--- a/mercurial/context.py	Tue May 09 19:16:48 2017 -0700
+++ b/mercurial/context.py	Tue May 09 20:23:21 2017 -0700
@@ -687,7 +687,9 @@
              in the repo,
     workingfilectx: a filecontext that represents files from the working
                     directory,
-    memfilectx: a filecontext that represents files in-memory."""
+    memfilectx: a filecontext that represents files in-memory,
+    overlayfilectx: duplicate another filecontext with some fields overridden.
+    """
     @propertycache
     def _filelog(self):
         return self._repo.file(self._path)
@@ -2078,6 +2080,77 @@
         """wraps repo.wwrite"""
         self._data = data
 
+class overlayfilectx(committablefilectx):
+    """Like memfilectx but take an original filectx and optional parameters to
+    override parts of it. This is useful when fctx.data() is expensive (i.e.
+    flag processor is expensive) and raw data, flags, and filenode could be
+    reused (ex. rebase or mode-only amend a REVIDX_EXTSTORED file).
+    """
+
+    def __init__(self, originalfctx, datafunc=None, path=None, flags=None,
+                 copied=None, ctx=None):
+        """originalfctx: filecontext to duplicate
+
+        datafunc: None or a function to override data (file content). It is a
+        function to be lazy. path, flags, copied, ctx: None or overridden value
+
+        copied could be (path, rev), or False. copied could also be just path,
+        and will be converted to (path, nullid). This simplifies some callers.
+        """
+
+        if path is None:
+            path = originalfctx.path()
+        if ctx is None:
+            ctx = originalfctx.changectx()
+            ctxmatch = lambda: True
+        else:
+            ctxmatch = lambda: ctx == originalfctx.changectx()
+
+        repo = originalfctx.repo()
+        flog = originalfctx.filelog()
+        super(overlayfilectx, self).__init__(repo, path, flog, ctx)
+
+        if copied is None:
+            copied = originalfctx.renamed()
+            copiedmatch = lambda: True
+        else:
+            if copied and not isinstance(copied, tuple):
+                # repo._filecommit will recalculate copyrev so nullid is okay
+                copied = (copied, nullid)
+            copiedmatch = lambda: copied == originalfctx.renamed()
+
+        # When data, copied (could affect data), ctx (could affect filelog
+        # parents) are not overridden, rawdata, rawflags, and filenode may be
+        # reused (repo._filecommit should double check filelog parents).
+        #
+        # path, flags are not hashed in filelog (but in manifestlog) so they do
+        # not affect reusable here.
+        #
+        # If ctx or copied is overridden to a same value with originalfctx,
+        # still consider it's reusable. originalfctx.renamed() may be a bit
+        # expensive so it's not called unless necessary. Assuming datafunc is
+        # always expensive, do not call it for this "reusable" test.
+        reusable = datafunc is None and ctxmatch() and copiedmatch()
+
+        if datafunc is None:
+            datafunc = originalfctx.data
+        if flags is None:
+            flags = originalfctx.flags()
+
+        self._datafunc = datafunc
+        self._flags = flags
+        self._copied = copied
+
+        if reusable:
+            # copy extra fields from originalfctx
+            attrs = ['rawdata', 'rawflags', '_filenode', '_filerev']
+            for attr in attrs:
+                if util.safehasattr(originalfctx, attr):
+                    setattr(self, attr, getattr(originalfctx, attr))
+
+    def data(self):
+        return self._datafunc()
+
 class metadataonlyctx(committablectx):
     """Like memctx but it's reusing the manifest of different commit.
     Intended to be used by lightweight operations that are creating