add test for streaming corrupt blobs

We do not have many tests for handling corrupt objects. This
new test at least checks that we detect a byte error in a
corrupt blob object while streaming it out with cat-file.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1060-object-corruption.sh

@@ -0,0 +1,34 @@
+#!/bin/sh
+
+test_description='see how we handle various forms of corruption'
+. ./test-lib.sh
+
+# convert "1234abcd" to ".git/objects/12/34abcd"
+obj_to_file() {
+	echo "$(git rev-parse --git-dir)/objects/$(git rev-parse "$1" | sed 's,..,&/,')"
+}
+
+# Convert byte at offset "$2" of object "$1" into '\0'
+corrupt_byte() {
+	obj_file=$(obj_to_file "$1") &&
+	chmod +w "$obj_file" &&
+	printf '\0' | dd of="$obj_file" bs=1 seek="$2" conv=notrunc
+}
+
+test_expect_success 'setup corrupt repo' '
+	git init bit-error &&
+	(
+		cd bit-error &&
+		test_commit content &&
+		corrupt_byte HEAD:content.t 10
+	)
+'
+
+test_expect_success 'streaming a corrupt blob fails' '
+	(
+		cd bit-error &&
+		test_must_fail git cat-file blob HEAD:content.t
+	)
+'
+
+test_done