configitems: add alias support in config

Aliases define optional alternatives to existing options. For example the old
option ui.user was deprecated and replaced by ui.username. With this mechanism,
it's even possible to create an alias to an option in a different section.

Add ui.user as alias to ui.username as an example of this concept.

The old alternates principle in ui.config is removed as it was used only for
this option.

mercurial/configitems.py

@@ -32,12 +32,14 b' class configitem(object):'
     :section: the official config section where to find this item,
        :name: the official name within the section,
     :default: default value for this item,
+    :alias: optional list of tuples as alternatives.
     """
 
-    def __init__(self, section, name, default=None):
+    def __init__(self, section, name, default=None, alias=()):
         self.section = section
         self.name = name
         self.default = default
+        self.alias = list(alias)
 
 coreitems = {}
 
@@ -194,6 +196,9 b" coreconfigitem('ui', 'interactive',"
 coreconfigitem('ui', 'quiet',
     default=False,
 )
+coreconfigitem('ui', 'username',
+    alias=[('ui', 'user')]
+)
 # Windows defaults to a limit of 512 open files. A buffer of 128
 # should give us enough headway.
 coreconfigitem('worker', 'backgroundclosemaxqueue',

mercurial/ui.py

@@ -448,10 +448,12 b' class ui(object):'
 
     def _config(self, section, name, default=_unset, untrusted=False):
         value = default
-        if isinstance(name, list):
-            alternates = name
-        else:
             item = self._knownconfig.get(section, {}).get(name)
+        alternates = [(section, name)]
+
+        if item is not None:
+            alternates.extend(item.alias)
+
             if default is _unset:
                 if item is None:
                     value = default
@@ -465,21 +467,20 b' class ui(object):'
                 msg %= (section, name, default)
                 self.develwarn(msg, 2, 'warn-config-default')
 
-            alternates = [name]
-
-        for n in alternates:
-            candidate = self._data(untrusted).get(section, n, None)
+        for s, n in alternates:
+            candidate = self._data(untrusted).get(s, n, None)
             if candidate is not None:
                 value = candidate
+                section = s
                 name = n
                 break
 
         if self.debugflag and not untrusted and self._reportuntrusted:
-            for n in alternates:
-                uvalue = self._ucfg.get(section, n)
+            for s, n in alternates:
+                uvalue = self._ucfg.get(s, n)
                 if uvalue is not None and uvalue != value:
                     self.debug("ignoring untrusted configuration option "
-                               "%s.%s = %s\n" % (section, n, uvalue))
+                               "%s.%s = %s\n" % (s, n, uvalue))
         return value
 
     def configsuboptions(self, section, name, default=_unset, untrusted=False):
@@ -744,7 +745,7 b' class ui(object):'
         """
         user = encoding.environ.get("HGUSER")
         if user is None:
-            user = self.config("ui", ["username", "user"])
+            user = self.config("ui", "username")
             if user is not None:
                 user = os.path.expandvars(user)
         if user is None:

tests/test-config.t

@@ -178,3 +178,36 b' config affected by environment variables'
 
   $ PAGER=p1 hg config --debug --config pager.pager=p2 | grep 'pager\.pager'
   --config: pager.pager=p2
+
+verify that aliases are evaluated as well
+
+  $ hg init aliastest
+  $ cd aliastest
+  $ cat > .hg/hgrc << EOF
+  > [ui]
+  > user = repo user
+  > EOF
+  $ touch index
+  $ unset HGUSER
+  $ hg ci -Am test
+  adding index
+  $ hg log --template '{author}\n'
+  repo user
+  $ cd ..
+
+alias has lower priority
+
+  $ hg init aliaspriority
+  $ cd aliaspriority
+  $ cat > .hg/hgrc << EOF
+  > [ui]
+  > user = alias user
+  > username = repo user
+  > EOF
+  $ touch index
+  $ unset HGUSER
+  $ hg ci -Am test
+  adding index
+  $ hg log --template '{author}\n'
+  repo user
+  $ cd ..