Re: [PATCH v8] read-cache: call verify_hdr() in a background thread

[Junio C Hamano <gitster@pobox.com>]

git@jeffhostetler.com writes:

> From: Jeff Hostetler <jeffhost@microsoft.com>
>
> Version 8 of this patch converts the unit test to use
> perl to corrupt the index checksum (rather than altering
> a filename) and also verifies the fsck error message as
> suggested in response to v7 on the mailing list.
>
> If there are no other suggestions, I think this version
> should be considered final.

Oops.  The earlier one has already been in 'master' for a few days.
Let's make this an incremental update.  

Is the description in the following something you are OK with (so
that I can add your sign-off)?

Thanks.

-- >8 --
From: Jeff Hostetler <jeffhost@microsoft.com>
Date: Tue, 25 Apr 2017 18:41:09 +0000
Subject: t1450: avoid use of "sed" on the index, which is a binary file

The previous step added a path zzzzzzzz to the index, and then used
"sed" to replace this string to yyyyyyyy to create a test case where
the checksum at the end of the file does not match the contents.

Unfortunately, use of "sed" on a non-text file is not portable.
Instead, use a Perl script that seeks to the end and modifies the
last byte of the file (where we _know_ stores the trailing
checksum).


---
 t/t1450-fsck.sh | 33 ++++++++++++++++++++++++++-------
 1 file changed, 26 insertions(+), 7 deletions(-)

diff --git a/t/t1450-fsck.sh b/t/t1450-fsck.sh
index 677e15a7a4..eff1cd68e9 100755
--- a/t/t1450-fsck.sh
+++ b/t/t1450-fsck.sh
@@ -689,16 +689,35 @@ test_expect_success 'bogus head does not fallback to all heads' '
 	! grep $blob out
 '
 
+# Corrupt the checksum on the index.
+# Add 1 to the last byte in the SHA.
+corrupt_index_checksum () {
+    perl -w -e '
+	use Fcntl ":seek";
+	open my $fh, "+<", ".git/index" or die "open: $!";
+	binmode $fh;
+	seek $fh, -1, SEEK_END or die "seek: $!";
+	read $fh, my $in_byte, 1 or die "read: $!";
+
+	$in_value = unpack("C", $in_byte);
+	$out_value = ($in_value + 1) & 255;
+
+	$out_byte = pack("C", $out_value);
+
+	seek $fh, -1, SEEK_END or die "seek: $!";
+	print $fh $out_byte;
+	close $fh or die "close: $!";
+    '
+}
+
+# Corrupt the checksum on the index and then
+# verify that only fsck notices.
 test_expect_success 'detect corrupt index file in fsck' '
 	cp .git/index .git/index.backup &&
 	test_when_finished "mv .git/index.backup .git/index" &&
-	echo zzzzzzzz >zzzzzzzz &&
-	git add zzzzzzzz &&
-	sed -e "s/zzzzzzzz/yyyyyyyy/" .git/index >.git/index.yyy &&
-	mv .git/index.yyy .git/index &&
-	# Confirm that fsck detects invalid checksum
-	test_must_fail git fsck --cache &&
-	# Confirm that status no longer complains about invalid checksum
+	corrupt_index_checksum &&
+	test_must_fail git fsck --cache 2>expect &&
+	grep "bad index file" expect &&
 	git status
 '