tests: overwrite bytes in files using a perl script instead of dd

The dd in my build environment on Windows crashes unpredictably. Work it
around by rewriting most instances with a helper function that uses perl
behind the scenes.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>

Author: j6t

t/t1060-object-corruption.sh

@@ -13,7 +13,7 @@ obj_to_file() {
 corrupt_byte() {
 	obj_file=$(obj_to_file "$1") &&
 	chmod +w "$obj_file" &&
-	printf '\0' | dd of="$obj_file" bs=1 seek="$2" conv=notrunc
+	printf '\0' | test_overwrite_bytes "$obj_file" "$2"
 }
 
 test_expect_success 'setup corrupt repo' '

t/t5300-pack-object.sh

@@ -290,7 +290,7 @@ test_expect_success 'verify-pack catches mismatched .idx and .pack files' '
 
 test_expect_success 'verify-pack catches a corrupted pack signature' '
 	cat test-1-${packname_1}.pack >test-3.pack &&
-	echo | dd of=test-3.pack count=1 bs=1 conv=notrunc seek=2 &&
+	echo | test_overwrite_bytes test-3.pack 2 &&
 	if git verify-pack test-3.idx
 	then false
 	else :;
@@ -299,7 +299,7 @@ test_expect_success 'verify-pack catches a corrupted pack signature' '
 
 test_expect_success 'verify-pack catches a corrupted pack version' '
 	cat test-1-${packname_1}.pack >test-3.pack &&
-	echo | dd of=test-3.pack count=1 bs=1 conv=notrunc seek=7 &&
+	echo | test_overwrite_bytes test-3.pack 7 &&
 	if git verify-pack test-3.idx
 	then false
 	else :;
@@ -308,7 +308,7 @@ test_expect_success 'verify-pack catches a corrupted pack version' '
 
 test_expect_success 'verify-pack catches a corrupted type/size of the 1st packed object data' '
 	cat test-1-${packname_1}.pack >test-3.pack &&
-	echo | dd of=test-3.pack count=1 bs=1 conv=notrunc seek=12 &&
+	echo | test_overwrite_bytes test-3.pack 12 &&
 	if git verify-pack test-3.idx
 	then false
 	else :;
@@ -319,7 +319,7 @@ test_expect_success 'verify-pack catches a corrupted sum of the index file itsel
 	l=$(wc -c <test-3.idx) &&
 	l=$(expr $l - 20) &&
 	cat test-1-${packname_1}.pack >test-3.pack &&
-	printf "%20s" "" | dd of=test-3.idx count=20 bs=1 conv=notrunc seek=$l &&
+	printf "%20s" "" | test_overwrite_bytes test-3.idx $l &&
 	if git verify-pack test-3.pack
 	then false
 	else :;

t/t5302-pack-index.sh

@@ -236,8 +236,9 @@ test_expect_success \
 	obj=$(git hash-object file_001) &&
 	nr=$(index_obj_nr ".git/objects/pack/pack-${pack1}.idx" $obj) &&
 	chmod +w ".git/objects/pack/pack-${pack1}.idx" &&
-	printf xxxx | dd of=".git/objects/pack/pack-${pack1}.idx" conv=notrunc \
-		bs=1 count=4 seek=$((8 + 256 * 4 + $(wc -l <obj-list) * rawsz + $nr * 4)) &&
+	printf xxxx |
+		test_overwrite_bytes ".git/objects/pack/pack-${pack1}.idx" \
+			$((8 + 256 * 4 + $(wc -l <obj-list) * rawsz + $nr * 4)) &&
 	 ( while read obj
 	   do git cat-file -p $obj >/dev/null || exit 1
 	   done <obj-list ) &&

t/t5303-pack-corruption-resilience.sh

@@ -57,7 +57,7 @@ do_corrupt_object() {
     ofs=$(git show-index < ${pack}.idx | grep $1 | cut -f1 -d" ") &&
     ofs=$(($ofs + $2)) &&
     chmod +w ${pack}.pack &&
-    dd of=${pack}.pack bs=1 conv=notrunc seek=$ofs &&
+    test_overwrite_bytes "${pack}.pack" "$ofs" &&
     test_must_fail git verify-pack ${pack}.pack
 }
 

t/t5313-pack-bounds-checks.sh

@@ -28,7 +28,7 @@ do_pack () {
 }
 
 munge () {
-	printf "$3" | dd of="$1" bs=1 conv=notrunc seek=$2
+	printf "$3" | test_overwrite_bytes "$1" "$2"
 }
 
 # Offset in a v2 .idx to its initial and extended offset tables. For an index

t/test-lib-functions.sh

@@ -1606,6 +1606,22 @@ test_skip_or_die () {
 	error "$2"
 }
 
+# Overwrite bytes at an offset in a file
+# $1 ... the file to modify
+# $2 ... byte offset into file
+# stdin ... new bytes
+test_overwrite_bytes () {
+	perl -e '
+		$fname = shift @ARGV;
+		$offset = shift @ARGV;
+		$bytes = <>;
+		open my $fh, "+<", $fname	or die "open $fname: $!\n";
+		seek($fh, $offset, 0)		or die "seek $fname: $!\n";
+		syswrite($fh, $bytes)		or die "write $fname: $!\n";
+		close $fh			or die "close $fname: $!\n";
+	' "$@"
+}
+
 # Like "env FOO=BAR some-program", but run inside a subshell, which means
 # it also works for shell functions (though those functions cannot impact
 # the environment outside of the test_env invocation).