From 3e5a188f1d5b48dcc0bc73ad520925cdb846dfaf Mon Sep 17 00:00:00 2001
From: Johan Herland <johan@herland.net>
Date: Sun, 30 May 2010 15:37:17 +0200
Subject: [PATCH] diff.c: Ensure "index $from..$to" line contains unambiguous
 SHA1s

In the metainfo section of git diffs there's an "index" line providing
abbreviated (unless --full-index is used) blob SHA1s from the
pre-/post-images used to generate the diff. These provide hints that
can be used to reconstruct a 3-way merge when applying the patch
(see the --3way option to 'git am' for more details).

In order for this to work, however, the blob SHA1s must not be
abbreviated into ambiguity.

This patch eliminates the possible ambiguity by using find_unique_abbrev()
to produce the abbreviated SHA1s (instead of blind abbreviation by way of
"%.*s").

A testcase verifying the fix is also included.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 diff.c                              |  6 ++---
 t/t4044-diff-index-unique-abbrev.sh | 35 +++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+), 3 deletions(-)
 create mode 100755 t/t4044-diff-index-unique-abbrev.sh

diff --git a/diff.c b/diff.c
index 494f5601e..1aefa6637 100644
--- a/diff.c
+++ b/diff.c
@@ -2419,9 +2419,9 @@ static void fill_metainfo(struct strbuf *msg,
 			    (!fill_mmfile(&mf, two) && diff_filespec_is_binary(two)))
 				abbrev = 40;
 		}
-		strbuf_addf(msg, "index %.*s..%.*s",
-			    abbrev, sha1_to_hex(one->sha1),
-			    abbrev, sha1_to_hex(two->sha1));
+		strbuf_addf(msg, "index %s..",
+			    find_unique_abbrev(one->sha1, abbrev));
+		strbuf_addstr(msg, find_unique_abbrev(two->sha1, abbrev));
 		if (one->mode == two->mode)
 			strbuf_addf(msg, " %06o", one->mode);
 		strbuf_addch(msg, '\n');
diff --git a/t/t4044-diff-index-unique-abbrev.sh b/t/t4044-diff-index-unique-abbrev.sh
new file mode 100755
index 000000000..d5ce72be6
--- /dev/null
+++ b/t/t4044-diff-index-unique-abbrev.sh
@@ -0,0 +1,35 @@
+#!/bin/sh
+
+test_description='test unique sha1 abbreviation on "index from..to" line'
+. ./test-lib.sh
+
+cat >expect_initial <<EOF
+100644 blob 51d2738463ea4ca66f8691c91e33ce64b7d41bb1	foo
+EOF
+
+cat >expect_update <<EOF
+100644 blob 51d2738efb4ad8a1e40bed839ab8e116f0a15e47	foo
+EOF
+
+test_expect_success 'setup' '
+	echo 4827 > foo &&
+	git add foo &&
+	git commit -m "initial" &&
+	git cat-file -p HEAD: > actual &&
+	test_cmp expect_initial actual &&
+	echo 11742 > foo &&
+	git commit -a -m "update" &&
+	git cat-file -p HEAD: > actual &&
+	test_cmp expect_update actual
+'
+
+cat >expect <<EOF
+index 51d27384..51d2738e 100644
+EOF
+
+test_expect_success 'diff does not produce ambiguous index line' '
+	git diff HEAD^..HEAD | grep index > actual &&
+	test_cmp expect actual
+'
+
+test_done
-- 
2.30.2