bundle: test unbundling with incomplete history

When verifying a bundle, Git checks first that all prerequisite commits
exist in the object store, then adds an additional check: those
prerequisite commits must be reachable from references in the
repository.

This check is stronger than what is checked for refs being added during
'git fetch', which simply guarantees that the new refs have a complete
history up to the point where it intersects with the current reachable
history.

However, we also do not have any tests that check the behavior under
this condition. Create a test that demonstrates its behavior.

In order to construct a broken history, perform a shallow clone of a
repository with a linear history, but whose default branch ('base') has
a single commit, so dropping the shallow markers leaves a complete
history from that reference. However, the 'tip' reference adds a
shallow commit whose parent is missing in the cloned repository. Trying
to unbundle a bundle with the 'tip' as a prerequisite will succeed past
the object store check and move into the reachability check.

The two errors that are reported are of this form:

  error: Could not read <missing-commit>
  fatal: Failed to traverse parents of commit <present-commit>

These messages are not particularly helpful for the person running the
unbundle command, but they do prevent the command from succeeding.

Signed-off-by: Derrick Stolee <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

t/t6020-bundle-misc.sh

@@ -559,4 +559,44 @@ test_expect_success 'cloning from filtered bundle has useful error' '
 	grep "cannot clone from filtered bundle" err
 '
 
+test_expect_success 'verify catches unreachable, broken prerequisites' '
+	test_when_finished rm -rf clone-from clone-to &&
+	git init clone-from &&
+	(
+		cd clone-from &&
+		git checkout -b base &&
+		test_commit A &&
+		git checkout -b tip &&
+		git commit --allow-empty -m "will drop by shallow" &&
+		git commit --allow-empty -m "will keep by shallow" &&
+		git commit --allow-empty -m "for bundle, not clone" &&
+		git bundle create tip.bundle tip~1..tip &&
+		git reset --hard HEAD~1 &&
+		git checkout base
+	) &&
+	BAD_OID=$(git -C clone-from rev-parse tip~1) &&
+	TIP_OID=$(git -C clone-from rev-parse tip) &&
+	git clone --depth=1 --no-single-branch \
+		"file://$(pwd)/clone-from" clone-to &&
+	(
+		cd clone-to &&
+
+		# Set up broken history by removing shallow markers
+		git update-ref -d refs/remotes/origin/tip &&
+		rm .git/shallow &&
+
+		# Verify should fail
+		test_must_fail git bundle verify \
+			../clone-from/tip.bundle 2>err &&
+		grep "Could not read $BAD_OID" err &&
+		grep "Failed to traverse parents of commit $TIP_OID" err &&
+
+		# Unbundling should fail
+		test_must_fail git bundle unbundle \
+			../clone-from/tip.bundle 2>err &&
+		grep "Could not read $BAD_OID" err &&
+		grep "Failed to traverse parents of commit $TIP_OID" err
+	)
+'
+
 test_done