Re: [PATCH v4 4/4] sparse-index: update do_read_index to ensure correct sparsity

Elijah Newren wrote:

On Fri, Oct 29, 2021 at 6:56 AM Victoria Dye via GitGitGadget
[off-list ref] wrote:

quoted

From: Victoria Dye <redacted>

If `command_requires_full_index` is false, ensure correct in-core index
sparsity on read by calling `ensure_correct_sparsity`. This change is meant
to update the how the index is read in a command after sparse index-related

s/update the how/update how/ ?

This was probably the result of mixing up "update the way" and "update how".
I'll go with the latter.

quoted

repository settings are modified. Previously, for example, if `index.sparse`
were changed from `true` to `false`, the in-core index on the next command
would be sparse. The index would only be expanded to full when it was next
written to disk.

By adding a call to `ensure_correct_sparsity`, the in-core index now matches
the sparsity dictated by the relevant repository settings as soon as it is
read into memory, rather than when it is later written to disk.

I split up reading this series across different days, and when I got
here, my first reaction was "Okay, but why would you want that?
Sounds like extra work for no gain."  I went and looked up the cover
letter and saw you mentioned that this "introduces the ability to
enable/disable the sparse index on a command-by-command basis (e.g.,
allowing a user to troubleshoot a sparse-aware command with '-c
index.sparse=false' [1]).  That seems like a good reason to me, and
sounds like it belongs in this commit message.  But it sounds like you
had other reasons in mind.  If so, could you share them; I'm having
difficulty understanding how this would make a difference other than
in the troubleshooting scenario.

The troubleshooting scenario was what inspired this change in the first
place, but it applies any time someone changes repository settings outside
of `git sparse-checkout init`. One relevant scenario I can imagine is if a
user enables `index.sparse, commands would not use the sparse index until a
command that *writes* the index is executed:

$ git config index.sparse true
$ git diff      # read full index, full in-core, no write
$ git add .     # read full index, full in-core, write sparse
$ git diff      # read sparse index, sparse in-core, no write

Another scenario might be enabling `index.sparse`, then running a command
that turns out to be surprisingly slow because it's operating on a full
index in-core (i.e., much slower than the convert_to_sparse + command with
sparse index would be). 

These are definitely edge cases - they rely on manual config changes, and
they're only really noticeable 1) if the config change is immediately
followed by index read-only commands and 2) if the first index-writing
command after the config change is slow. That said, on the off chance that a
user (or future developer) encounter one of these scenarios, I think it'll
be helpful to have `index.sparse` take effect "immediately" after the config
change.

I'll include these (and the troubleshooting) examples in the updated commit
message.

quoted

Helped-by: Junio C Hamano [off-list ref]
Co-authored-by: Derrick Stolee [off-list ref]
Signed-off-by: Victoria Dye <redacted>
---
 read-cache.c                             |  8 ++++++
 t/t1092-sparse-checkout-compatibility.sh | 31 ++++++++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/read-cache.c b/read-cache.c
index a78b88a41bf..b3772ba70a1 100644
--- a/read-cache.c
+++ b/read-cache.c

@@ -2337,9 +2337,17 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)

        if (!istate->repo)
                istate->repo = the_repository;
+
+       /*
+        * If the command explicitly requires a full index, force it
+        * to be full. Otherwise, correct the sparsity based on repository
+        * settings and other properties of the index (if necessary).
+        */
        prepare_repo_settings(istate->repo);
        if (istate->repo->settings.command_requires_full_index)
                ensure_full_index(istate);
+       else
+               ensure_correct_sparsity(istate);

        return istate->cache_nr;

diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
index ca91c6a67f8..59accde1fa3 100755
--- a/t/t1092-sparse-checkout-compatibility.sh
+++ b/t/t1092-sparse-checkout-compatibility.sh

@@ -694,6 +694,37 @@ test_expect_success 'sparse-index is expanded and converted back' '
        test_region index ensure_full_index trace2.txt
 '

+test_expect_success 'index.sparse disabled inline uses full index' '
+       init_repos &&
+
+       # When index.sparse is disabled inline with `git status`, the
+       # index is expanded at the beginning of the execution then never
+       # converted back to sparse. It is then written to disk as a full index.
+       rm -f trace2.txt &&
+       GIT_TRACE2_EVENT="$(pwd)/trace2.txt" GIT_TRACE2_EVENT_NESTING=10 \
+               git -C sparse-index -c index.sparse=false status &&
+       ! test_region index convert_to_sparse trace2.txt &&
+       test_region index ensure_full_index trace2.txt &&
+
+       # Since index.sparse is set to true at a repo level, the index
+       # is converted from full to sparse when read, then never expanded
+       # over the course of `git status`. It is written to disk as a sparse
+       # index.
+       rm -f trace2.txt &&
+       GIT_TRACE2_EVENT="$(pwd)/trace2.txt" GIT_TRACE2_EVENT_NESTING=10 \
+               git -C sparse-index status &&
+       test_region index convert_to_sparse trace2.txt &&
+       ! test_region index ensure_full_index trace2.txt &&
+
+       # Now that the index has been written to disk as sparse, it is not
+       # converted to sparse (or expanded to full) when read by `git status`.
+       rm -f trace2.txt &&
+       GIT_TRACE2_EVENT="$(pwd)/trace2.txt" GIT_TRACE2_EVENT_NESTING=10 \
+               git -C sparse-index status &&
+       ! test_region index convert_to_sparse trace2.txt &&
+       ! test_region index ensure_full_index trace2.txt
+'
+
 ensure_not_expanded () {
        rm -f trace2.txt &&
        echo >>sparse-index/untracked.txt &&
--

gitgitgadget

The rest of the patch looks fine.