t: introduce tests for unexpected object types

Call an object's type "unexpected" when the actual type of an object
does not match Git's contextual expectation. For example, a tree entry
whose mode differs from the object's actual type, or a commit's parent
which is not another commit, and so on.

This can manifest itself in various unfortunate ways, including Git
SIGSEGV-ing under specific conditions. Consider the following example:
Git traverses a blob (say, via `git rev-list`), and then tries to read
out a tree-entry which lists that object as something other than a blob.
In this case, `lookup_blob()` will return NULL, and the subsequent
dereference will result in a SIGSEGV.

Introduce tests that present objects of "unexpected" type in the above
fashion to 'git rev-list'. Mark as failures the combinations that are
already broken (i.e., they exhibit the segfault described above). In the
cases that are not broken (i.e., they have NULL-ness checks or similar),
mark these as expecting success.

We might hit an unexpected type in two different ways (imagine we have a
tree entry that claims to be a tree but actually points to a blob):

- when we call lookup_tree(), we might find that we've already seen
the object referenced as a blob, in which case we'd get NULL. We
can exercise this with "git rev-list --objects $blob $tree", which
guarantees that the blob will have been parsed before we look in
the tree. These tests are marked as "seen" in the test script.

- we call lookup_tree() successfully, but when we try to read the
object, we find out it's something else. We construct our tests
such that $blob is not otherwise mentioned in $tree. These tests
are marked as "lone" in the script.

We should check that we behave sensibly in both cases (especially
because it is easy for a malicious actor to provoke one case or the
other).

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/t/t6102-rev-list-unexpected-objects.sh b/t/t6102-rev-list-unexpected-objects.sh
new file mode 100755 (executable)
index 0000000..15072ec
--- /dev/null
+++ b/t/t6102-rev-list-unexpected-objects.sh
@@ -0,0 +1,123 @@
+#!/bin/sh
+
+test_description='git rev-list should handle unexpected object types'
+
+. ./test-lib.sh
+
+test_expect_success 'setup well-formed objects' '
+       blob="$(printf "foo" | git hash-object -w --stdin)" &&
+       tree="$(printf "100644 blob $blob\tfoo" | git mktree)" &&
+       commit="$(git commit-tree $tree -m "first commit")" &&
+       git cat-file commit $commit >good-commit
+'
+
+test_expect_success 'setup unexpected non-blob entry' '
+       printf "100644 foo\0$(echo $tree | hex2oct)" >broken-tree &&
+       broken_tree="$(git hash-object -w --literally -t tree broken-tree)"
+'
+
+test_expect_failure 'traverse unexpected non-blob entry (lone)' '
+       test_must_fail git rev-list --objects $broken_tree
+'
+
+test_expect_failure 'traverse unexpected non-blob entry (seen)' '
+       test_must_fail git rev-list --objects $tree $broken_tree
+'
+
+test_expect_success 'setup unexpected non-tree entry' '
+       printf "40000 foo\0$(echo $blob | hex2oct)" >broken-tree &&
+       broken_tree="$(git hash-object -w --literally -t tree broken-tree)"
+'
+
+test_expect_failure 'traverse unexpected non-tree entry (lone)' '
+       test_must_fail git rev-list --objects $broken_tree
+'
+
+test_expect_failure 'traverse unexpected non-tree entry (seen)' '
+       test_must_fail git rev-list --objects $blob $broken_tree
+'
+
+test_expect_success 'setup unexpected non-commit parent' '
+       sed "/^author/ { h; s/.*/parent $blob/; G; }" <good-commit \
+               >broken-commit &&
+       broken_commit="$(git hash-object -w --literally -t commit \
+               broken-commit)"
+'
+
+test_expect_success 'traverse unexpected non-commit parent (lone)' '
+       test_must_fail git rev-list --objects $broken_commit >output 2>&1 &&
+       test_i18ngrep "not a commit" output
+'
+
+test_expect_success 'traverse unexpected non-commit parent (seen)' '
+       test_must_fail git rev-list --objects $commit $broken_commit \
+               >output 2>&1 &&
+       test_i18ngrep "not a commit" output
+'
+
+test_expect_success 'setup unexpected non-tree root' '
+       sed -e "s/$tree/$blob/" <good-commit >broken-commit &&
+       broken_commit="$(git hash-object -w --literally -t commit \
+               broken-commit)"
+'
+
+test_expect_failure 'traverse unexpected non-tree root (lone)' '
+       test_must_fail git rev-list --objects $broken_commit
+'
+
+test_expect_failure 'traverse unexpected non-tree root (seen)' '
+       test_must_fail git rev-list --objects $blob $broken_commit
+'
+
+test_expect_success 'setup unexpected non-commit tag' '
+       git tag -a -m "tagged commit" tag $commit &&
+       git cat-file tag tag >good-tag &&
+       test_when_finished "git tag -d tag" &&
+       sed -e "s/$commit/$blob/" <good-tag >broken-tag &&
+       tag=$(git hash-object -w --literally -t tag broken-tag)
+'
+
+test_expect_success 'traverse unexpected non-commit tag (lone)' '
+       test_must_fail git rev-list --objects $tag
+'
+
+test_expect_success 'traverse unexpected non-commit tag (seen)' '
+       test_must_fail git rev-list --objects $blob $tag >output 2>&1 &&
+       test_i18ngrep "not a commit" output
+'
+
+test_expect_success 'setup unexpected non-tree tag' '
+       git tag -a -m "tagged tree" tag $tree &&
+       git cat-file tag tag >good-tag &&
+       test_when_finished "git tag -d tag" &&
+       sed -e "s/$tree/$blob/" <good-tag >broken-tag &&
+       tag=$(git hash-object -w --literally -t tag broken-tag)
+'
+
+test_expect_success 'traverse unexpected non-tree tag (lone)' '
+       test_must_fail git rev-list --objects $tag
+'
+
+test_expect_success 'traverse unexpected non-tree tag (seen)' '
+       test_must_fail git rev-list --objects $blob $tag >output 2>&1 &&
+       test_i18ngrep "not a tree" output
+'
+
+test_expect_success 'setup unexpected non-blob tag' '
+       git tag -a -m "tagged blob" tag $blob &&
+       git cat-file tag tag >good-tag &&
+       test_when_finished "git tag -d tag" &&
+       sed -e "s/$blob/$commit/" <good-tag >broken-tag &&
+       tag=$(git hash-object -w --literally -t tag broken-tag)
+'
+
+test_expect_failure 'traverse unexpected non-blob tag (lone)' '
+       test_must_fail git rev-list --objects $tag
+'
+
+test_expect_success 'traverse unexpected non-blob tag (seen)' '
+       test_must_fail git rev-list --objects $commit $tag >output 2>&1 &&
+       test_i18ngrep "not a blob" output
+'
+
+test_done