[PATCH 03/27] sparse-index: API protection strategy

From

Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>

Date

Mar 16, 2021, 21:16 UTC

Message-ID

<bbf19f8a2be599a3451469731eed2eada7d3456a.1615929436.git.gitgitgadget@gmail.com>

In-Reply-To

<pull.906.git.1615929435.gitgitgadget@gmail.com>

From: Derrick Stolee <dstolee@microsoft.com>

Looking for non-zero stage:
* builtin/add.c:chmod_pathspec()
* builtin/merge.c:count_unmerged_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>
---
 Documentation/technical/sparse-index.txt | 32 +++++++++++++++++++++---
 1 file changed, 29 insertions(+), 3 deletions(-)

diff --git a/Documentation/technical/sparse-index.txt b/Documentation/technical/sparse-index.txt
index aa116406a016..7ab51bf6c441 100644
--- a/Documentation/technical/sparse-index.txt
+++ b/Documentation/technical/sparse-index.txt
@@ -82,9 +82,35 @@ also introduce other features that have been considered for improving the
 index, as well.
 
 Next, consumers of the index will be guarded against operating on a
-sparse-index by inserting calls to `ensure_full_index()` or
-`expand_index_to_path()`. After these guards are in place, we can begin
-leaving sparse-directory entries in the in-memory index structure.
+sparse-index by inserting calls to `ensure_full_index()` before iterating
+over all cache entries. If a specific path is requested, then those will
+be protected from within the `index_file_exists()` and `index_name_pos()`
+API calls: they will call `ensure_full_index()` if necessary.
+
+During a scan of the codebase, not every iteration of the cache entries
+needs an `ensure_full_index()` check. The basic reasons include:
+
+1. The loop is scanning for entries with non-zero stage. These entries
+   are not collapsed into a sparse-directory entry.
+
+2. The loop is scanning for submodules. These entries are not collapsed
+   into a sparse-directory entry.
+
+3. The loop is part of the index API, especially around reading or
+   writing the format.
+
+4. The loop is checking for correct order of cache entries and that is
+   correct if and only if the sparse-directory entries are in the correct
+   location.
+
+5. The loop ignores entries with the `SKIP_WORKTREE` bit set, or is
+   otherwise already aware of sparse directory entries.
+
+6. The sparse-index is disabled at this point when using the split-index
+   feature, so no effort is made to protect the split-index API.
+
+After these guards are in place, we can begin leaving sparse-directory
+entries in the in-memory index structure.
 
 Even after inserting these guards, we will keep expanding sparse-indexes
 for most Git commands using the `command_requires_full_index` repository

-- 
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