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+#!/bin/sh
+
+test_description='test handling of inter-pack delta cycles during repack
+
+The goal here is to create a situation where we have two blobs, A and B, with A
+as a delta against B in one pack, and vice versa in the other. Then if we can
+persuade a full repack to find A from one pack and B from the other, that will
+give us a cycle when we attempt to reuse those deltas.
+
+The trick is in the "persuade" step, as it depends on the internals of how
+pack-objects picks which pack to reuse the deltas from. But we can assume
+that it does so in one of two general strategies:
+
+ 1. Using a static ordering of packs. In this case, no inter-pack cycles can
+    happen. Any objects with a delta relationship must be present in the same
+    pack (i.e., no "--thin" packs on disk), so we will find all related objects
+    from that pack. So assuming there are no cycles within a single pack (and
+    we avoid generating them via pack-objects or importing them via
+    index-pack), then our result will have no cycles.
+
+    So this case should pass the tests no matter how we arrange things.
+
+ 2. Picking the next pack to examine based on locality (i.e., where we found
+    something else recently).
+
+    In this case, we want to make sure that we find the delta versions of A and
+    B and not their base versions. We can do this by putting two blobs in each
+    pack. The first is a "dummy" blob that can only be found in the pack in
+    question.  And then the second is the actual delta we want to find.
+
+    The two blobs must be present in the same tree, not present in other trees,
+    and the dummy pathname must sort before the delta path.
+
+The setup below focuses on case 2. We have two commits HEAD and HEAD^, each
+which has two files: "dummy" and "file". Then we can make two packs which
+contain:
+
+  [pack one]
+  HEAD:dummy
+  HEAD:file  (as delta against HEAD^:file)
+  HEAD^:file (as base)
+
+  [pack two]
+  HEAD^:dummy
+  HEAD^:file (as delta against HEAD:file)
+  HEAD:file  (as base)
+
+Then no matter which order we start looking at the packs in, we know that we
+will always find a delta for "file", because its lookup will always come
+immediately after the lookup for "dummy".
+'
+
+TEST_PASSES_SANITIZE_LEAK=true
+. ./test-lib.sh
+
+# Create a pack containing the tree $1 and blob $1:file, with
+# the latter stored as a delta against $2:file.
+#
+# We convince pack-objects to make the delta in the direction of our choosing
+# by marking $2 as a preferred-base edge. That results in $1:file as a thin
+# delta, and index-pack completes it by adding $2:file as a base.
+#
+# Note that the two variants of "file" must be similar enough to convince git
+# to create the delta.
+make_pack () {
+	{
+		printf '%s\n' "-$(git rev-parse $2)"
+		printf '%s dummy\n' "$(git rev-parse $1:dummy)"
+		printf '%s file\n' "$(git rev-parse $1:file)"
+	} |
+	git pack-objects --stdout |
+	git index-pack --stdin --fix-thin
+}
+
+test_expect_success 'setup' '
+	test-tool genrandom base 4096 >base &&
+	for i in one two
+	do
+		# we want shared content here to encourage deltas...
+		cp base file &&
+		echo $i >>file &&
+
+		# ...whereas dummy should be short, because we do not want
+		# deltas that would create duplicates when we --fix-thin
+		echo $i >dummy &&
+
+		git add file dummy &&
+		test_tick &&
+		git commit -m $i ||
+		return 1
+	done &&
+
+	make_pack HEAD^ HEAD &&
+	make_pack HEAD HEAD^
+'
+
+test_expect_success 'repack' '
+	# We first want to check that we do not have any internal errors,
+	# and also that we do not hit the last-ditch cycle-breaking code
+	# in write_object(), which will issue a warning to stderr.
+	git repack -ad 2>stderr &&
+	test_must_be_empty stderr &&
+
+	# And then double-check that the resulting pack is usable (i.e.,
+	# we did not fail to notice any cycles). We know we are accessing
+	# the objects via the new pack here, because "repack -d" will have
+	# removed the others.
+	git cat-file blob HEAD:file >/dev/null &&
+	git cat-file blob HEAD^:file >/dev/null
+'
+
+test_done