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diff --git a/t/t5320-delta-islands.sh b/t/t5320-delta-islands.sh
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+#!/bin/sh
+
+test_description='exercise delta islands'
+
+TEST_PASSES_SANITIZE_LEAK=true
+. ./test-lib.sh
+
+# returns true iff $1 is a delta based on $2
+is_delta_base () {
+	delta_base=$(echo "$1" | git cat-file --batch-check='%(deltabase)') &&
+	echo >&2 "$1 has base $delta_base" &&
+	test "$delta_base" = "$2"
+}
+
+# generate a commit on branch $1 with a single file, "file", whose
+# content is mostly based on the seed $2, but with a unique bit
+# of content $3 appended. This should allow us to see whether
+# blobs of different refs delta against each other.
+commit() {
+	blob=$({ test-tool genrandom "$2" 10240 && echo "$3"; } |
+	       git hash-object -w --stdin) &&
+	tree=$(printf '100644 blob %s\tfile\n' "$blob" | git mktree) &&
+	commit=$(echo "$2-$3" | git commit-tree "$tree" ${4:+-p "$4"}) &&
+	git update-ref "refs/heads/$1" "$commit" &&
+	eval "$1"'=$(git rev-parse $1:file)' &&
+	eval "echo >&2 $1=\$$1"
+}
+
+test_expect_success 'setup commits' '
+	commit one seed 1 &&
+	commit two seed 12
+'
+
+# Note: This is heavily dependent on the "prefer larger objects as base"
+# heuristic.
+test_expect_success 'vanilla repack deltas one against two' '
+	git repack -adf &&
+	is_delta_base $one $two
+'
+
+test_expect_success 'island repack with no island definition is vanilla' '
+	git repack -adfi &&
+	is_delta_base $one $two
+'
+
+test_expect_success 'island repack with no matches is vanilla' '
+	git -c "pack.island=refs/foo" repack -adfi &&
+	is_delta_base $one $two
+'
+
+test_expect_success 'separate islands disallows delta' '
+	git -c "pack.island=refs/heads/(.*)" repack -adfi &&
+	! is_delta_base $one $two &&
+	! is_delta_base $two $one
+'
+
+test_expect_success 'same island allows delta' '
+	git -c "pack.island=refs/heads" repack -adfi &&
+	is_delta_base $one $two
+'
+
+test_expect_success 'coalesce same-named islands' '
+	git \
+		-c "pack.island=refs/(.*)/one" \
+		-c "pack.island=refs/(.*)/two" \
+		repack -adfi &&
+	is_delta_base $one $two
+'
+
+test_expect_success 'island restrictions drop reused deltas' '
+	git repack -adfi &&
+	is_delta_base $one $two &&
+	git -c "pack.island=refs/heads/(.*)" repack -adi &&
+	! is_delta_base $one $two &&
+	! is_delta_base $two $one
+'
+
+test_expect_success 'island regexes are left-anchored' '
+	git -c "pack.island=heads/(.*)" repack -adfi &&
+	is_delta_base $one $two
+'
+
+test_expect_success 'island regexes follow last-one-wins scheme' '
+	git \
+		-c "pack.island=refs/heads/(.*)" \
+		-c "pack.island=refs/heads/" \
+		repack -adfi &&
+	is_delta_base $one $two
+'
+
+test_expect_success 'setup shared history' '
+	commit root shared root &&
+	commit one shared 1 root &&
+	commit two shared 12-long root
+'
+
+# We know that $two will be preferred as a base from $one,
+# because we can transform it with a pure deletion.
+#
+# We also expect $root as a delta against $two by the "longest is base" rule.
+test_expect_success 'vanilla delta goes between branches' '
+	git repack -adf &&
+	is_delta_base $one $two &&
+	is_delta_base $root $two
+'
+
+# Here we should allow $one to base itself on $root; even though
+# they are in different islands, the objects in $root are in a superset
+# of islands compared to those in $one.
+#
+# Similarly, $two can delta against $root by our rules. And unlike $one,
+# in which we are just allowing it, the island rules actually put $root
+# as a possible base for $two, which it would not otherwise be (due to the size
+# sorting).
+test_expect_success 'deltas allowed against superset islands' '
+	git -c "pack.island=refs/heads/(.*)" repack -adfi &&
+	is_delta_base $one $root &&
+	is_delta_base $two $root
+'
+
+# We are going to test the packfile order here, so we again have to make some
+# assumptions. We assume that "$root", as part of our core "one", must come
+# before "$two". This should be guaranteed by the island code. However, for
+# this test to fail without islands, we are also assuming that it would not
+# otherwise do so. This is true by the current write order, which will put
+# commits (and their contents) before their parents.
+test_expect_success 'island core places core objects first' '
+	cat >expect <<-EOF &&
+	$root
+	$two
+	EOF
+	git -c "pack.island=refs/heads/(.*)" \
+	    -c "pack.islandcore=one" \
+	    repack -adfi &&
+	git verify-pack -v .git/objects/pack/*.pack |
+	cut -d" " -f1 |
+	grep -E "$root|$two" >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'unmatched island core is not fatal' '
+	git -c "pack.islandcore=one" repack -adfi
+'
+
+test_done