[PATCH v3 0/4] cache parent project's gitdir in submodules

Since v2, mostly documentation changes and a handful of small nits from Junio and Jonathan Tan. Thanks for the reviews, both.

CI run here: https://github.com/nasamuffin/git/actions/runs/1147984974

 - Emily

Emily Shaffer (4):
  t7400-submodule-basic: modernize inspect() helper
  introduce submodule.superprojectGitDir record
  submodule: record superproject gitdir during absorbgitdirs
  submodule: record superproject gitdir during 'update'

 Documentation/config/submodule.txt | 15 ++++++++++
 builtin/submodule--helper.c        |  4 +++
 git-submodule.sh                   | 10 +++++++
 submodule.c                        | 10 +++++++
 t/t7400-submodule-basic.sh         | 48 ++++++++++++++----------------
 t/t7406-submodule-update.sh        | 10 +++++++
 t/t7412-submodule-absorbgitdirs.sh |  9 +++++-
 7 files changed, 79 insertions(+), 27 deletions(-)

Range-diff against v2:
1:  a6718eea80 ! 1:  f1236dc9d7 t7400-submodule-basic: modernize inspect() helper
    @@ t/t7400-submodule-basic.sh: test_expect_success 'setup - repository to add submo
     +	git -C "$dir" clean -n -d -x >untracked
      }
      
    -+
      test_expect_success 'submodule add' '
    - 	echo "refs/heads/main" >expect &&
    - 
     @@ t/t7400-submodule-basic.sh: test_expect_success 'submodule add' '
      	) &&
      
2:  4cebe7bcb5 ! 2:  2caf9eb8ee introduce submodule.superprojectGitDir cache
    @@ Metadata
     Author: Emily Shaffer <emilyshaffer@google.com>
     
      ## Commit message ##
    -    introduce submodule.superprojectGitDir cache
    +    introduce submodule.superprojectGitDir record
     
         Teach submodules a reference to their superproject's gitdir. This allows
         us to A) know that we're running from a submodule, and B) have a
    @@ Commit message
         superproject directory around on the filesystem without breaking the
         submodule's cache.
     
    -    Since this cached value is only introduced during new submodule creation
    +    Since this hint value is only introduced during new submodule creation
         via `git submodule add`, though, there is more work to do to allow the
    -    cache to be created at other times.
    +    record to be created at other times.
    +
    +    If the new config is present, we can do some optional value-added
    +    behavior, like letting "git status" print additional info about the
    +    submodule's status in relation to its superproject, or like letting the
    +    superproject and submodule share an additional config file separate from
    +    either one's local config.
     
         Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
    +    Helped-by: Junio C Hamano <gitster@pobox.com>
     
      ## Documentation/config/submodule.txt ##
     @@ Documentation/config/submodule.txt: submodule.alternateErrorStrategy::
    @@ Documentation/config/submodule.txt: submodule.alternateErrorStrategy::
      	clone proceeds as if no alternate was specified.
     +
     +submodule.superprojectGitDir::
    -+	The relative path from the submodule's worktree  to the superproject's
    -+	gitdir. This config should only be present in projects which are
    -+	submodules, but is not guaranteed to be present in every submodule. It
    -+	is set automatically during submodule creation.
    ++	The relative path from the submodule's worktree to its superproject's
    ++	gitdir. When Git is run in a repository, it usually makes no difference
    ++	whether this repository is standalone or a submodule, but if this
    ++	configuration variable is present, additional behavior may be possible,
    ++	such as "git status" printing additional information about this
    ++	submodule's status with respect to its superproject. This config should
    ++	only be present in projects which are submodules, but is not guaranteed
    ++	to be present in every submodule, so only optional value-added behavior
    ++	should be linked to it. It is set automatically during
    ++	submodule creation.
     ++
    -+	In situations where more than one superproject references the same
    -+	submodule worktree, the value of this config and the behavior of
    -+	operations which use it are undefined. To reference a single project
    -+	from multiple superprojects, it is better to create a worktree of the
    -+	submodule for each superproject.
    ++	Because of this configuration variable, it is forbidden to use the
    ++	same submodule worktree shared by multiple superprojects.
     
      ## builtin/submodule--helper.c ##
     @@ builtin/submodule--helper.c: static int module_clone(int argc, const char **argv, const char *prefix)
    @@ t/t7400-submodule-basic.sh: test_expect_success 'setup - repository to add submo
     +	git -C "$sub_dir" clean -n -d -x >untracked
      }
      
    - 
    + test_expect_success 'submodule add' '
     @@ t/t7400-submodule-basic.sh: test_expect_success 'submodule add' '
      	) &&
      
3:  df97a9c2bb ! 3:  d278568a8e submodule: cache superproject gitdir during absorbgitdirs
    @@ Metadata
     Author: Emily Shaffer <emilyshaffer@google.com>
     
      ## Commit message ##
    -    submodule: cache superproject gitdir during absorbgitdirs
    +    submodule: record superproject gitdir during absorbgitdirs
     
    -    Already during 'git submodule add' we cache a pointer to the
    +    Already during 'git submodule add' we record a pointer to the
         superproject's gitdir. However, this doesn't help brand-new
         submodules created with 'git init' and later absorbed with 'git
         submodule absorbgitdir'. Let's start adding that pointer during 'git
4:  a3f3be58ad ! 4:  6866c36620 submodule: cache superproject gitdir during 'update'
    @@ Metadata
     Author: Emily Shaffer <emilyshaffer@google.com>
     
      ## Commit message ##
    -    submodule: cache superproject gitdir during 'update'
    +    submodule: record superproject gitdir during 'update'
     
    -    A cached path to the superproject's gitdir might be added during 'git
    -    submodule add', but in some cases - like submodules which were created
    -    before 'git submodule add' learned to cache that info - it might be
    -    useful to update the cache. Let's do it during 'git submodule update',
    -    when we already have a handle to the superproject while calling
    +    A recorded hint path to the superproject's gitdir might be added during
    +    'git submodule add', but in some cases - like submodules which were
    +    created before 'git submodule add' learned to record that info - it might
    +    be useful to update the hint. Let's do it during 'git submodule
    +    update', when we already have a handle to the superproject while calling
         operations on the submodules.
     
         Signed-off-by: Emily Shaffer <emilyshaffer@google.com>