templater: drop symbols which should be overridden by new 'ctx' (issue5612)

This problem is caused by impedance mismatch between the templater and the
formatter interface, which is that the template keywords are generally
evaluated dynamically, but the formatter puts static values into a template
mapping.

This patch avoids the problem by removing conflicting values from a mapping
dict when a 'ctx' is switched.

mercurial/formatter.py

@@ -504,6 +504,10 b' class templateresources(templater.resour'
             'ui': ui,
         }
 
+    def availablekeys(self, context, mapping):
+        return {k for k, g in self._gettermap.iteritems()
+                if g(self, context, mapping, k) is not None}
+
     def knownkeys(self):
         return self._knownkeys
 

mercurial/templater.py

@@ -563,6 +563,10 b' class resourcemapper(object):'
     __metaclass__ = abc.ABCMeta
 
     @abc.abstractmethod
+    def availablekeys(self, context, mapping):
+        """Return a set of available resource keys based on the given mapping"""
+
+    @abc.abstractmethod
     def knownkeys(self):
         """Return a set of supported resource keys"""
 
@@ -571,6 +575,9 b' class resourcemapper(object):'
         """Return a resource for the key if available; otherwise None"""
 
 class nullresourcemapper(resourcemapper):
+    def availablekeys(self, context, mapping):
+        return set()
+
     def knownkeys(self):
         return set()
 
@@ -616,10 +623,23 b' class engine(object):'
     def overlaymap(self, origmapping, newmapping):
         """Create combined mapping from the original mapping and partial
         mapping to override the original"""
-        mapping = origmapping.copy()
+        # do not copy symbols which overrides the defaults depending on
+        # new resources, so the defaults will be re-evaluated (issue5612)
+        knownres = self._resources.knownkeys()
+        newres = self._resources.availablekeys(self, newmapping)
+        mapping = {k: v for k, v in origmapping.iteritems()
+                   if (k in knownres  # not a symbol per self.symbol()
+                       or newres.isdisjoint(self._defaultrequires(k)))}
         mapping.update(newmapping)
         return mapping
 
+    def _defaultrequires(self, key):
+        """Resource keys required by the specified default symbol function"""
+        v = self._defaults.get(key)
+        if v is None or not callable(v):
+            return ()
+        return getattr(v, '_requires', ())
+
     def symbol(self, mapping, key):
         """Resolve symbol to value or function; None if nothing found"""
         v = None

tests/test-identify.t

@@ -63,6 +63,16 b' test template keywords and functions whi'
   $ hg id -T '{parents % "{rev} {node|shortest} {desc}\n"}'
   0 cb9a a
 
+test nested template: '{tags}'/'{node}' constants shouldn't override the
+default keywords, but '{id}' persists because there's no default keyword
+for '{id}' (issue5612)
+
+  $ hg id -T '{tags}\n'
+  tip
+  $ hg id -T '{revset("null:.") % "{rev}:{node|short} {tags} {id}\n"}'
+  -1:000000000000  cb9a9f314b8b
+  0:cb9a9f314b8b tip cb9a9f314b8b
+
 with modifications
 
   $ echo b > a