Stop relying on SHA1 fallback for `the_hash_algo`

Hi,

this is the third version of my patch series that stops relying on the SHA1 fallback configured for the_hash_algo. Instead, any callers that access the_hash_algo without it being initialized will now crash hard. This is designed to catch various subtle bugs going forward.

The only change compared to v3 of this patch series is a rebase on top of the following two topics:

- ps/the-index-is-no-more, which causes a conflict in the final patch.

- jc/no-default-attr-tree-in-bare, which causes a conflict in the fourth patch.

Interestingly, the range diff does not surface the second resolved conflict. This is because the context is actually the same, but when applying the patch in question you would get a conflict. To aid with this I decided to generate patches with --unified=4, which surfaces the previously-conflicting hunk in compute_default_attr_source().

Patrick

Patrick Steinhardt (13):
path: harden validation of HEAD with non-standard hashes
path: move validate_headref() to its only user
parse-options-cb: only abbreviate hashes when hash algo is known
attr: don't recompute default attribute source
attr: fix BUG() when parsing attrs outside of repo
remote-curl: fix parsing of detached SHA256 heads builtin/rev-parse: allow shortening to more than 40 hex characters builtin/blame: don't access potentially unitialized the_hash_algo builtin/bundle: abort "verify" early when there is no repository builtin/diff: explicitly set hash algo when there is no repo builtin/shortlog: don't set up revisions without repo oss-fuzz/commit-graph: set up hash algorithm
repository: stop setting SHA1 as the default object hash

 attr.c                       | 31 +++++++++++++++------
 builtin/blame.c              |  5 ++--
 builtin/bundle.c             |  5 ++++
 builtin/diff.c               |  9 ++++++
 builtin/rev-parse.c          |  5 ++--
 builtin/shortlog.c           |  2 +-
 oss-fuzz/fuzz-commit-graph.c |  1 +
 parse-options-cb.c           |  3 +-
 path.c                       | 53 ------------------------------------
 path.h                       |  1 -
 remote-curl.c                | 19 ++++++++++++-
 repository.c                 | 20 --------------
 setup.c                      | 53 ++++++++++++++++++++++++++++++++++++
 t/t0003-attributes.sh        | 15 ++++++++++
 t/t0040-parse-options.sh     | 17 ++++++++++++
 t/t1500-rev-parse.sh         |  6 ++++
 t/t5550-http-fetch-dumb.sh   | 15 ++++++++++
 17 files changed, 168 insertions(+), 92 deletions(-)

Range-diff against v3:

 1:  5134f35cda =  1:  5ee372c2d8 path: harden validation of HEAD with non-standard hashes
 2:  589b6a99ef =  2:  ece0ab94a8 path: move `validate_headref()` to its only user
 3:  9a63c445d2 =  3:  1a0859eaf1 parse-options-cb: only abbreviate hashes when hash algo is known
 4:  929bacbfce =  4:  1204a34216 attr: don't recompute default attribute source
 5:  8f20aec1ee =  5:  f098ce9690 attr: fix BUG() when parsing attrs outside of repo
 6:  53439067a1 =  6:  e8876052ad remote-curl: fix parsing of detached SHA256 heads
 7:  1f74960760 =  7:  6116030310 builtin/rev-parse: allow shortening to more than 40 hex characters
 8:  2d985abca1 =  8:  872ded113e builtin/blame: don't access potentially unitialized `the_hash_algo`
 9:  f3b23d28aa =  9:  5b4a21d2ce builtin/bundle: abort "verify" early when there is no repository
10:  7577b6b96c = 10:  e7254ae757 builtin/diff: explicitly set hash algo when there is no repo
11:  509c79d1d3 = 11:  c21b15ba17 builtin/shortlog: don't set up revisions without repo
12:  660f976129 = 12:  f37beb0e89 oss-fuzz/commit-graph: set up hash algorithm
13:  95909c2da5 ! 13:  950b08bc78 repository: stop setting SHA1 as the default object hash

@@ Commit message

## repository.c ##
-@@ repository.c: void initialize_the_repository(void)
- the_repo.parsed_objects = parsed_object_pool_new();
-
- index_state_init(&the_index, the_repository);
+@@ repository.c: void initialize_repository(struct repository *repo)
+ repo->parsed_objects = parsed_object_pool_new();
+ ALLOC_ARRAY(repo->index, 1);
+ index_state_init(repo->index, repo);
-
-- repo_set_hash_algo(&the_repo, GIT_HASH_SHA1);
+- /*
+- * Unfortunately, we need to keep this hack around for the time being:
+- *
+- * - Not setting up the hash algorithm for the_repository leads to
+- * crashes because the_hash_algo is a macro that expands to
+- * the_repository->hash_algo. So if Git commands try to access
+- * the_hash_algo without a Git directory we crash.
+- *
+- * - Setting up the hash algorithm to be SHA1 by default breaks other
+- * commands when running with SHA256.
+- *
+- * This is another point in case why having global state is a bad idea.
+- * Eventually, we should remove this hack and stop setting the hash
+- * algorithm in this function altogether. Instead, it should only ever
+- * be set via our repository setup procedures. But that requires more
+- * work.
+- */
+- if (repo == the_repository)
+- repo_set_hash_algo(repo, GIT_HASH_SHA1);
}

static void expand_base_dir(char **out, const char *in,

2.45.0