util: add a way to issue deprecation warning without a UI object

Our current deprecation warning mechanism relies on ui object. They are case
where we cannot have access to the UI object. On a general basis we avoid using
the python mechanism for deprecation warning because up to Python 2.6 it is
exposing warning to unsuspecting user who cannot do anything to deal with them.

So we build a "safe" strategy to hide this warnings behind a flag in an
environment variable. The test runner set this flag so that tests show these
warning.  This will help us marker API as deprecated for extensions to update
their code.

mercurial/util.py

@@ -38,6 +38,7 b' import tempfile'
 import textwrap
 import time
 import traceback
+import warnings
 import zlib
 
 from . import (
@@ -156,6 +157,31 b' def bitsfrom(container):'
         bits |= bit
     return bits
 
+# python 2.6 still have deprecation warning enabled by default. We do not want
+# to display anything to standard user so detect if we are running test and
+# only use python deprecation warning in this case.
+_dowarn = bool(encoding.environ.get('HGEMITWARNINGS'))
+if _dowarn:
+    # explicitly unfilter our warning for python 2.7
+    #
+    # The option of setting PYTHONWARNINGS in the test runner was investigated.
+    # However, module name set through PYTHONWARNINGS was exactly matched, so
+    # we cannot set 'mercurial' and have it match eg: 'mercurial.scmutil'. This
+    # makes the whole PYTHONWARNINGS thing useless for our usecase.
+    warnings.filterwarnings('default', '', DeprecationWarning, 'mercurial')
+    warnings.filterwarnings('default', '', DeprecationWarning, 'hgext')
+    warnings.filterwarnings('default', '', DeprecationWarning, 'hgext3rd')
+
+def nouideprecwarn(msg, version, stacklevel=1):
+    """Issue an python native deprecation warning
+
+    This is a noop outside of tests, use 'ui.deprecwarn' when possible.
+    """
+    if _dowarn:
+        msg += ("\n(compatibility will be dropped after Mercurial-%s,"
+                " update your code.)") % version
+        warnings.warn(msg, DeprecationWarning, stacklevel + 1)
+
 DIGESTS = {
     'md5': hashlib.md5,
     'sha1': hashlib.sha1,

tests/run-tests.py

@@ -884,6 +884,7 b' class Test(unittest.TestCase):'
         env = os.environ.copy()
         if sysconfig is not None:
             env['PYTHONUSERBASE'] = sysconfig.get_config_var('userbase')
+        env['HGEMITWARNINGS'] = '1'
         env['TESTTMP'] = self._testtmp
         env['HOME'] = self._testtmp
         # This number should match portneeded in _getport

tests/test-devel-warnings.t

@@ -3,7 +3,7 b''
   > """A small extension that tests our developer warnings
   > """
   > 
-  > from mercurial import cmdutil, repair
+  > from mercurial import cmdutil, repair, util
   > 
   > cmdtable = {}
   > command = cmdutil.command(cmdtable)
@@ -58,6 +58,9 b''
   >     def foobar(ui):
   >         ui.deprecwarn('foorbar is deprecated, go shopping', '42.1337')
   >     foobar(ui)
+  > @command('nouiwarning', [], '')
+  > def nouiwarning(ui, repo):
+  >     util.nouideprecwarn('this is a test', '13.37')
   > EOF
 
   $ cat << EOF >> $HGRCPATH
@@ -163,4 +166,15 b' Test programming error failure:'
   Traceback (most recent call last):
   mercurial.error.ProgrammingError: transaction requires locking
 
+Old style deprecation warning
+
+  $ hg nouiwarning
+  $TESTTMP/buggylocking.py:61: DeprecationWarning: this is a test
+  (compatibility will be dropped after Mercurial-13.37, update your code.)
+    util.nouideprecwarn('this is a test', '13.37')
+
+(disabled outside of test run)
+
+  $ HGEMITWARNINGS= hg nouiwarning
+
   $ cd ..