rcutil: add a method to convert environment variables to config items

Handling config and environ priorities has been messy. Partially because we
don't have config layers - you either get all configs (sys + user), or none.

Ideally, environ like $EDITOR, $PAGER should be able to override the system
configs "ui.editor", "pager.pager". This patch provides the ability to
convert them into config items, so they can be inserted into the middle
config layer between system rc and user rc.

mercurial/rcutil.py

@@ -1,68 +1,90 b''
 # rcutil.py - utilities about config paths, special config sections etc.
 #
 #  Copyright Mercurial Contributors
 #
 # 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
 
 from . import (
     encoding,
     osutil,
     pycompat,
     util,
 )
 
 if pycompat.osname == 'nt':
     from . import scmwindows as scmplatform
 else:
     from . import scmposix as scmplatform
 
 systemrcpath = scmplatform.systemrcpath
 userrcpath = scmplatform.userrcpath
 
 def _expandrcpath(path):
     '''path could be a file or a directory. return a list of file paths'''
     p = util.expandpath(path)
     if os.path.isdir(p):
         join = os.path.join
         return [join(p, f) for f, k in osutil.listdir(p) if f.endswith('.rc')]
     return [p]
 
+def envrcitems(env=None):
+    '''Return [(section, name, value, source)] config items.
+
+    The config items are extracted from environment variables specified by env,
+    used to override systemrc, but not userrc.
+
+    If env is not provided, encoding.environ will be used.
+    '''
+    if env is None:
+        env = encoding.environ
+    checklist = [
+        ('EDITOR', 'ui', 'editor'),
+        ('VISUAL', 'ui', 'editor'),
+        ('PAGER', 'pager', 'pager'),
+    ]
+    result = []
+    for envname, section, configname in checklist:
+        if envname not in env:
+            continue
+        result.append((section, configname, env[envname], '$%s' % envname))
+    return result
+
 def defaultrcpath():
     '''return rc paths in default.d'''
     path = []
     defaultpath = os.path.join(util.datapath, 'default.d')
     if os.path.isdir(defaultpath):
         path = _expandrcpath(defaultpath)
     return path
 
 _rccomponents = None
 
 def rccomponents():
     '''return an ordered [(type, obj)] about where to load configs.
 
     respect $HGRCPATH. if $HGRCPATH is empty, only .hg/hgrc of current repo is
     used. if $HGRCPATH is not set, the platform default will be used.
 
     if a directory is provided, *.rc files under it will be used.
 
     type could be either 'path' or 'items', if type is 'path', obj is a string,
     and is the config file path. if type is 'items', obj is a list of (section,
     name, value, source) that should fill the config directly.
     '''
     global _rccomponents
     if _rccomponents is None:
         if 'HGRCPATH' in encoding.environ:
             _rccomponents = []
             for p in encoding.environ['HGRCPATH'].split(pycompat.ospathsep):
                 if not p:
                     continue
                 _rccomponents.extend(('path', p) for p in _expandrcpath(p))
         else:
             paths = defaultrcpath() + systemrcpath() + userrcpath()
             _rccomponents = [('path', os.path.normpath(p)) for p in paths]
     return _rccomponents