registrar: define revsetpredicate to decorate revset predicate

registrar: define revsetpredicate to decorate revset predicate

revsetpredicate is used to replace revset.predicate and
revset.extpredicate in subsequent patches.

This patch also adds loadpredicate() to revset, because this
combination helps to figure out how the name of safe predicate is put
into safesymbols.

This patch still uses safesymbols set to examine whether the predicate
corresponded to the 'name' is safe from DoS attack or not, because
just setting func._safe property needs changes below for such
examination.

  before:
      name in revset.safesymbols

  after:
      getattr(revset.symbols.get(name, None), '_safe', False)

"automatic registration" described in help doc of revsetpredicate
class will be achieved by the subsequent patch, which lists
loadpredicate() up in dispatch.extraloaders.

--- a/mercurial/registrar.py	Tue Mar 08 23:04:53 2016 +0900
+++ b/mercurial/registrar.py	Tue Mar 08 23:04:53 2016 +0900
@@ -205,3 +205,36 @@
         """Execute exra setup for registered function, if needed
         """
         pass
+
+class revsetpredicate(_funcregistrarbase):
+    """Decorator to register revset predicate
+
+    Usage::
+
+        revsetpredicate = registrar.revsetpredicate()
+
+        @revsetpredicate('mypredicate(arg1, arg2[, arg3])')
+        def mypredicatefunc(repo, subset, x):
+            '''Explanation of this revset predicate ....
+            '''
+            pass
+
+    The first string argument is used also in online help.
+
+    Optional argument 'safe' indicates whether a predicate is safe for
+    DoS attack (False by default).
+
+    'revsetpredicate' instance in example above can be used to
+    decorate multiple functions.
+
+    Decorated functions are registered automatically at loading
+    extension, if an instance named as 'revsetpredicate' is used for
+    decorating in extension.
+
+    Otherwise, explicit 'revset.loadpredicate()' is needed.
+    """
+    _getname = _funcregistrarbase._parsefuncdecl
+    _docformat = "``%s``\n    %s"
+
+    def _extrasetup(self, name, func, safe=False):
+        func._safe = safe

--- a/mercurial/revset.py	Tue Mar 08 23:04:53 2016 +0900
+++ b/mercurial/revset.py	Tue Mar 08 23:04:53 2016 +0900
@@ -3628,5 +3628,13 @@
         p = q
     return '\n'.join('  ' * l + s for l, s in lines)
 
+def loadpredicate(ui, extname, registrarobj):
+    """Load revset predicates from specified registrarobj
+    """
+    for name, func in registrarobj._table.iteritems():
+        symbols[name] = func
+        if func._safe:
+            safesymbols.add(name)
+
 # tell hggettext to extract docstrings from these functions:
 i18nfunctions = symbols.values()