Thanks again! As mentioned, I've taken the amendments Ævar posted in
[1] and fixed the bug mentioned there. The only things I've changed
were some punctuation and changing the base commit to upstream's
ab/submodule-cleanup (which is based off a slightly older 'master').

Note for Junio: this version is based on ab/submodule-cleanup.

= Description

This series is a refactor of "git submodule--helper update" that replaces
"--recursive-prefix" with "--super-prefix" (see Background). This was
initially motivated by:

 * Junio's suggestion to simplify the code [2] (in response to a memory leak
   found by Johannes Schindelin [3]).
 * A desire to use the module_list API + get_submodule_displaypath() outside
   of builtin/submodule--helper.c (I expect to use this to check out
   branches in each submodule).

But it also happens to remove some overly complicated/duplicated code that
was literally converted from shell :)

= Background

When recursing into nested submodules, Git commands keep track of the path
from the superproject to the submodule in order to print a "display path" to
the user, e.g.

  Submodule path '../super/sub/nested-sub': checked out 'abcdef'

For historical reasons, "git submodule--helper update" uses
"--recursive-prefix" for this purpose, but it should use "--super-prefix"
instead because:

 * That's what every other command uses (not just submodule--helper
   subcommands).
 * Using the "--super-prefix" helper functions makes the "git
   submodule--helper update" code simpler

= Patch organization

1/7 and 5/7 were contributed by Ævar (thanks!)

* Patches 1-2/7 makes sure that display paths are only computed using
  display path helper functions ([do_]get_submodule_displaypath()) and
  fixes some display paths that we never realized were broken.
* Patches 3-4/7 simplify and deduplicate some display path computations.
* Patch 5/7 removes SUPPORT_SUPER_PREFIX where it's not needed.
  * This doesn't affect correctness, but we want to do this eventually, so
    do it now to make 6/7 a bit cleaner.
* Patch 6/7 replaces "--recursive-prefix" with "--super-prefix", making
  do_get_submodule_displaypath() obsolete.
* Patch 7/7 removes do_get_submodule_displaypath().

= Series history

Changes in v4:
* Split patch 1 so that the missing test coverage is introduced in its
  own patch
* Fix a stale commit message in 1/6 (now patch 2/7)
* Use test_cmp instead of "grep -F"
* Style fixes and improvements

Changes in v3:
* None (resend of v2 because v2 accidentally included
  ab/submodule-cleanup)

Changes in v2:
 * Rebase onto ab/submodule-cleanup (previously master)
 * Cherry pick https://github.com/avar/git/commit/f445c57490d into 4/6
 * Style fixes in .c and tests

= Future work

At the end of this series, get_submodule_displaypath() can be moved to
submodule.h, which would make submodule.c:get_super_prefix_or_empty()
obsolete. I have a patch that demonstrates this (CI run: [4]), but I decided
to keep this series more focused on "git submodule--helper update" so that
it's easier to review.

[1] https://lore.kernel.org/git/220701.865ykhdboc.gmgdl@evledraar.gmail.com (local)
[2] https://lore.kernel.org/git/xmqq35g5xmmv.fsf@gitster.g (local)
[3] https://lore.kernel.org/git/877a45867ae368bf9e053caedcb6cf421e02344d.1655336146.git.gitgitgadget@gmail.com (local)
[4] https://github.com/chooglen/git/actions/runs/2572557584

Glen Choo (6):
  submodule--helper tests: add missing "display path" coverage
  submodule--helper update: use display path helper
  submodule--helper: don't recreate recursive prefix
  submodule--helper: use correct display path helper
  submodule--helper update: use --super-prefix
  submodule--helper: remove display path helper

Ævar Arnfjörð Bjarmason (1):
  submodule--helper: remove unused SUPPORT_SUPER_PREFIX flags

 builtin/submodule--helper.c | 86 ++++++++++---------------------------
 t/t7406-submodule-update.sh | 62 ++++++++++++++++++++++++++
 2 files changed, 84 insertions(+), 64 deletions(-)

Range-diff against v3:
1:  8c82518d33 ! 1:  1f2ef5f6a2 submodule--helper update: use display path helper
    @@ Metadata
     Author: Glen Choo [off-list ref]
     
      ## Commit message ##
    -    submodule--helper update: use display path helper
    +    submodule--helper tests: add missing "display path" coverage
     
         There are two locations in prepare_to_clone_next_submodule() that
    -    manually calculate the submodule display path, but should just use
    -    do_get_submodule_displaypath() for consistency.
    -
    -    Do this replacement and reorder the code slightly to avoid computing
    -    the display path twice.
    -
    -    This code was never tested, and adding tests shows that both these sites
    -    have been computing the display path incorrectly ever since they were
    -    introduced in 48308681b0 (git submodule update: have a dedicated helper
    -    for cloning, 2016-02-29) [1]:
    -
    -    - The first hunk puts a "/" between recursive_prefix and ce->name, but
    -      recursive_prefix already ends with "/".
    -    - The second hunk calls relative_path() on recursive_prefix and
    -      ce->name, but relative_path() only makes sense when both paths share
    -      the same base directory. This is never the case here:
    -      - recursive_prefix is the path from the topmost superproject to the
    -        current submodule
    -      - ce->name is the path from the root of the current submodule to its
    -        submodule.
    -      so, e.g. recursive_prefix="super" and ce->name="submodule" produces
    -      displayname="../super" instead of "super/submodule".
    -
    -    [1] I verified this by applying the tests to 48308681b0.
    +    manually calculate the submodule display path. As discussed in the
    +    next commit the "Skipping" output isn't exactly what we want, but
    +    let's test how we behave now, before changing the existing behavior.
     
         Signed-off-by: Glen Choo [off-list ref]
    -
    - ## builtin/submodule--helper.c ##
    -@@ builtin/submodule--helper.c: static int prepare_to_clone_next_submodule(const struct cache_entry *ce,
    - 	const char *update_string;
    - 	enum submodule_update_type update_type;
    - 	char *key;
    --	struct strbuf displaypath_sb = STRBUF_INIT;
    - 	struct strbuf sb = STRBUF_INIT;
    --	const char *displaypath = NULL;
    -+	char *displaypath;
    - 	int needs_cloning = 0;
    - 	int need_free_url = 0;
    - 
    -+	displaypath = do_get_submodule_displaypath(ce->name,
    -+						   suc->update_data->prefix,
    -+						   suc->update_data->recursive_prefix);
    -+
    - 	if (ce_stage(ce)) {
    --		if (suc->update_data->recursive_prefix)
    --			strbuf_addf(&sb, "%s/%s", suc->update_data->recursive_prefix, ce->name);
    --		else
    --			strbuf_addstr(&sb, ce->name);
    --		strbuf_addf(out, _("Skipping unmerged submodule %s"), sb.buf);
    -+		strbuf_addf(out, _("Skipping unmerged submodule %s"), displaypath);
    - 		strbuf_addch(out, '\n');
    - 		goto cleanup;
    - 	}
    - 
    - 	sub = submodule_from_path(the_repository, null_oid(), ce->name);
    - 
    --	if (suc->update_data->recursive_prefix)
    --		displaypath = relative_path(suc->update_data->recursive_prefix,
    --					    ce->name, &displaypath_sb);
    --	else
    --		displaypath = ce->name;
    --
    - 	if (!sub) {
    - 		next_submodule_warn_missing(suc, out, displaypath);
    - 		goto cleanup;
    -@@ builtin/submodule--helper.c: static int prepare_to_clone_next_submodule(const struct cache_entry *ce,
    - 					      "--no-single-branch");
    - 
    - cleanup:
    --	strbuf_release(&displaypath_sb);
    -+	free(displaypath);
    - 	strbuf_release(&sb);
    - 	if (need_free_url)
    - 		free((void*)url);
    +    Signed-off-by: Ævar Arnfjörð Bjarmason [off-list ref]
     
      ## t/t7406-submodule-update.sh ##
     @@ t/t7406-submodule-update.sh: test_expect_success 'submodule update --filter sets partial clone settings' '
    @@ t/t7406-submodule-update.sh: test_expect_success 'submodule update --filter sets
     +	# because of its unmerged state
     +	test_config -C top-cloned submodule.middle.update !true &&
     +	git -C top-cloned submodule update --recursive 2>actual.err &&
    -+	grep -F "Skipping unmerged submodule middle/bottom" actual.err
    ++	cat >expect.err <<-\EOF &&
    ++	Skipping unmerged submodule middle//bottom
    ++	EOF
    ++	test_cmp expect.err actual.err
     +'
     +
     +test_expect_success 'submodule update --recursive skip submodules with strategy=none' '
    @@ t/t7406-submodule-update.sh: test_expect_success 'submodule update --filter sets
     +	test_commit -C top-cloned/middle/bottom downstream_commit &&
     +	git -C top-cloned/middle config submodule.bottom.update none &&
     +	git -C top-cloned submodule update --recursive 2>actual.err &&
    -+	grep -F "Skipping submodule ${SQ}middle/bottom${SQ}" actual.err
    ++	cat >expect.err <<-\EOF &&
    ++	Skipping submodule '\''../middle/'\''
    ++	EOF
    ++	test_cmp expect.err actual.err
     +'
     +
      test_done
-:  ---------- > 2:  146b88eaa3 submodule--helper update: use display path helper
2:  102ada974d ! 3:  a744640cd3 submodule--helper: don't recreate recursive prefix
    @@ builtin/submodule--helper.c: static void update_data_to_args(struct update_data
     -	if (update_data->recursive_prefix)
     -		strvec_pushl(args, "--recursive-prefix",
     -			     update_data->recursive_prefix, NULL);
    -+	if (update_data->displaypath)
    -+		strvec_pushf(args, "--recursive-prefix=%s/",
    -+			     update_data->displaypath);
    ++	if (update_data->displaypath) {
    ++		strvec_push(args, "--recursive-prefix");
    ++		strvec_pushf(args, "%s/", update_data->displaypath);
    ++	}
      	if (update_data->quiet)
      		strvec_push(args, "--quiet");
      	if (update_data->force)
3:  b6dda65084 = 4:  0b45f2ff2b submodule--helper: use correct display path helper
4:  aa5d389bb8 = 5:  963c8ba07b submodule--helper: remove unused SUPPORT_SUPER_PREFIX flags
5:  4dcfb9889f ! 6:  a777fcf905 submodule--helper update: use --super-prefix
    @@ builtin/submodule--helper.c: struct submodule_update_clone {
      	enum submodule_update_type update_default;
      	struct object_id suboid;
     @@ builtin/submodule--helper.c: static int prepare_to_clone_next_submodule(const struct cache_entry *ce,
    - 
    - 	displaypath = do_get_submodule_displaypath(ce->name,
    - 						   suc->update_data->prefix,
    --						   suc->update_data->recursive_prefix);
    -+						   get_super_prefix());
    - 
    - 	if (ce_stage(ce)) {
    - 		strbuf_addf(out, _("Skipping unmerged submodule %s"), displaypath);
    + 	char *key;
    + 	struct update_data *ud = suc->update_data;
    + 	char *displaypath = do_get_submodule_displaypath(ce->name, ud->prefix,
    +-							 ud->recursive_prefix);
    ++							 get_super_prefix());
    + 	struct strbuf sb = STRBUF_INIT;
    + 	int needs_cloning = 0;
    + 	int need_free_url = 0;
     @@ builtin/submodule--helper.c: static void update_data_to_args(struct update_data *update_data, struct strvec *
      {
      	enum submodule_update_type update_type = update_data->update_default;
      
     -	strvec_pushl(args, "submodule--helper", "update", "--recursive", NULL);
     -	strvec_pushf(args, "--jobs=%d", update_data->max_jobs);
    - 	if (update_data->displaypath)
    --		strvec_pushf(args, "--recursive-prefix=%s/",
    -+		strvec_pushf(args, "--super-prefix=%s/",
    - 			     update_data->displaypath);
    + 	if (update_data->displaypath) {
    +-		strvec_push(args, "--recursive-prefix");
    ++		strvec_push(args, "--super-prefix");
    + 		strvec_pushf(args, "%s/", update_data->displaypath);
    + 	}
     +	strvec_pushl(args, "submodule--helper", "update", "--recursive", NULL);
     +	strvec_pushf(args, "--jobs=%d", update_data->max_jobs);
      	if (update_data->quiet)
6:  109c55236d ! 7:  d47ea17cc2 submodule--helper: remove display path helper
    @@ builtin/submodule--helper.c: static void init_submodule(const char *path, const
      	sub = submodule_from_path(the_repository, null_oid(), path);
      
     @@ builtin/submodule--helper.c: static int prepare_to_clone_next_submodule(const struct cache_entry *ce,
    + 	enum submodule_update_type update_type;
    + 	char *key;
    + 	struct update_data *ud = suc->update_data;
    +-	char *displaypath = do_get_submodule_displaypath(ce->name, ud->prefix,
    +-							 get_super_prefix());
    ++	char *displaypath = get_submodule_displaypath(ce->name, ud->prefix);
    + 	struct strbuf sb = STRBUF_INIT;
      	int needs_cloning = 0;
      	int need_free_url = 0;
    - 
    --	displaypath = do_get_submodule_displaypath(ce->name,
    --						   suc->update_data->prefix,
    --						   get_super_prefix());
    -+	displaypath =
    -+		get_submodule_displaypath(ce->name, suc->update_data->prefix);
    - 
    - 	if (ce_stage(ce)) {
    - 		strbuf_addf(out, _("Skipping unmerged submodule %s"), displaypath);
     @@ builtin/submodule--helper.c: static int update_submodule(struct update_data *update_data)
      {
      	ensure_core_worktree(update_data->sm_path);

base-commit: 5b893f7d81eb7feb43662ed8663e2af76a76b4c8
-- 
2.37.0.rc0.161.g10f37bed90-goog