absolute_import
+  hgext/convert/p4.py not using absolute_import
+  hgext/convert/subversion.py not using absolute_import
+  hgext/convert/transport.py not using absolute_import
+  hgext/eol.py not using absolute_import
+  hgext/extdiff.py not using absolute_import
+  hgext/factotum.py not using absolute_import
+  hgext/fetch.py not using absolute_import
+  hgext/gpg.py not using absolute_import
+  hgext/graphlog.py not using absolute_import
+  hgext/hgcia.py not using absolute_import
+  hgext/hgk.py not using absolute_import
+  hgext/highlight/__init__.py not using absolute_import
+  hgext/highlight/highlight.py not using absolute_import
+  hgext/histedit.py not using absolute_import
+  hgext/keyword.py not using absolute_import
+  hgext/largefiles/__init__.py not using absolute_import
+  hgext/largefiles/basestore.py not using absolute_import
+  hgext/largefiles/lfcommands.py not using absolute_import
+  hgext/largefiles/lfutil.py not using absolute_import
+  hgext/largefiles/localstore.py not using absolute_import
+  hgext/largefiles/overrides.py not using absolute_import
+  hgext/largefiles/proto.py not using absolute_import
+  hgext/largefiles/remotestore.py not using absolute_import
+  hgext/largefiles/reposetup.py not using absolute_import
+  hgext/largefiles/uisetup.py not using absolute_import
+  hgext/largefiles/wirestore.py not using absolute_import
+  hgext/mq.py not using absolute_import
+  hgext/notify.py not using absolute_import
+  hgext/pager.py not using absolute_import
+  hgext/patchbomb.py not using absolute_import
+  hgext/purge.py not using absolute_import
+  hgext/rebase.py not using absolute_import
+  hgext/record.py not using absolute_import
+  hgext/relink.py not using absolute_import
+  hgext/schemes.py not using absolute_import
+  hgext/share.py not using absolute_import
+  hgext/shelve.py not using absolute_import
+  hgext/strip.py not using absolute_import
+  hgext/transplant.py not using absolute_import
+  hgext/win32mbcs.py not using absolute_import
+  hgext/win32text.py not using absolute_import
+  hgext/zeroconf/Zeroconf.py not using absolute_import
+  hgext/zeroconf/Zeroconf.py requires print_function
+  hgext/zeroconf/__init__.py not using absolute_import
+  i18n/check-translation.py not using absolute_import
+  i18n/polib.py not using absolute_import
+  mercurial/byterange.py not using absolute_import
+  mercurial/cmdutil.py not using absolute_import
+  mercurial/commands.py not using absolute_import
+  mercurial/commandserver.py not using absolute_import
+  mercurial/context.py not using absolute_import
+  mercurial/destutil.py not using absolute_import
+  mercurial/dirstate.py not using absolute_import
+  mercurial/dispatch.py requires print_function
+  mercurial/encoding.py not using absolute_import
+  mercurial/exchange.py not using absolute_import
+  mercurial/help.py not using absolute_import
+  mercurial/httpclient/__init__.py not using absolute_import
+  mercurial/httpclient/_readers.py not using absolute_import
+  mercurial/httpclient/socketutil.py not using absolute_import
+  mercurial/httpconnection.py not using absolute_import
+  mercurial/keepalive.py not using absolute_import
+  mercurial/keepalive.py requires print_function
+  mercurial/localrepo.py not using absolute_import
+  mercurial/lsprof.py requires print_function
+  mercurial/lsprofcalltree.py not using absolute_import
+  mercurial/lsprofcalltree.py requires print_function
+  mercurial/mail.py requires print_function
+  mercurial/manifest.py not using absolute_import
+  mercurial/mdiff.py not using absolute_import
+  mercurial/obsolete.py not using absolute_import
+  mercurial/patch.py not using absolute_import
+  mercurial/pure/__init__.py not using absolute_import
+  mercurial/pure/base85.py not using absolute_import
+  mercurial/pure/bdiff.py not using absolute_import
+  mercurial/pure/diffhelpers.py not using absolute_import
+  mercurial/pure/mpatch.py not using absolute_import
+  mercurial/pure/osutil.py not using absolute_import
+  mercurial/pure/parsers.py not using absolute_import
+  mercurial/pvec.py not using absolute_import
+  mercurial/py3kcompat.py not using absolute_import
+  mercurial/revlog.py not using absolute_import
+  mercurial/scmposix.py not using absolute_import
+  mercurial/scmutil.py not using absolute_import
+  mercurial/scmwindows.py not using absolute_import
+  mercurial/similar.py not using absolute_import
+  mercurial/store.py not using absolute_import
+  mercurial/util.py not using absolute_import
+  mercurial/windows.py not using absolute_import
+  setup.py not using absolute_import
+  tests/autodiff.py not using absolute_import
+  tests/dumbhttp.py not using absolute_import
+  tests/fakedirstatewritetime.py not using absolute_import
+  tests/fakepatchtime.py not using absolute_import
+  tests/filterpyflakes.py not using absolute_import
+  tests/filterpyflakes.py requires print_function
+  tests/generate-working-copy-states.py not using absolute_import
+  tests/generate-working-copy-states.py requires print_function
+  tests/get-with-headers.py not using absolute_import
+  tests/get-with-headers.py requires print_function
+  tests/heredoctest.py not using absolute_import
+  tests/heredoctest.py requires print_function
+  tests/hghave.py not using absolute_import
+  tests/hgweberror.py not using absolute_import
+  tests/hypothesishelpers.py not using absolute_import
+  tests/hypothesishelpers.py requires print_function
+  tests/killdaemons.py not using absolute_import
+  tests/md5sum.py not using absolute_import
+  tests/mockblackbox.py not using absolute_import
+  tests/printenv.py not using absolute_import
+  tests/readlink.py not using absolute_import
+  tests/readlink.py requires print_function
+  tests/revlog-formatv0.py not using absolute_import
+  tests/run-tests.py not using absolute_import
+  tests/seq.py not using absolute_import
+  tests/seq.py requires print_function
+  tests/silenttestrunner.py not using absolute_import
+  tests/silenttestrunner.py requires print_function
+  tests/sitecustomize.py not using absolute_import
+  tests/svn-safe-append.py not using absolute_import
+  tests/svnxml.py not using absolute_import
+  tests/test-ancestor.py not using absolute_import
+  tests/test-ancestor.py requires print_function
+  tests/test-atomictempfile.py not using absolute_import
+  tests/test-batching.py not using absolute_import
+  tests/test-batching.py requires print_function
+  tests/test-bdiff.py not using absolute_import
+  tests/test-bdiff.py requires print_function
+  tests/test-context.py not using absolute_import
+  tests/test-context.py requires print_function
+  tests/test-demandimport.py not using absolute_import
+  tests/test-demandimport.py requires print_function
+  tests/test-dispatch.py not using absolute_import
+  tests/test-dispatch.py requires print_function
+  tests/test-doctest.py not using absolute_import
+  tests/test-duplicateoptions.py not using absolute_import
+  tests/test-duplicateoptions.py requires print_function
+  tests/test-filecache.py not using absolute_import
+  tests/test-filecache.py requires print_function
+  tests/test-filelog.py not using absolute_import
+  tests/test-filelog.py requires print_function
+  tests/test-hg-parseurl.py not using absolute_import
+  tests/test-hg-parseurl.py requires print_function
+  tests/test-hgweb-auth.py not using absolute_import
+  tests/test-hgweb-auth.py requires print_function
+  tests/test-hgwebdir-paths.py not using absolute_import
+  tests/test-hybridencode.py not using absolute_import
+  tests/test-hybridencode.py requires print_function
+  tests/test-lrucachedict.py not using absolute_import
+  tests/test-lrucachedict.py requires print_function
+  tests/test-manifest.py not using absolute_import
+  tests/test-minirst.py not using absolute_import
+  tests/test-minirst.py requires print_function
+  tests/test-parseindex2.py not using absolute_import
+  tests/test-parseindex2.py requires print_function
+  tests/test-pathencode.py not using absolute_import
+  tests/test-pathencode.py requires print_function
+  tests/test-propertycache.py not using absolute_import
+  tests/test-propertycache.py requires print_function
+  tests/test-revlog-ancestry.py not using absolute_import
+  tests/test-revlog-ancestry.py requires print_function
+  tests/test-run-tests.py not using absolute_import
+  tests/test-simplemerge.py not using absolute_import
+  tests/test-status-inprocess.py not using absolute_import
+  tests/test-status-inprocess.py requires print_function
+  tests/test-symlink-os-yes-fs-no.py not using absolute_import
+  tests/test-trusted.py not using absolute_import
+  tests/test-trusted.py requires print_function
+  tests/test-ui-color.py not using absolute_import
+  tests/test-ui-color.py requires print_function
+  tests/test-ui-config.py not using absolute_import
+  tests/test-ui-config.py requires print_function
+  tests/test-ui-verbosity.py not using absolute_import
+  tests/test-ui-verbosity.py requires print_function
+  tests/test-url.py not using absolute_import
+  tests/test-url.py requires print_function
+  tests/test-walkrepo.py not using absolute_import
+  tests/test-walkrepo.py requires print_function
+  tests/test-wireproto.py not using absolute_import
+  tests/test-wireproto.py requires print_function
+  tests/tinyproxy.py not using absolute_import
+  tests/tinyproxy.py requires print_function