[PATCH stable] patch: fix segfault against unified diffs which start line is zero

[Yuya Nishihara]

# HG changeset patch
# User Yuya Nishihara <yuya at tcha.org>
# Date 1336806601 -32400
# Branch stable
# Node ID 2910f6a0748829896685bfd64877409fce33d813
# Parent  e6dfbc5df76fe96f73ca204b1d10cd27a814e890
patch: fix segfault against unified diffs which start line is zero

Since 2b1ec74c961f, if a chunk starts with "@@ -0,1", oldstart turns into
a negative value. Because diffhelpers.testhunk() doesn't expect negative bstart,
it bypasses "alen > blen - bstart" condition and segfaults at
"PyList_GET_ITEM(b, i + bstart)".

diff --git a/mercurial/patch.py b/mercurial/patch.py
--- a/mercurial/patch.py
+++ b/mercurial/patch.py
@@ -1014,9 +1014,9 @@ class hunk(object):
         oldstart = self.starta + top
         newstart = self.startb + top
         # zero length hunk ranges already have their start decremented
-        if self.lena:
+        if self.lena and oldstart > 0:
             oldstart -= 1
-        if self.lenb:
+        if self.lenb and newstart > 0:
             newstart -= 1
         return old, oldstart, new, newstart
 
diff --git a/tests/test-import.t b/tests/test-import.t
--- a/tests/test-import.t
+++ b/tests/test-import.t
@@ -997,6 +997,26 @@ import a unified diff with no lines of c
   c3
   c4
 
+no segfault while importing a unified diff which start line is zero but chunk
+size is non-zero
+
+  $ hg init startlinezero
+  $ cd startlinezero
+  $ echo foo > foo
+  $ hg commit -Amfoo
+  adding foo
+
+  $ hg import --no-commit - << EOF
+  > diff a/foo b/foo
+  > --- a/foo
+  > +++ b/foo
+  > @@ -0,1 +0,1 @@
+  >  foo
+  > EOF
+  applying patch from stdin
+
+  $ cd ..
+
 Test corner case involving fuzz and skew
 
   $ hg init morecornercases