# HG changeset patch
# User Matt Mackall <mpm@selenic.com>
# Date 1191449847 18000
# Node ID 7530334bf301d743df9838af503bbc694271f563
# Parent  ff32b2725651c8fd17782a01ef43109c25eb9bbe
revlog: generate trivial deltas against null revision

To avoid extra memory usage and performance issues with large files,
generate a trivial delta header for deltas against the null revision
rather than calling the usual delta generator.

We append the delta header to meta rather than prepending it to data
to avoid a large allocate and copy.

diff --git a/mercurial/mdiff.py b/mercurial/mdiff.py
--- a/mercurial/mdiff.py
+++ b/mercurial/mdiff.py
@@ -245,6 +245,9 @@ def patch(a, bin):
 def get_matching_blocks(a, b):
     return [(d[0], d[2], d[1] - d[0]) for d in bdiff.blocks(a, b)]
 
+def trivialdiffheader(length):
+    return struct.pack(">lll", 0, 0, length)
+
 patches = mpatch.patches
 patchedsize = mpatch.patchedsize
 textdiff = bdiff.bdiff
diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -1087,9 +1087,13 @@ class revlog(object):
             if infocollect is not None:
                 infocollect(nb)
 
-            d = self.revdiff(a, b)
             p = self.parents(nb)
             meta = nb + p[0] + p[1] + lookup(nb)
+            if a == -1:
+                d = self.revision(nb)
+                meta += mdiff.trivialdiffheader(len(d))
+            else:
+                d = self.revdiff(a, b)
             yield changegroup.genchunk("%s%s" % (meta, d))
 
         yield changegroup.closechunk()