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Table of contents:

  * Terminology
  * Purpose of sparse-checkouts
  * Usecases of primary concern
  * Oversimplified mental models ("Cliff Notes" for this document!)
  * Desired behavior
  * Behavior classes
  * Subcommand-dependent defaults
  * Sparse specification vs. sparsity patterns
  * Implementation Questions
  * Implementation Goals/Plans
  * Known bugs
  * Reference Emails


=== Terminology ===

cone mode: one of two modes for specifying the desired subset of files
        in a sparse-checkout.  In cone-mode, the user specifies
        directories (getting both everything under that directory as
        well as everything in leading directories), while in non-cone
        mode, the user specifies gitignore-style patterns.  Controlled
        by the --[no-]cone option to sparse-checkout init|set.

SKIP_WORKTREE: When tracked files do not match the sparse specification and
        are removed from the working tree, the file in the index is marked
        with a SKIP_WORKTREE bit.  Note that if a tracked file has the
        SKIP_WORKTREE bit set but the file is later written by the user to
        the working tree anyway, the SKIP_WORKTREE bit will be cleared at
        the beginning of any subsequent Git operation.

        Most sparse checkout users are unaware of this implementation
        detail, and the term should generally be avoided in user-facing
        descriptions and command flags.  Unfortunately, prior to the
        `sparse-checkout` subcommand this low-level detail was exposed,
        and as of time of writing, is still exposed in various places.

sparse-checkout: a subcommand in git used to reduce the files present in
        the working tree to a subset of all tracked files.  Also, the
        name of the file in the $GIT_DIR/info directory used to track
        the sparsity patterns corresponding to the user's desired
        subset.

sparse cone: see cone mode

sparse directory: An entry in the index corresponding to a directory, which
        appears in the index instead of all the files under that directory
        that would normally appear.  See also sparse-index.  Something that
        can cause confusion is that the "sparse directory" does NOT match
        the sparse specification, i.e. the directory is NOT present in the
        working tree.  May be renamed in the future (e.g. to "skipped
        directory").

sparse index: A special mode for sparse-checkout that also makes the
        index sparse by recording a directory entry in lieu of all the
        files underneath that directory (thus making that a "skipped
        directory" which unfortunately has also been called a "sparse
        directory"), and does this for potentially multiple
        directories.  Controlled by the --[no-]sparse-index option to
        init|set|reapply.

sparsity patterns: patterns from $GIT_DIR/info/sparse-checkout used to
        define the set of files of interest.  A warning: It is easy to
        over-use this term (or the shortened "patterns" term), for two
        reasons: (1) users in cone mode specify directories rather than
        patterns (their directories are transformed into patterns, but
        users may think you are talking about non-cone mode if you use the
        word "patterns"), and (b) the sparse specification might
        transiently differ in the working tree or index from the sparsity
        patterns (see "Sparse specification vs. sparsity patterns").

sparse specification: The set of paths in the user's area of focus.  This
        is typically just the tracked files that match the sparsity
        patterns, but the sparse specification can temporarily differ and
        include additional files.  (See also "Sparse specification
        vs. sparsity patterns")

        * When working with history, the sparse specification is exactly
          the set of files matching the sparsity patterns.
        * When interacting with the working tree, the sparse specification
          is the set of tracked files with a clear SKIP_WORKTREE bit or
          tracked files present in the working copy.
        * When modifying or showing results from the index, the sparse
          specification is the set of files with a clear SKIP_WORKTREE bit
          or that differ in the index from HEAD.
        * If working with the index and the working copy, the sparse
          specification is the union of the paths from above.

vivifying: When a command restores a tracked file to the working tree (and
        hopefully also clears the SKIP_WORKTREE bit in the index for that
        file), this is referred to as "vivifying" the file.


=== Purpose of sparse-checkouts ===

sparse-checkouts exist to allow users to work with a subset of their
files.

You can think of sparse-checkouts as subdividing "tracked" files into two
categories -- a sparse subset, and all the rest.  Implementationally, we
mark "all the rest" in the index with a SKIP_WORKTREE bit and leave them
out of the working tree.  The SKIP_WORKTREE files are still tracked, just
not present in the working tree.

In the past, sparse-checkouts were defined by "SKIP_WORKTREE means the file
is missing from the working tree but pretend the file contents match HEAD".
That was not only bogus (it actually meant the file missing from the
working tree matched the index rather than HEAD), but it was also a
low-level detail which only provided decent behavior for a few commands.
There were a surprising number of ways in which that guiding principle gave
command results that violated user expectations, and as such was a bad
mental model.  However, it persisted for many years and may still be found
in some corners of the code base.

Anyway, the idea of "working with a subset of files" is simple enough, but
there are multiple different high-level usecases which affect how some Git
subcommands should behave.  Further, even if we only considered one of
those usecases, sparse-checkouts can modify different subcommands in over a
half dozen different ways.  Let's start by considering the high level
usecases:

  A) Users are _only_ interested in the sparse portion of the repo

  A*) Users are _only_ interested in the sparse portion of the repo
      that they have downloaded so far

  B) Users want a sparse working tree, but are working in a larger whole

  C) sparse-checkout is a behind-the-scenes implementation detail allowing
     Git to work with a specially crafted in-house virtual file system;
     users are actually working with a "full" working tree that is
     lazily populated, and sparse-checkout helps with the lazy population
     piece.

It may be worth explaining each of these in a bit more detail:


  (Behavior A) Users are _only_ interested in the sparse portion of the repo

These folks might know there are other things in the repository, but
don't care.  They are uninterested in other parts of the repository, and
only want to know about changes within their area of interest.  Showing
them other files from history (e.g. from diff/log/grep/etc.)  is a
usability annoyance, potentially a huge one since other changes in
history may dwarf the changes they are interested in.

Some of these users also arrive at this usecase from wanting to use partial
clones together with sparse checkouts (in a way where they have downloaded
blobs within the sparse specification) and do disconnected development.
Not only do these users generally not care about other parts of the
repository, but consider it a blocker for Git commands to try to operate on
those.  If commands attempt to access paths in history outside the sparsity
specification, then the partial clone will attempt to download additional
blobs on demand, fail, and then fail the user's command.  (This may be
unavoidable in some cases, e.g. when `git merge` has non-trivial changes to
reconcile outside the sparse specification, but we should limit how often
users are forced to connect to the network.)

Also, even for users using partial clones that do not mind being
always connected to the network, the need to download blobs as
side-effects of various other commands (such as the printed diffstat
after a merge or pull) can lead to worries about local repository size
growing unnecessarily[10].

  (Behavior A*) Users are _only_ interested in the sparse portion of the repo
      that they have downloaded so far (a variant on the first usecase)

This variant is driven by folks who using partial clones together with
sparse checkouts and do disconnected development (so far sounding like a
subset of behavior A users) and doing so on very large repositories.  The
reason for yet another variant is that downloading even just the blobs
through history within their sparse specification may be too much, so they
only download some.  They would still like operations to succeed without
network connectivity, though, so things like `git log -S${SEARCH_TERM} -p`
or `git grep ${SEARCH_TERM} OLDREV ` would need to be prepared to provide
partial results that depend on what happens to have been downloaded.

This variant could be viewed as Behavior A with the sparse specification
for history querying operations modified from "sparsity patterns" to
"sparsity patterns limited to the blobs we have already downloaded".

  (Behavior B) Users want a sparse working tree, but are working in a
      larger whole

Stolee described this usecase this way[11]:

"I'm also focused on users that know that they are a part of a larger
whole. They know they are operating on a large repository but focus on
what they need to contribute their part. I expect multiple "roles" to
use very different, almost disjoint parts of the codebase. Some other
"architect" users operate across the entire tree or hop between different
sections of the codebase as necessary. In this situation, I'm wary of
scoping too many features to the sparse-checkout definition, especially
"git log," as it can be too confusing to have their view of the codebase
depend on your "point of view."

People might also end up wanting behavior B due to complex inter-project
dependencies.  The initial attempts to use sparse-checkouts usually involve
the directories you are directly interested in plus what those directories
depend upon within your repository.  But there's a monkey wrench here: if
you have integration tests, they invert the hierarchy: to run integration
tests, you need not only what you are interested in and its in-tree
dependencies, you also need everything that depends upon what you are
interested in or that depends upon one of your dependencies...AND you need
all the in-tree dependencies of that expanded group.  That can easily
change your sparse-checkout into a nearly dense one.

Naturally, that tends to kill the benefits of sparse-checkouts.  There are
a couple solutions to this conundrum: either avoid grabbing in-repo
dependencies (maybe have built versions of your in-repo dependencies pulled
from a CI cache somewhere), or say that users shouldn't run integration
tests directly and instead do it on the CI server when they submit a code
review.  Or do both.  Regardless of whether you stub out your in-repo
dependencies or stub out the things that depend upon you, there is
certainly a reason to want to query and be aware of those other stubbed-out
parts of the repository, particularly when the dependencies are complex or
change relatively frequently.  Thus, for such uses, sparse-checkouts can be
used to limit what you directly build and modify, but these users do not
necessarily want their sparse checkout paths to limit their queries of
versions in history.

Some people may also be interested in behavior B over behavior A simply as
a performance workaround: if they are using non-cone mode, then they have
to deal with its inherent quadratic performance problems.  In that mode,
every operation that checks whether paths match the sparsity specification
can be expensive.  As such, these users may only be willing to pay for
those expensive checks when interacting with the working copy, and may
prefer getting "unrelated" results from their history queries over having
slow commands.

  (Behavior C) sparse-checkout is an implementational detail supporting a
               special VFS.

This usecase goes slightly against the traditional definition of
sparse-checkout in that it actually tries to present a full or dense
checkout to the user.  However, this usecase utilizes the same underlying
technical underpinnings in a new way which does provide some performance
advantages to users.  The basic idea is that a company can have an in-house
Git-aware Virtual File System which pretends all files are present in the
working tree, by intercepting all file system accesses and using those to
fetch and write accessed files on demand via partial clones.  The VFS uses
sparse-checkout to prevent Git from writing or paying attention to many
files, and manually updates the sparse checkout patterns itself based on
user access and modification of files in the working tree.  See commit
ecc7c8841d ("repo_read_index: add config to expect files outside sparse
patterns", 2022-02-25) and the link at [17] for a more detailed description
of such a VFS.

The biggest difference here is that users are completely unaware that the
sparse-checkout machinery is even in use.  The sparse patterns are not
specified by the user but rather are under the complete control of the VFS
(and the patterns are updated frequently and dynamically by it).  The user
will perceive the checkout as dense, and commands should thus behave as if
all files are present.


=== Usecases of primary concern ===

Most of the rest of this document will focus on Behavior A and Behavior
B.  Some notes about the other two cases and why we are not focusing on
them:

  (Behavior A*)

Supporting this usecase is estimated to be difficult and a lot of work.
There are no plans to implement it currently, but it may be a potential
future alternative.  Knowing about the existence of additional alternatives
may affect our choice of command line flags (e.g. if we need tri-state or
quad-state flags rather than just binary flags), so it was still important
to at least note.

Further, I believe the descriptions below for Behavior A are probably still
valid for this usecase, with the only exception being that it redefines the
sparse specification to restrict it to already-downloaded blobs.  The hard
part is in making commands capable of respecting that modified definition.

  (Behavior C)

This usecase violates some of the early sparse-checkout documented
assumptions (since files marked as SKIP_WORKTREE will be displayed to users
as present in the working tree).  That violation may mean various
sparse-checkout related behaviors are not well suited to this usecase and
we may need tweaks -- to both documentation and code -- to handle it.
However, this usecase is also perhaps the simplest model to support in that
everything behaves like a dense checkout with a few exceptions (e.g. branch
checkouts and switches write fewer things, knowing the VFS will lazily
write the rest on an as-needed basis).

Since there is no publicly available VFS-related code for folks to try,
the number of folks who can test such a usecase is limited.

The primary reason to note the Behavior C usecase is that as we fix things
to better support Behaviors A and B, there may be additional places where
we need to make tweaks allowing folks in this usecase to get the original
non-sparse treatment.  For an example, see ecc7c8841d ("repo_read_index:
add config to expect files outside sparse patterns", 2022-02-25).  The
secondary reason to note Behavior C, is so that folks taking advantage of
Behavior C do not assume they are part of the Behavior B camp and propose
patches that break things for the real Behavior B folks.


=== Oversimplified mental models ===

An oversimplification of the differences in the above behaviors is:

  Behavior A: Restrict worktree and history operations to sparse specification
  Behavior B: Restrict worktree operations to sparse specification; have any
              history operations work across all files
  Behavior C: Do not restrict either worktree or history operations to the
              sparse specification...with the exception of branch checkouts or
              switches which avoid writing files that will match the index so
              they can later lazily be populated instead.


=== Desired behavior ===

As noted previously, despite the simple idea of just working with a subset
of files, there are a range of different behavioral changes that need to be
made to different subcommands to work well with such a feature.  See
[1,2,3,4,5,6,7,8,9,10] for various examples.  In particular, at [2], we saw
that mere composition of other commands that individually worked correctly
in a sparse-checkout context did not imply that the higher level command
would work correctly; it sometimes requires further tweaks.  So,
understanding these differences can be beneficial.

* Commands behaving the same regardless of high-level use-case

  * commands that only look at files within the sparsity specification

      * diff (without --cached or REVISION arguments)
      * grep (without --cached or REVISION arguments)
      * diff-files

  * commands that restore files to the working tree that match sparsity
    patterns, and remove unmodified files that don't match those
    patterns:

      * switch
      * checkout (the switch-like half)
      * read-tree
      * reset --hard

  * commands that write conflicted files to the working tree, but otherwise
    will omit writing files to the working tree that do not match the
    sparsity patterns:

      * merge
      * rebase
      * cherry-pick
      * revert

      * `am` and `apply --cached` should probably be in this section but
        are buggy (see the "Known bugs" section below)

    The behavior for these commands somewhat depends upon the merge
    strategy being used:
      * `ort` behaves as described above
      * `recursive` tries to not vivify files unnecessarily, but does sometimes
        vivify files without conflicts.
      * `octopus` and `resolve` will always vivify any file changed in the merge
        relative to the first parent, which is rather suboptimal.

    It is also important to note that these commands WILL update the index
    outside the sparse specification relative to when the operation began,
    BUT these commands often make a commit just before or after such that
    by the end of the operation there is no change to the index outside the
    sparse specification.  Of course, if the operation hits conflicts or
    does not make a commit, then these operations clearly can modify the
    index outside the sparse specification.

    Finally, it is important to note that at least the first four of these
    commands also try to remove differences between the sparse
    specification and the sparsity patterns (much like the commands in the
    previous section).

  * commands that always ignore sparsity since commits must be full-tree

      * archive
      * bundle
      * commit
      * format-patch
      * fast-export
      * fast-import
      * commit-tree

  * commands that write any modified file to the working tree (conflicted
    or not, and whether those paths match sparsity patterns or not):

      * stash
      * apply (without `--index` or `--cached`)

* Commands that may slightly differ for behavior A vs. behavior B:

  Commands in this category behave mostly the same between the two
  behaviors, but may differ in verbosity and types of warning and error
  messages.

  * commands that make modifications to which files are tracked:
      * add
      * rm
      * mv
      * update-index

    The fact that files can move between the 'tracked' and 'untracked'
    categories means some commands will have to treat untracked files
    differently.  But if we have to treat untracked files differently,
    then additional commands may also need changes:

      * status
      * clean

    In particular, `status` may need to report any untracked files outside
    the sparsity specification as an erroneous condition (especially to
    avoid the user trying to `git add` them, forcing `git add` to display
    an error).

    It's not clear to me exactly how (or even if) `clean` would change,
    but it's the other command that also affects untracked files.

    `update-index` may be slightly special.  Its --[no-]skip-worktree flag
    may need to ignore the sparse specification by its nature.  Also, its
    current --[no-]ignore-skip-worktree-entries default is totally bogus.

  * commands for manually tweaking paths in both the index and the working tree
      * `restore`
      * the restore-like half of `checkout`

    These commands should be similar to add/rm/mv in that they should
    only operate on the sparse specification by default, and require a
    special flag to operate on all files.

    Also, note that these commands currently have a number of issues (see
    the "Known bugs" section below)

* Commands that significantly differ for behavior A vs. behavior B:

  * commands that query history
      * diff (with --cached or REVISION arguments)
      * grep (with --cached or REVISION arguments)
      * show (when given commit arguments)
      * blame (only matters when one or more -C flags are passed)
        * and annotate
      * log
      * whatchanged
      * ls-files
      * diff-index
      * diff-tree
      * ls-tree

    Note: for log and whatchanged, revision walking logic is unaffected
    but displaying of patches is affected by scoping the command to the
    sparse-checkout.  (The fact that revision walking is unaffected is
    why rev-list, shortlog, show-branch, and bisect are not in this
    list.)

    ls-files may be slightly special in that e.g. `git ls-files -t` is
    often used to see what is sparse and what is not.  Perhaps -t should
    always work on the full tree?

* Commands I don't know how to classify

  * range-diff

    Is this like `log` or `format-patch`?

  * cherry

    See range-diff

* Commands unaffected by sparse-checkouts

  * shortlog
  * show-branch
  * rev-list
  * bisect

  * branch
  * describe
  * fetch
  * gc
  * init
  * maintenance
  * notes
  * pull (merge & rebase have the necessary changes)
  * push
  * submodule
  * tag

  * config
  * filter-branch (works in separate checkout without sparse-checkout setup)
  * pack-refs
  * prune
  * remote
  * repack
  * replace

  * bugreport
  * count-objects
  * fsck
  * gitweb
  * help
  * instaweb
  * merge-tree (doesn't touch worktree or index, and merges always compute full-tree)
  * rerere
  * verify-commit
  * verify-tag

  * commit-graph
  * hash-object
  * index-pack
  * mktag
  * mktree
  * multi-pack-index
  * pack-objects
  * prune-packed
  * symbolic-ref
  * unpack-objects
  * update-ref
  * write-tree (operates on index, possibly optimized to use sparse dir entries)

  * for-each-ref
  * get-tar-commit-id
  * ls-remote
  * merge-base (merges are computed full tree, so merge base should be too)
  * name-rev
  * pack-redundant
  * rev-parse
  * show-index
  * show-ref
  * unpack-file
  * var
  * verify-pack

  * <Everything under 'Interacting with Others' in 'git help --all'>
  * <Everything under 'Low-level...Syncing' in 'git help --all'>
  * <Everything under 'Low-level...Internal Helpers' in 'git help --all'>
  * <Everything under 'External commands' in 'git help --all'>

* Commands that might be affected, but who cares?

  * merge-file
  * merge-index
  * gitk?


=== Behavior classes ===

From the above there are a few classes of behavior:

  * "restrict"

    Commands in this class only read or write files in the working tree
    within the sparse specification.

    When moving to a new commit (e.g. switch, reset --hard), these commands
    may update index files outside the sparse specification as of the start
    of the operation, but by the end of the operation those index files
    will match HEAD again and thus those files will again be outside the
    sparse specification.

    When paths are explicitly specified, these paths are intersected with
    the sparse specification and will only operate on such paths.
    (e.g. `git restore [--staged] -- '*.png'`, `git reset -p -- '*.md'`)

    Some of these commands may also attempt, at the end of their operation,
    to cull transient differences between the sparse specification and the
    sparsity patterns (see "Sparse specification vs. sparsity patterns" for
    details, but this basically means either removing unmodified files not
    matching the sparsity patterns and marking those files as
    SKIP_WORKTREE, or vivifying files that match the sparsity patterns and
    marking those files as !SKIP_WORKTREE).

  * "restrict modulo conflicts"

    Commands in this class generally behave like the "restrict" class,
    except that:
      (1) they will ignore the sparse specification and write files with
          conflicts to the working tree (thus temporarily expanding the
          sparse specification to include such files.)
      (2) they are grouped with commands which move to a new commit, since
          they often create a commit and then move to it, even though we
          know there are many exceptions to moving to the new commit.  (For
          example, the user may rebase a commit that becomes empty, or have
          a cherry-pick which conflicts, or a user could run `merge
          --no-commit`, and we also view `apply --index` kind of like `am
          --no-commit`.)  As such, these commands can make changes to index
          files outside the sparse specification, though they'll mark such
          files with SKIP_WORKTREE.

  * "restrict also specially applied to untracked files"

    Commands in this class generally behave like the "restrict" class,
    except that they have to handle untracked files differently too, often
    because these commands are dealing with files changing state between
    'tracked' and 'untracked'.  Often, this may mean printing an error
    message if the command had nothing to do, but the arguments may have
    referred to files whose tracked-ness state could have changed were it
    not for the sparsity patterns excluding them.

  * "no restrict"

    Commands in this class ignore the sparse specification entirely.

  * "restrict or no restrict dependent upon behavior A vs. behavior B"

    Commands in this class behave like "no restrict" for folks in the
    behavior B camp, and like "restrict" for folks in the behavior A camp.
    However, when behaving like "restrict" a warning of some sort might be
    provided that history queries have been limited by the sparse-checkout
    specification.


=== Subcommand-dependent defaults ===

Note that we have different defaults depending on the command for the
desired behavior :

  * Commands defaulting to "restrict":
    * diff-files
    * diff (without --cached or REVISION arguments)
    * grep (without --cached or REVISION arguments)
    * switch
    * checkout (the switch-like half)
    * reset (<commit>)

    * restore
    * checkout (the restore-like half)
    * checkout-index
    * reset (with pathspec)

    This behavior makes sense; these interact with the working tree.

  * Commands defaulting to "restrict modulo conflicts":
    * merge
    * rebase
    * cherry-pick
    * revert

    * am
    * apply --index (which is kind of like an `am --no-commit`)

    * read-tree (especially with -m or -u; is kind of like a --no-commit merge)
    * reset (<tree-ish>, due to similarity to read-tree)

    These also interact with the working tree, but require slightly
    different behavior either so that (a) conflicts can be resolved or (b)
    because they are kind of like a merge-without-commit operation.

    (See also the "Known bugs" section below regarding `am` and `apply`)

  * Commands defaulting to "no restrict":
    * archive
    * bundle
    * commit
    * format-patch
    * fast-export
    * fast-import
    * commit-tree

    * stash
    * apply (without `--index`)

    These have completely different defaults and perhaps deserve the most
    detailed explanation:

    In the case of commands in the first group (format-patch,
    fast-export, bundle, archive, etc.), these are commands for
    communicating history, which will be broken if they restrict to a
    subset of the repository.  As such, they operate on full paths and
    have no `--restrict` option for overriding.  Some of these commands may
    take paths for manually restricting what is exported, but it needs to
    be very explicit.

    In the case of stash, it needs to vivify files to avoid losing the
    user's changes.

    In the case of apply without `--index`, that command needs to update
    the working tree without the index (or the index without the working
    tree if `--cached` is passed), and if we restrict those updates to the
    sparse specification then we'll lose changes from the user.

  * Commands defaulting to "restrict also specially applied to untracked files":
    * add
    * rm
    * mv
    * update-index
    * status
    * clean (?)

    Our original implementation for the first three of these commands was
    "no restrict", but it had some severe usability issues:
      * `git add <somefile>` if honored and outside the sparse
        specification, can result in the file randomly disappearing later
        when some subsequent command is run (since various commands
        automatically clean up unmodified files outside the sparse
        specification).
      * `git rm '*.jpg'` could very negatively surprise users if it deletes
        files outside the range of the user's interest.
      * `git mv` has similar surprises when moving into or out of the cone,
        so best to restrict by default

    So, we switched `add` and `rm` to default to "restrict", which made
    usability problems much less severe and less frequent, but we still got
    complaints because commands like:
        git add <file-outside-sparse-specification>
        git rm <file-outside-sparse-specification>
    would silently do nothing.  We should instead print an error in those
    cases to get usability right.

    update-index needs to be updated to match, and status and maybe clean
    also need to be updated to specially handle untracked paths.

    There may be a difference in here between behavior A and behavior B in
    terms of verboseness of errors or additional warnings.

  * Commands falling under "restrict or no restrict dependent upon behavior
    A vs. behavior B"

    * diff (with --cached or REVISION arguments)
    * grep (with --cached or REVISION arguments)
    * show (when given commit arguments)
    * blame (only matters when one or more -C flags passed)
      * and annotate
    * log
      * and variants: shortlog, gitk, show-branch, whatchanged, rev-list
    * ls-files
    * diff-index
    * diff-tree
    * ls-tree

    For now, we default to behavior B for these, which want a default of
    "no restrict".

    Note that two of these commands -- diff and grep -- also appeared in a
    different list with a default of "restrict", but only when limited to
    searching the working tree.  The working tree vs. history distinction
    is fundamental in how behavior B operates, so this is expected.  Note,
    though, that for diff and grep with --cached, when doing "restrict"
    behavior, the difference between sparse specification and sparsity
    patterns is important to handle.

    "restrict" may make more sense as the long term default for these[12].
    Also, supporting "restrict" for these commands might be a fair amount
    of work to implement, meaning it might be implemented over multiple
    releases.  If that behavior were the default in the commands that
    supported it, that would force behavior B users to need to learn to
    slowly add additional flags to their commands, depending on git
    version, to get the behavior they want.  That gradual switchover would
    be painful, so we should avoid it at least until it's fully
    implemented.


=== Sparse specification vs. sparsity patterns ===

In a well-behaved situation, the sparse specification is given directly
by the $GIT_DIR/info/sparse-checkout file.  However, it can transiently
diverge for a few reasons:

    * needing to resolve conflicts (merging will vivify conflicted files)
    * running Git commands that implicitly vivify files (e.g. "git stash apply")
    * running Git commands that explicitly vivify files (e.g. "git checkout
      --ignore-skip-worktree-bits FILENAME")
    * other commands that write to these files (perhaps a user copies it
      from elsewhere)

For the last item, note that we do automatically clear the SKIP_WORKTREE
bit for files that are present in the working tree.  This has been true
since 82386b4496 ("Merge branch 'en/present-despite-skipped'",
2022-03-09)

However, such a situation is transient because:

   * Such transient differences can and will be automatically removed as
     a side-effect of commands which call unpack_trees() (checkout,
     merge, reset, etc.).
   * Users can also request such transient differences be corrected via
     running `git sparse-checkout reapply`.  Various places recommend
     running that command.
   * Additional commands are also welcome to implicitly fix these
     differences; we may add more in the future.

While we avoid dropping unstaged changes or files which have conflicts,
we otherwise aggressively try to fix these transient differences.  If
users want these differences to persist, they should run the `set` or
`add` subcommands of `git sparse-checkout` to reflect their intended
sparse specification.

However, when we need to do a query on history restricted to the
"relevant subset of files" such a transiently expanded sparse
specification is ignored.  There are a couple reasons for this:

   * The behavior wanted when doing something like
         git grep expression REVISION
     is roughly what the users would expect from
         git checkout REVISION && git grep expression
     (modulo a "REVISION:" prefix), which has a couple ramifications:

   * REVISION may have paths not in the current index, so there is no
     path we can consult for a SKIP_WORKTREE setting for those paths.

   * Since `checkout` is one of those commands that tries to remove
     transient differences in the sparse specification, it makes sense
     to use the corrected sparse specification
     (i.e. $GIT_DIR/info/sparse-checkout) rather than attempting to
     consult SKIP_WORKTREE anyway.

So, a transiently expanded (or restricted) sparse specification applies to
the working tree, but not to history queries where we always use the
sparsity patterns.  (See [16] for an early discussion of this.)

Similar to a transiently expanded sparse specification of the working tree
based on additional files being present in the working tree, we also need
to consider additional files being modified in the index.  In particular,
if the user has staged changes to files (relative to HEAD) that do not
match the sparsity patterns, and the file is not present in the working
tree, we still want to consider the file part of the sparse specification
if we are specifically performing a query related to the index (e.g. git
diff --cached [REVISION], git diff-index [REVISION], git restore --staged
--source=REVISION -- PATHS, etc.)  Note that a transiently expanded sparse
specification for the index usually only matters under behavior A, since
under behavior B index operations are lumped with history and tend to
operate full-tree.


=== Implementation Questions ===

  * Do the options --scope={sparse,all} sound good to others?  Are there better
    options?
    * Names in use, or appearing in patches, or previously suggested:
      * --sparse/--dense
      * --ignore-skip-worktree-bits
      * --ignore-skip-worktree-entries
      * --ignore-sparsity
      * --[no-]restrict-to-sparse-paths
      * --full-tree/--sparse-tree
      * --[no-]restrict
      * --scope={sparse,all}
      * --focus/--unfocus
      * --limit/--unlimited
    * Rationale making me lean slightly towards --scope={sparse,all}:
      * We want a name that works for many commands, so we need a name that
        does not conflict
      * We know that we have more than two possible usecases, so it is best
        to avoid a flag that appears to be binary.
      * --scope={sparse,all} isn't overly long and seems relatively
        explanatory
      * `--sparse`, as used in add/rm/mv, is totally backwards for
        grep/log/etc.  Changing the meaning of `--sparse` for these
        commands would fix the backwardness, but possibly break existing
        scripts.  Using a new name pairing would allow us to treat
        `--sparse` in these commands as a deprecated alias.
      * There is a different `--sparse`/`--dense` pair for commands using
        revision machinery, so using that naming might cause confusion
      * There is also a `--sparse` in both pack-objects and show-branch, which
        don't conflict but do suggest that `--sparse` is overloaded
      * The name --ignore-skip-worktree-bits is a double negative, is
        quite a mouthful, refers to an implementation detail that many
        users may not be familiar with, and we'd need a negation for it
        which would probably be even more ridiculously long.  (But we
        can make --ignore-skip-worktree-bits a deprecated alias for
        --no-restrict.)

  * If a config option is added (sparse.scope?) what should the values and
    description be?  "sparse" (behavior A), "worktree-sparse-history-dense"
    (behavior B), "dense" (behavior C)?  There's a risk of confusion,
    because even for Behaviors A and B we want some commands to be
    full-tree and others to operate sparsely, so the wording may need to be
    more tied to the usecases and somehow explain that.  Also, right now,
    the primary difference we are focusing is just the history-querying
    commands (log/diff/grep).  Previous config suggestion here: [13]

  * Is `--no-expand` a good alias for ls-files's `--sparse` option?
    (`--sparse` does not map to either `--scope=sparse` or `--scope=all`,
    because in non-cone mode it does nothing and in cone-mode it shows the
    sparse directory entries which are technically outside the sparse
    specification)

  * Under Behavior A:
    * Does ls-files' `--no-expand` override the default `--scope=all`, or
      does it need an extra flag?
    * Does ls-files' `-t` option imply `--scope=all`?
    * Does update-index's `--[no-]skip-worktree` option imply `--scope=all`?

  * sparse-checkout: once behavior A is fully implemented, should we take
    an interim measure to ease people into switching the default?  Namely,
    if folks are not already in a sparse checkout, then require
    `sparse-checkout init/set` to take a
    `--set-scope=(sparse|worktree-sparse-history-dense|dense)` flag (which
    would set sparse.scope according to the setting given), and throw an
    error if the flag is not provided?  That error would be a great place
    to warn folks that the default may change in the future, and get them
    used to specifying what they want so that the eventual default switch
    is seamless for them.


=== Implementation Goals/Plans ===

 * Get buy-in on this document in general.

 * Figure out answers to the 'Implementation Questions' sections (above)

 * Fix bugs in the 'Known bugs' section (below)

 * Provide some kind of method for backfilling the blobs within the sparse
   specification in a partial clone

 [Below here is kind of spitballing since the first two haven't been resolved]

 * update-index: flip the default to --no-ignore-skip-worktree-entries,
   nuke this stupid "Oh, there's a bug?  Let me add a flag to let users
   request that they not trigger this bug." flag

 * Flags & Config
   * Make `--sparse` in add/rm/mv a deprecated alias for `--scope=all`
   * Make `--ignore-skip-worktree-bits` in checkout-index/checkout/restore
     a deprecated aliases for `--scope=all`
   * Create config option (sparse.scope?), tie it to the "Cliff notes"
     overview

   * Add --scope=sparse (and --scope=all) flag to each of the history querying
     commands.  IMPORTANT: make sure diff machinery changes don't mess with
     format-patch, fast-export, etc.

=== Known bugs ===

This list used to be a lot longer (see e.g. [1,2,3,4,5,6,7,8,9]), but we've
been working on it.

0. Behavior A is not well supported in Git.  (Behavior B didn't used to
   be either, but was the easier of the two to implement.)

1. am and apply:

   apply, without `--index` or `--cached`, relies on files being present
   in the working copy, and also writes to them unconditionally.  As
   such, it should first check for the files' presence, and if found to
   be SKIP_WORKTREE, then clear the bit and vivify the paths, then do
   its work.  Currently, it just throws an error.

   apply, with either `--cached` or `--index`, will not preserve the
   SKIP_WORKTREE bit.  This is fine if the file has conflicts, but
   otherwise SKIP_WORKTREE bits should be preserved for --cached and
   probably also for --index.

   am, if there are no conflicts, will vivify files and fail to preserve
   the SKIP_WORKTREE bit.  If there are conflicts and `-3` is not
   specified, it will vivify files and then complain the patch doesn't
   apply.  If there are conflicts and `-3` is specified, it will vivify
   files and then complain that those vivified files would be
   overwritten by merge.

2. reset --hard:

   reset --hard provides confusing error message (works correctly, but
   misleads the user into believing it didn't):

    $ touch addme
    $ git add addme
    $ git ls-files -t
    H addme
    H tracked
    S tracked-but-maybe-skipped
    $ git reset --hard                           # usually works great
    error: Path 'addme' not uptodate; will not remove from working tree.
    HEAD is now at bdbbb6f third
    $ git ls-files -t
    H tracked
    S tracked-but-maybe-skipped
    $ ls -1
    tracked

    `git reset --hard` DID remove addme from the index and the working tree, contrary
    to the error message, but in line with how reset --hard should behave.

3. read-tree

   `read-tree` doesn't apply the 'SKIP_WORKTREE' bit to *any* of the
   entries it reads into the index, resulting in all your files suddenly
   appearing to be "deleted".

4. Checkout, restore:

   These command do not handle path & revision arguments appropriately:

    $ ls
    tracked
    $ git ls-files -t
    H tracked
    S tracked-but-maybe-skipped
    $ git status --porcelain
    $ git checkout -- '*skipped'
    error: pathspec '*skipped' did not match any file(s) known to git
    $ git ls-files -- '*skipped'
    tracked-but-maybe-skipped
    $ git checkout HEAD -- '*skipped'
    error: pathspec '*skipped' did not match any file(s) known to git
    $ git ls-tree HEAD | grep skipped
    100644 blob 276f5a64354b791b13840f02047738c77ad0584f        tracked-but-maybe-skipped
    $ git status --porcelain
    $ git checkout HEAD~1 -- '*skipped'
    $ git ls-files -t
    H tracked
    H tracked-but-maybe-skipped
    $ git status --porcelain
    M  tracked-but-maybe-skipped
    $ git checkout HEAD -- '*skipped'
    $ git status --porcelain
    $

    Note that checkout without a revision (or restore --staged) fails to
    find a file to restore from the index, even though ls-files shows
    such a file certainly exists.

    Similar issues occur with HEAD (--source=HEAD in restore's case),
    but suddenly works when HEAD~1 is specified.  And then after that it
    will work with HEAD specified, even though it didn't before.

    Directories are also an issue:

    $ git sparse-checkout set nomatches
    $ git status
    On branch main
    You are in a sparse checkout with 0% of tracked files present.

    nothing to commit, working tree clean
    $ git checkout .
    error: pathspec '.' did not match any file(s) known to git
    $ git checkout HEAD~1 .
    Updated 1 path from 58916d9
    $ git ls-files -t
    S tracked
    H tracked-but-maybe-skipped

5. checkout and restore --staged, continued:

   These commands do not correctly scope operations to the sparse
   specification, and make it worse by not setting important SKIP_WORKTREE
   bits:

   $ git restore --source OLDREV --staged outside-sparse-cone/
   $ git status --porcelain
   MD outside-sparse-cone/file1
   MD outside-sparse-cone/file2
   MD outside-sparse-cone/file3

   We can add a --scope=all mode to `git restore` to let it operate outside
   the sparse specification, but then it will be important to set the
   SKIP_WORKTREE bits appropriately.

6. Performance issues; see:
    https://lore.kernel.org/git/CABPp-BEkJQoKZsQGCYioyga_uoDQ6iBeW+FKr8JhyuuTMK1RDw@mail.gmail.com/


=== Reference Emails ===

Emails that detail various bugs we've had in sparse-checkout:

[1] (Original descriptions of behavior A & behavior B)
    https://lore.kernel.org/git/CABPp-BGJ_Nvi5TmgriD9Bh6eNXE2EDq2f8e8QKXAeYG3BxZafA@mail.gmail.com/
[2] (Fix stash applications in sparse checkouts; bugs from behavioral differences)
    https://lore.kernel.org/git/ccfedc7140dbf63ba26a15f93bd3885180b26517.1606861519.git.gitgitgadget@gmail.com/
[3] (Present-despite-skipped entries)
    https://lore.kernel.org/git/11d46a399d26c913787b704d2b7169cafc28d639.1642175983.git.gitgitgadget@gmail.com/
[4] (Clone --no-checkout interaction)
    https://lore.kernel.org/git/pull.801.v2.git.git.1591324899170.gitgitgadget@gmail.com/ (clone --no-checkout)
[5] (The need for update_sparsity() and avoiding `read-tree -mu HEAD`)
    https://lore.kernel.org/git/3a1f084641eb47515b5a41ed4409a36128913309.1585270142.git.gitgitgadget@gmail.com/
[6] (SKIP_WORKTREE is advisory, not mandatory)
    https://lore.kernel.org/git/844306c3e86ef67591cc086decb2b760e7d710a3.1585270142.git.gitgitgadget@gmail.com/
[7] (`worktree add` should copy sparsity settings from current worktree)
    https://lore.kernel.org/git/c51cb3714e7b1d2f8c9370fe87eca9984ff4859f.1644269584.git.gitgitgadget@gmail.com/
[8] (Avoid negative surprises in add, rm, and mv)
    https://lore.kernel.org/git/cover.1617914011.git.matheus.bernardino@usp.br/
    https://lore.kernel.org/git/pull.1018.v4.git.1632497954.gitgitgadget@gmail.com/
[9] (Move from out-of-cone to in-cone)
    https://lore.kernel.org/git/20220630023737.473690-6-shaoxuan.yuan02@gmail.com/
    https://lore.kernel.org/git/20220630023737.473690-4-shaoxuan.yuan02@gmail.com/
[10] (Unnecessarily downloading objects outside sparse specification)
     https://lore.kernel.org/git/CAOLTT8QfwOi9yx_qZZgyGa8iL8kHWutEED7ok_jxwTcYT_hf9Q@mail.gmail.com/

[11] (Stolee's comments on high-level usecases)
     https://lore.kernel.org/git/1a1e33f6-3514-9afc-0a28-5a6b85bd8014@gmail.com/

[12] Others commenting on eventually switching default to behavior A:
  * https://lore.kernel.org/git/xmqqh719pcoo.fsf@gitster.g/
  * https://lore.kernel.org/git/xmqqzgeqw0sy.fsf@gitster.g/
  * https://lore.kernel.org/git/a86af661-cf58-a4e5-0214-a67d3a794d7e@github.com/

[13] Previous config name suggestion and description
  * https://lore.kernel.org/git/CABPp-BE6zW0nJSStcVU=_DoDBnPgLqOR8pkTXK3dW11=T01OhA@mail.gmail.com/

[14] Tangential issue: switch to cone mode as default sparse specification mechanism:
  https://lore.kernel.org/git/a1b68fd6126eb341ef3637bb93fedad4309b36d0.1650594746.git.gitgitgadget@gmail.com/

[15] Lengthy email on grep behavior, covering what should be searched:
  * https://lore.kernel.org/git/CABPp-BGVO3QdbfE84uF_3QDF0-y2iHHh6G5FAFzNRfeRitkuHw@mail.gmail.com/

[16] Email explaining sparsity patterns vs. SKIP_WORKTREE and history operations,
     search for the parenthetical comment starting "We do not check".
    https://lore.kernel.org/git/CABPp-BFsCPPNOZ92JQRJeGyNd0e-TCW-LcLyr0i_+VSQJP+GCg@mail.gmail.com/

[17] https://lore.kernel.org/git/20220207190320.2960362-1-jonathantanmy@google.com/