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Edit and expand the sparse-index design document with the plan for guarding index operations with ensure_full_index(). Notably, the plan has changed to not have an expand_to_path() method in favor of checking for a sparse-directory hit inside of the index_path_pos() API. The changes that follow this one will incrementally add ensure_full_index() guards to iterations over all cache entries. Some iterations over the cache entries are not protected due to a few categories listed in the document. Since these are not being modified, here is a short list of the files and methods that will not receive these guards: Looking for non-zero stage: * builtin/add.c:chmod_pathspec() * builtin/merge.c:count_unmerged_entries() * merge-ort.c:record_conflicted_index_entries() * read-cache.c:unmerged_index() * rerere.c:check_one_conflict(), find_conflict(), rerere_remaining() * revision.c:prepare_show_merge() * sequencer.c:append_conflicts_hint() * wt-status.c:wt_status_collect_changes_initial() Looking for submodules: * builtin/submodule--helper.c:module_list_compute() * submodule.c: several methods * worktree.c:validate_no_submodules() Part of the index API: * name-hash.c: lazy init methods * preload-index.c:preload_thread(), preload_index() * read-cache.c: file format methods Checking for correct order of cache entries: * read-cache.c:check_ce_order() Ignores SKIP_WORKTREE entries or already aware: * unpack-trees.c:mark_new_skip_worktree() * wt-status.c:wt_status_check_sparse_checkout() Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Reviewed-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
208 lines
9.3 KiB
Text
208 lines
9.3 KiB
Text
Git Sparse-Index Design Document
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================================
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The sparse-checkout feature allows users to focus a working directory on
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a subset of the files at HEAD. The cone mode patterns, enabled by
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`core.sparseCheckoutCone`, allow for very fast pattern matching to
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discover which files at HEAD belong in the sparse-checkout cone.
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Three important scale dimensions for a Git working directory are:
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* `HEAD`: How many files are present at `HEAD`?
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* Populated: How many files are within the sparse-checkout cone.
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* Modified: How many files has the user modified in the working directory?
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We will use big-O notation -- O(X) -- to denote how expensive certain
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operations are in terms of these dimensions.
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These dimensions are ordered by their magnitude: users (typically) modify
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fewer files than are populated, and we can only populate files at `HEAD`.
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Problems occur if there is an extreme imbalance in these dimensions. For
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example, if `HEAD` contains millions of paths but the populated set has
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only tens of thousands, then commands like `git status` and `git add` can
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be dominated by operations that require O(`HEAD`) operations instead of
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O(Populated). Primarily, the cost is in parsing and rewriting the index,
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which is filled primarily with files at `HEAD` that are marked with the
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`SKIP_WORKTREE` bit.
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The sparse-index intends to take these commands that read and modify the
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index from O(`HEAD`) to O(Populated). To do this, we need to modify the
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index format in a significant way: add "sparse directory" entries.
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With cone mode patterns, it is possible to detect when an entire
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directory will have its contents outside of the sparse-checkout definition.
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Instead of listing all of the files it contains as individual entries, a
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sparse-index contains an entry with the directory name, referencing the
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object ID of the tree at `HEAD` and marked with the `SKIP_WORKTREE` bit.
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If we need to discover the details for paths within that directory, we
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can parse trees to find that list.
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At time of writing, sparse-directory entries violate expectations about the
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index format and its in-memory data structure. There are many consumers in
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the codebase that expect to iterate through all of the index entries and
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see only files. In fact, these loops expect to see a reference to every
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staged file. One way to handle this is to parse trees to replace a
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sparse-directory entry with all of the files within that tree as the index
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is loaded. However, parsing trees is slower than parsing the index format,
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so that is a slower operation than if we left the index alone. The plan is
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to make all of these integrations "sparse aware" so this expansion through
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tree parsing is unnecessary and they use fewer resources than when using a
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full index.
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The implementation plan below follows four phases to slowly integrate with
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the sparse-index. The intention is to incrementally update Git commands to
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interact safely with the sparse-index without significant slowdowns. This
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may not always be possible, but the hope is that the primary commands that
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users need in their daily work are dramatically improved.
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Phase I: Format and initial speedups
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------------------------------------
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During this phase, Git learns to enable the sparse-index and safely parse
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one. Protections are put in place so that every consumer of the in-memory
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data structure can operate with its current assumption of every file at
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`HEAD`.
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At first, every index parse will call a helper method,
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`ensure_full_index()`, which scans the index for sparse-directory entries
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(pointing to trees) and replaces them with the full list of paths (with
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blob contents) by parsing tree objects. This will be slower in all cases.
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The only noticeable change in behavior will be that the serialized index
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file contains sparse-directory entries.
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To start, we use a new required index extension, `sdir`, to allow
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inserting sparse-directory entries into indexes with file format
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versions 2, 3, and 4. This prevents Git versions that do not understand
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the sparse-index from operating on one, while allowing tools that do not
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understand the sparse-index to operate on repositories as long as they do
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not interact with the index. A new format, index v5, will be introduced
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that includes sparse-directory entries by default. It might also
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introduce other features that have been considered for improving the
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index, as well.
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Next, consumers of the index will be guarded against operating on a
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sparse-index by inserting calls to `ensure_full_index()` or
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`expand_index_to_path()`. If a specific path is requested, then those will
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be protected from within the `index_file_exists()` and `index_name_pos()`
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API calls: they will call `ensure_full_index()` if necessary. The
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intention here is to preserve existing behavior when interacting with a
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sparse-checkout. We don't want a change to happen by accident, without
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tests. Many of these locations may not need any change before removing the
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guards, but we should not do so without tests to ensure the expected
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behavior happens.
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It may be desirable to _change_ the behavior of some commands in the
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presence of a sparse index or more generally in any sparse-checkout
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scenario. In such cases, these should be carefully communicated and
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tested. No such behavior changes are intended during this phase.
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During a scan of the codebase, not every iteration of the cache entries
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needs an `ensure_full_index()` check. The basic reasons include:
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1. The loop is scanning for entries with non-zero stage. These entries
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are not collapsed into a sparse-directory entry.
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2. The loop is scanning for submodules. These entries are not collapsed
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into a sparse-directory entry.
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3. The loop is part of the index API, especially around reading or
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writing the format.
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4. The loop is checking for correct order of cache entries and that is
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correct if and only if the sparse-directory entries are in the correct
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location.
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5. The loop ignores entries with the `SKIP_WORKTREE` bit set, or is
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otherwise already aware of sparse directory entries.
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6. The sparse-index is disabled at this point when using the split-index
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feature, so no effort is made to protect the split-index API.
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Even after inserting these guards, we will keep expanding sparse-indexes
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for most Git commands using the `command_requires_full_index` repository
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setting. This setting will be on by default and disabled one builtin at a
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time until we have sufficient confidence that all of the index operations
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are properly guarded.
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To complete this phase, the commands `git status` and `git add` will be
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integrated with the sparse-index so that they operate with O(Populated)
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performance. They will be carefully tested for operations within and
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outside the sparse-checkout definition.
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Phase II: Careful integrations
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------------------------------
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This phase focuses on ensuring that all index extensions and APIs work
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well with a sparse-index. This requires significant increases to our test
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coverage, especially for operations that interact with the working
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directory outside of the sparse-checkout definition. Some of these
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behaviors may not be the desirable ones, such as some tests already
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marked for failure in `t1092-sparse-checkout-compatibility.sh`.
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The index extensions that may require special integrations are:
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* FS Monitor
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* Untracked cache
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While integrating with these features, we should look for patterns that
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might lead to better APIs for interacting with the index. Coalescing
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common usage patterns into an API call can reduce the number of places
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where sparse-directories need to be handled carefully.
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Phase III: Important command speedups
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-------------------------------------
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At this point, the patterns for testing and implementing sparse-directory
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logic should be relatively stable. This phase focuses on updating some of
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the most common builtins that use the index to operate as O(Populated).
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Here is a potential list of commands that could be valuable to integrate
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at this point:
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* `git commit`
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* `git checkout`
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* `git merge`
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* `git rebase`
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Hopefully, commands such as `git merge` and `git rebase` can benefit
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instead from merge algorithms that do not use the index as a data
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structure, such as the merge-ORT strategy. As these topics mature, we
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may enable the ORT strategy by default for repositories using the
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sparse-index feature.
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Along with `git status` and `git add`, these commands cover the majority
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of users' interactions with the working directory. In addition, we can
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integrate with these commands:
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* `git grep`
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* `git rm`
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These have been proposed as some whose behavior could change when in a
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repo with a sparse-checkout definition. It would be good to include this
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behavior automatically when using a sparse-index. Some clarity is needed
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to make the behavior switch clear to the user.
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This phase is the first where parallel work might be possible without too
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much conflicts between topics.
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Phase IV: The long tail
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-----------------------
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This last phase is less a "phase" and more "the new normal" after all of
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the previous work.
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To start, the `command_requires_full_index` option could be removed in
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favor of expanding only when hitting an API guard.
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There are many Git commands that could use special attention to operate as
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O(Populated), while some might be so rare that it is acceptable to leave
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them with additional overhead when a sparse-index is present.
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Here are some commands that might be useful to update:
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* `git sparse-checkout set`
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* `git am`
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* `git clean`
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* `git stash`
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