Patchwork [7,of,7] templatekw: add 'requires' flag to switch to exception-safe interface

diff --git a/mercurial/formatter.py b/mercurial/formatter.py
--- a/mercurial/formatter.py
+++ b/mercurial/formatter.py
@@ -383,9 +383,7 @@  class templateformatter(baseformatter):
             return
         ref = self._parts[part]
 
-        # TODO: add support for filectx. probably each template keyword or
-        # function will have to declare dependent resources. e.g.
-        # @templatekeyword(..., requires=('ctx',))
+        # TODO: add support for filectx
         props = {}
         # explicitly-defined fields precede templatekw
         props.update(item)
diff --git a/mercurial/registrar.py b/mercurial/registrar.py
--- a/mercurial/registrar.py
+++ b/mercurial/registrar.py
@@ -283,6 +283,14 @@  class templatekeyword(_templateregistrar
 
         templatekeyword = registrar.templatekeyword()
 
+        # new API (since Mercurial 4.6)
+        @templatekeyword('mykeyword', requires={'repo', 'ctx'})
+        def mykeywordfunc(context, mapping):
+            '''Explanation of this template keyword ....
+            '''
+            pass
+
+        # old API
         @templatekeyword('mykeyword')
         def mykeywordfunc(repo, ctx, templ, cache, revcache, **args):
             '''Explanation of this template keyword ....
@@ -291,6 +299,11 @@  class templatekeyword(_templateregistrar
 
     The first string argument is used also in online help.
 
+    Optional argument 'requires' should be a collection of resource names
+    which the template keyword depends on. This also serves as a flag to
+    switch to the new API. If 'requires' is unspecified, all template
+    keywords and resources are expanded to the function arguments.
+
     'templatekeyword' instance in example above can be used to
     decorate multiple functions.
 
@@ -301,6 +314,9 @@  class templatekeyword(_templateregistrar
     Otherwise, explicit 'templatekw.loadkeyword()' is needed.
     """
 
+    def _extrasetup(self, name, func, requires=None):
+        func._requires = requires
+
 class templatefilter(_templateregistrarbase):
     """Decorator to register template filer
 
diff --git a/mercurial/templatekw.py b/mercurial/templatekw.py
--- a/mercurial/templatekw.py
+++ b/mercurial/templatekw.py
@@ -341,21 +341,14 @@  defaulttempl = {
 # filecopy is preserved for compatibility reasons
 defaulttempl['filecopy'] = defaulttempl['file_copy']
 
-# keywords are callables like:
-# fn(repo, ctx, templ, cache, revcache, **args)
-# with:
-# repo - current repository instance
-# ctx - the changectx being displayed
-# templ - the templater instance
-# cache - a cache dictionary for the whole templater run
-# revcache - a cache dictionary for the current revision
+# keywords are callables (see registrar.templatekeyword for details)
 keywords = {}
-
 templatekeyword = registrar.templatekeyword(keywords)
 
-@templatekeyword('author')
-def showauthor(repo, ctx, templ, **args):
+@templatekeyword('author', requires={'ctx'})
+def showauthor(context, mapping):
     """String. The unmodified author of the changeset."""
+    ctx = context.resource(mapping, 'ctx')
     return ctx.user()
 
 @templatekeyword('bisect')
diff --git a/mercurial/templater.py b/mercurial/templater.py
--- a/mercurial/templater.py
+++ b/mercurial/templater.py
@@ -436,12 +436,18 @@  def runsymbol(context, mapping, key, def
             v = context.process(key, safemapping)
         except TemplateNotFound:
             v = default
-    if callable(v):
-        # TODO: templatekw functions will be updated to take (context, mapping)
-        # pair instead of **props
+    if callable(v) and getattr(v, '_requires', None) is None:
+        # old templatekw: expand all keywords and resources
         props = context._resources.copy()
         props.update(mapping)
         return v(**pycompat.strkwargs(props))
+    if callable(v):
+        # new templatekw
+        try:
+            return v(context, mapping)
+        except ResourceUnavailable:
+            # unsupported keyword is mapped to empty just like unknown keyword
+            return None
     return v
 
 def buildtemplate(exp, context):
diff --git a/tests/test-command-template.t b/tests/test-command-template.t
--- a/tests/test-command-template.t
+++ b/tests/test-command-template.t
@@ -214,6 +214,8 @@  Never crash on internal resource not ava
   abort: template resource not available: ctx
   [255]
 
+  $ hg config -T '{author}'
+
 Quoting for ui.logtemplate
 
   $ hg tip --config "ui.logtemplate={rev}\n"
diff --git a/tests/test-template-engine.t b/tests/test-template-engine.t
--- a/tests/test-template-engine.t
+++ b/tests/test-template-engine.t
@@ -9,6 +9,15 @@ 
   >         self._defaults = defaults
   >         self._resources = resources
   > 
+  >     def symbol(self, mapping, key):
+  >         return mapping[key]
+  > 
+  >     def resource(self, mapping, key):
+  >         v = self._resources[key]
+  >         if v is None:
+  >             v = mapping[key]
+  >         return v
+  > 
   >     def process(self, t, map):
   >         tmpl = self.loader(t)
   >         props = self._defaults.copy()
@@ -16,10 +25,12 @@ 
   >         for k, v in props.items():
   >             if k in ('templ', 'ctx', 'repo', 'revcache', 'cache', 'troubles'):
   >                 continue
-  >             if hasattr(v, '__call__'):
+  >             if callable(v) and getattr(v, '_requires', None) is None:
   >                 props = self._resources.copy()
   >                 props.update(map)
   >                 v = v(**props)
+  >             elif callable(v):
+  >                 v = v(self, props)
   >             v = templater.stringify(v)
   >             tmpl = tmpl.replace('{{%s}}' % k, v)
   >         yield tmpl