[PATCH v4] sparse-checkout.txt: new document with sparse-checkout directions

["Elijah Newren via GitGitGadget" <gitgitgadget@gmail.com>]

From: Elijah Newren <newren@gmail.com>

Once upon a time, Matheus wrote some patches to make
   git grep [--cached | <REVISION>] ...
restrict its output to the sparsity specification when working in a
sparse checkout[1].  That effort got derailed by two things:

  (1) The --sparse-index work just beginning which we wanted to avoid
      creating conflicts for
  (2) Never deciding on flag and config names and planned high level
      behavior for all commands.

More recently, Shaoxuan implemented a more limited form of Matheus'
patches that only affected --cached, using a different flag name,
but also changing the default behavior in line with what Matheus did.
This again highlighted the fact that we never decided on command line
flag names, config option names, and the big picture path forward.

The --sparse-index work has been mostly complete (or at least released
into production even if some small edges remain) for quite some time
now.  We have also had several discussions on flag and config names,
though we never came to solid conclusions.  Stolee once upon a time
suggested putting all these into some document in
Documentation/technical[3], which Victoria recently also requested[4].
I'm behind the times, but here's a patch attempting to finally do that.

[1] https://lore.kernel.org/git/5f3f7ac77039d41d1692ceae4b0c5df3bb45b74a.1612901326.git.matheus.bernardino@usp.br/
    (See his second link in that email in particular)
[2] https://lore.kernel.org/git/20220908001854.206789-2-shaoxuan.yuan02@gmail.com/
[3] https://lore.kernel.org/git/CABPp-BHwNoVnooqDFPAsZxBT9aR5Dwk5D9sDRCvYSb8akxAJgA@mail.gmail.com/
    (Scroll to the very end for the final few paragraphs)
[4] https://lore.kernel.org/git/cafcedba-96a2-cb85-d593-ef47c8c8397c@github.com/

Signed-off-by: Elijah Newren <newren@gmail.com>
---
    sparse-checkout.txt: new document with sparse-checkout directions
    
    v2 and v3 didn't get any reviews (I know, I know, this document is
    really long), but it's been nearly a month and this patch is still
    marked as "Needs Review", so I'm hoping sending a v4 will encourage
    feedback. I think it's good enough to accept and start iterating, but
    want to be sure others agree.
    
    As before, I think we're starting to converge on actual proposals;
    there's some areas we've agreed on, others we've compromised on, and
    some we've just figured out what the others were saying. The discussion
    has been very illuminating; thanks to everyone who has chimed in. I've
    tried to take my best stab at cleaning up and culling things that don't
    need to remain as open questions, but if I've mis-represented anyone or
    missed something, don't hesitate to speak up. Everything is still open
    for debate, even if not marked as a currently open question.
    
    Changes since v3:
    
     * A few minor wording cleanups here and there, and one paragraph moved
       to keep similar things together.
    
    Changes since v2:
    
     * Compromised with Stollee on log -- Behavior A only affects
       patch-related operations, not revision walking
     * Incorporated Junio's suggestions about untracked file handling
     * Added new usecases, one brought up by Martin, one by Stolee
     * Added new sections:
       * Usecases of primary concern
       * Oversimplified mental models ("Cliff Notes" for this document!)
     * Recategorization of a few commands based on discussion
     * Greater details on how index operations work under Behavior A, to
       avoid weird edge cases
     * Extended explanation of the sparse specification, particularly when
       index differs from HEAD
     * Switched proposed flag names to --scope={sparse,all} to avoid binary
       flags that are hard to extend
     * Switched proposed config option name (still need good values and
       descriptions for it, though)
     * Removed questions we seemed to have agreement on. Modified/extended
       some existing questions.
     * Added Stolee's sparse-backfill ideas to the plans
     * Additional Known bugs
     * Various wording improvements
     * Possibly other things I've missed.
    
    Changes since v1:
    
     * Added new sections:
       * "Terminology"
       * "Behavior classes"
       * "Sparse specification vs. sparsity patterns"
     * Tried to shuffle commands from unknown into appropriate sections
       based on feedback, but I got some conflicting feedback, so...who
       knows if thing are in the right place
     * More consistency in using "sparse specification" over other terms
     * Extra comments about how add/rm/mv operate on moving files across the
       tracked/untracked boundary
     * --restrict-but-warn should have been "restrict or error", but
       reworded even more heavily as part of "Behavior classes" section
     * Added extra questions based on feedback (--no-expand, update-index
       stuff, apply --index)
     * More details on apply/am bugs
     * Documented read-tree issue
     * A few cases of fixing line wrapping at <=80 chars
     * Added more alternate name suggestions for options instead of
       --[no-]restrict

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1367%2Fnewren%2Fsparse-checkout-directions-v4
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1367/newren/sparse-checkout-directions-v4
Pull-Request: https://github.com/gitgitgadget/git/pull/1367

Range-diff vs v3:

 1:  5923e75195c ! 1:  e09c7aa2396 sparse-checkout.txt: new document with sparse-checkout directions
     @@ Documentation/technical/sparse-checkout.txt (new)
      +	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 Git operation.
     ++	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
     @@ Documentation/technical/sparse-checkout.txt (new)
      +
      +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.  Controlled by the
     -+	--[no-]sparse-index option to init|set|reapply.  See also
     -+	"sparse directory".
     ++	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
     @@ Documentation/technical/sparse-checkout.txt (new)
      +
      +  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
     @@ Documentation/technical/sparse-checkout.txt (new)
      +     lazily populated, and sparse-checkout helps with the lazy population
      +     piece.
      +
     -+  A*) Users are _only_ interested in the sparse portion of the repo that
     -+      they have downloaded so far (a variant on the first usecase)
     -+
     -+
      +It may be worth explaining each of these in a bit more detail:
      +
      +
     @@ Documentation/technical/sparse-checkout.txt (new)
      +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 results from history (e.g. from diff/log/grep/etc.) is a
     ++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.
      +
     @@ Documentation/technical/sparse-checkout.txt (new)
      +after a merge or pull) can lead to worries about local repository size
      +growing unnecessarily[10].
      +
     -+  (Behavior B) Users want a sparse working tree, but are working in a larger whole
     ++  (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 ${SEARCH_TERM} OLDREV ` would need to be prepared to provide
     ++partial results that depend on what happens to have been downloaded.
     ++
     ++This variant could be viewed as Behavior A with the sparse specification
     ++for history querying operations modified from "sparsity patterns" to
     ++"sparsity patterns limited to the blobs we have already downloaded".
     ++
     ++  (Behavior B) Users want a sparse working tree, but are working in a
     ++      larger whole
      +
      +Stolee described this usecase this way[11]:
      +
     @@ Documentation/technical/sparse-checkout.txt (new)
      +will perceive the checkout as dense, and commands should thus behave as if
      +all files are present.
      +
     -+  (Behavior A*) Users are _only_ interested in the sparse portion of the repo
     -+      that they have downloaded so far (a variant on the first usecase)
     -+
     -+This variant is driven by folks who using partial clones together with
     -+sparse checkouts and do disconnected development (so far sounding like a
     -+subset of behavior A users) and doing so on very large repositories.  The
     -+reason for yet another variant is that downloading even just the blobs
     -+through history within their sparse specification may be too much, so they
     -+only download some.  They would still like operations to succeed without
     -+network connectivity, though, so things like `git log -S${SEARCH_TERM} -p`
     -+or `git grep ${SEARCH_TERM} OLDREV ` would need to be prepared to provide
     -+partial results.
     -+
     -+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".
     -+
      +
      +=== Usecases of primary concern ===
      +
     -+Most of the rest of this document will focus on the first two usecases:
     -+Behavior A and Behavior B.  Some notes about the other two cases and why we
     -+are not focusing on them:
     ++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*)
      +
     @@ Documentation/technical/sparse-checkout.txt (new)
      +      * 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):
     ++  * 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`)
     @@ Documentation/technical/sparse-checkout.txt (new)
      +    avoid the user trying to `git add` them, forcing `git add` to display
      +    an error).
      +
     -+    It's not clear to me exactly how (or if `clean` would change, but it's
     -+    the other command that also affects untracked files.
     ++    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
     @@ Documentation/technical/sparse-checkout.txt (new)
      +      * diff-tree
      +      * ls-tree
      +
     -+    Note: for log and whatchanged, only patch related parts are affected by
     -+    scoping the command to the sparse-checkout; the revision walking is
     -+    unaffected.  (The fact that revision walking is unaffected is why
     -+    rev-list, shortlog, show-branch, and bisect are not in this list.)
     ++    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
     @@ Documentation/technical/sparse-checkout.txt (new)
      +    * status
      +    * clean (?)
      +
     -+    Our original implementation for these commands was "no restrict", but
     -+    it had some severe usability issues:
     ++    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
     @@ Documentation/technical/sparse-checkout.txt (new)
      +    * diff (with --cached or REVISION arguments)
      +    * grep (with --cached or REVISION arguments)
      +    * show (when given commit arguments)
     -+    * bisect
      +    * blame (only matters when one or more -C flags passed)
      +      * and annotate
      +    * log
     @@ Documentation/technical/sparse-checkout.txt (new)
      +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.)
     ++--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 ===
     @@ Documentation/technical/sparse-checkout.txt (new)
      +     overview
      +
      +   * Add --scope=sparse (and --scope=all) flag to each of the history querying
     -+     commands.  IMPORATNT: make sure diff machinery changes don't mess with
     ++     commands.  IMPORTANT: make sure diff machinery changes don't mess with
      +     format-patch, fast-export, etc.
      +
      +=== Known bugs ===


 Documentation/technical/sparse-checkout.txt | 1103 +++++++++++++++++++
 1 file changed, 1103 insertions(+)
 create mode 100644 Documentation/technical/sparse-checkout.txt

diff --git a/Documentation/technical/sparse-checkout.txt b/Documentation/technical/sparse-checkout.txt
new file mode 100644
index 00000000000..fa0d01cbda4
--- /dev/null
+++ b/Documentation/technical/sparse-checkout.txt
@@ -0,0 +1,1103 @@
+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 publically 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/

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