Add revlog.strip to truncate away revisions.

This updates the revlog data structures for index and nodemap in place
so the .d and .i files don't need to be reread after stripping away a revision.

mercurial/revlog.py

@@ -71,6 +71,9 b' class lazyparser:'
         self.all = 0
         self.revlog = revlog
 
+    def trunc(self, pos):
+        self.l = pos/self.s
+
     def load(self, pos=None):
         if self.all: return
         if pos is not None:
@@ -104,8 +107,12 b' class lazyindex:'
         return self.p.index[pos]
     def __getitem__(self, pos):
         return self.p.index[pos] or self.load(pos)
+    def __delitem__(self, pos):
+        del self.p.index[pos]
     def append(self, e):
         self.p.index.append(e)
+    def trunc(self, pos):
+        self.p.trunc(pos)
 
 class lazymap:
     """a lazy version of the node map"""
@@ -140,6 +147,8 b' class lazymap:'
                 raise KeyError("node " + hex(key))
     def __setitem__(self, key, val):
         self.p.map[key] = val
+    def __delitem__(self, key):
+        del self.p.map[key]
 
 class RevlogError(Exception): pass
 
@@ -834,6 +843,36 b' class revlog:'
         ifh.close()
         return node
 
+    def strip(self, rev, minlink):
+        if self.count() == 0 or rev >= self.count():
+            return
+
+        # When stripping away a revision, we need to make sure it
+        # does not actually belong to an older changeset.
+        # The minlink parameter defines the oldest revision
+        # we're allowed to strip away.
+        while minlink > self.index[rev][3]:
+            rev += 1
+            if rev >= self.count():
+                return
+
+        # first truncate the files on disk
+        end = self.start(rev)
+        self.opener(self.datafile, "a").truncate(end)
+        end = rev * struct.calcsize(indexformat)
+        self.opener(self.indexfile, "a").truncate(end)
+
+        # then reset internal state in memory to forget those revisions
+        self.cache = None
+        for p in self.index[rev:]:
+            del self.nodemap[p[6]]
+        del self.index[rev:]
+
+        # truncating the lazyindex also truncates the lazymap.
+        if isinstance(self.index, lazyindex):
+            self.index.trunc(end)
+
+
     def checksize(self):
         expected = 0
         if self.count():