[PATCH 4 of 6] revset: precompute _descendants()

[Patrick Mezard]

# HG changeset patch
# User Patrick Mezard <patrick at mezard.eu>
# Date 1336509824 -7200
# Node ID 3d24bf6b15d03ed618b7a7b0e49e349e70a9c674
# Parent  dbeeb20515c9b9225f74fc8491d516a7b6b2f48a
revset: precompute _descendants()

On mercurial repository:

                             first line    total
  * hg log -r 0:tip --follow
  2.2                            0.124s   2.628s
  before                         0.153s   3.048s
  after                          0.150s   2.369s

diff --git a/mercurial/revset.py b/mercurial/revset.py
--- a/mercurial/revset.py
+++ b/mercurial/revset.py
@@ -484,19 +484,26 @@
             l.append(r)
     return l
 
+ at precomputed
 def _descendants(repo, subset, x, followfirst=False):
     args = getset(repo, range(len(repo)), x)
-    if not args:
-        return []
-    s = set(_revdescendants(repo, args, followfirst)) | set(args)
-    return [r for r in subset if r in s]
+    s = set()
+    if args:
+        s = set(_revdescendants(repo, args, followfirst)) | set(args)
+    def fn(repo, subset, x):
+        if not s:
+            return []
+        return [r for r in subset if r in s]
+    return fn
 
+ at precomputed
 def descendants(repo, subset, x):
     """``descendants(set)``
     Changesets which are descendants of changesets in set.
     """
     return _descendants(repo, subset, x)
 
+ at precomputed
 def _firstdescendants(repo, subset, x):
     # ``_firstdescendants(set)``
     # Like ``descendants(set)`` but follows only the first parents.