Re: [PATCH v2 4/5] split-index: don't compare stat data of entries already marked for split index
- From
SZEDER Gábor <szeder.dev@gmail.com>
- Date
- Sep 27, 2018, 13:43 UTC
- Message-ID
- <20180927134324.GI27036@localhost>
- In-Reply-To
- <20180927124434.30835-5-szeder.dev@gmail.com>
On Thu, Sep 27, 2018 at 02:44:33PM +0200, SZEDER Gábor wrote:
> split-index.c | 79 ++++++++++++++++++++++++++++++++++++++++----------- > 1 file changed, 62 insertions(+), 17 deletions(-)
I generated this patch with more context lines than usual, so the two conditions that I didn't add any comments to in this or in the next patch are fully visible.
Show 42 quoted lines
> diff --git a/split-index.c b/split-index.c
> index 548272ec33..7d8799f6b7 100644
> --- a/split-index.c
> +++ b/split-index.c
> @@ -204,19 +204,34 @@ void prepare_to_write_split_index(struct index_state *istate)
> * that are not marked with either CE_MATCHED or
> * CE_UPDATE_IN_BASE. If istate->cache[i] is a
> * duplicate, deduplicate it.
> */
> for (i = 0; i < istate->cache_nr; i++) {
> struct cache_entry *base;
> - /* namelen is checked separately */
> - const unsigned int ondisk_flags =
> - CE_STAGEMASK | CE_VALID | CE_EXTENDED_FLAGS;
> - unsigned int ce_flags, base_flags, ret;
> ce = istate->cache[i];
> - if (!ce->index)
> + if (!ce->index) {
> + /*
> + * During simple update index operations this
> + * is a cache entry that is not present in
> + * the shared index. It will be added to the
> + * split index.
> + *
> + * However, it might also represent a file
> + * that already has a cache entry in the
> + * shared index, but a new index has just
> + * been constructed by unpack_trees(), and
> + * this entry now refers to different content
> + * than what was recorded in the original
> + * index, e.g. during 'read-tree -m HEAD^' or
> + * 'checkout HEAD^'. In this case the
> + * original entry in the shared index will be
> + * marked as deleted, and this entry will be
> + * added to the split index.
> + */
> continue;
> + }
> if (ce->index > si->base->cache_nr) {
> ce->index = 0;
> continue;
> }This condition in the context above checks whether a cache entry refers to a non-existing entry in the shared index.
I don't understand the role of this condition, for two reasons:
- Under what circumstances can this condition be ever fulfilled?
I instrumented it and run the test suite repeatedly with
'GIT_TEST_SPLIT_INDEX=yes', but it has never been fulfilled. I
also tried to come up with all kinds of elaborate scenarios to
trigger it, but no joy, and code inspection didn't bring anything
either. - There are similar conditions in 'split-index.c' in the functions
mark_entry_for_delete() and replace_entry(); here is the one from
the latter, but they only differ in the error message: if (pos >= istate->cache_nr)
die("position for replacement %d exceeds base index size %d",
(int)pos, istate->cache_nr); (Note that this 'istate->cache_nr' here equals
to 'si->base->cache_nr'; see their caller merge_base_index().) The die() clearly indicates that fulfilling this condition is a
Bad Thing. These two functions are invoked to create a unified
view of the just read split and shared indexes, so the fulfillment
of this condition could indicate a corrupt index file, and
die()ing right away seems to be justified. Then why doesn't the condition in prepare_to_write_split_index()
die() as well?! After all if it were fulfilled, then it would
indicate a corruption in the current index_state, and writing a
new split index from corrupt data doesn't seem like a particularly
good idea.Show 9 quoted lines
> ce->ce_flags |= CE_MATCHED; /* or "shared" */
> base = si->base->cache[ce->index - 1];
> @@ -224,24 +239,54 @@ void prepare_to_write_split_index(struct index_state *istate)
> continue;
> if (ce->ce_namelen != base->ce_namelen ||
> strcmp(ce->name, base->name)) {
> ce->index = 0;
> continue;
> }I don't understand the role of this condition either, and just like the one discussed above, the test suite with 'GIT_TEST_SPLIT_INDEX=yes' seems to never fulfill it.
Show 63 quoted lines
> - ce_flags = ce->ce_flags;
> - base_flags = base->ce_flags;
> - /* only on-disk flags matter */
> - ce->ce_flags &= ondisk_flags;
> - base->ce_flags &= ondisk_flags;
> - ret = memcmp(&ce->ce_stat_data, &base->ce_stat_data,
> - offsetof(struct cache_entry, name) -
> - offsetof(struct cache_entry, ce_stat_data));
> - ce->ce_flags = ce_flags;
> - base->ce_flags = base_flags;
> - if (ret)
> - ce->ce_flags |= CE_UPDATE_IN_BASE;
> + /*
> + * This is the copy of a cache entry that is present
> + * in the shared index, created by unpack_trees()
> + * while it constructed a new index.
> + */
> + if (ce->ce_flags & CE_UPDATE_IN_BASE) {
> + /*
> + * Already marked for inclusion in the split
> + * index, either because the corresponding
> + * file was modified and the cached stat data
> + * was refreshed, or because the original
> + * entry already had a replacement entry in
> + * the split index.
> + * Nothing to do.
> + */
> + } else {
> + /*
> + * Thoroughly compare the cached data to see
> + * whether it should be marked for inclusion
> + * in the split index.
> + *
> + * This comparison might be unnecessary, as
> + * code paths modifying the cached data do
> + * set CE_UPDATE_IN_BASE as well.
> + */
> + const unsigned int ondisk_flags =
> + CE_STAGEMASK | CE_VALID |
> + CE_EXTENDED_FLAGS;
> + unsigned int ce_flags, base_flags, ret;
> + ce_flags = ce->ce_flags;
> + base_flags = base->ce_flags;
> + /* only on-disk flags matter */
> + ce->ce_flags &= ondisk_flags;
> + base->ce_flags &= ondisk_flags;
> + ret = memcmp(&ce->ce_stat_data, &base->ce_stat_data,
> + offsetof(struct cache_entry, name) -
> + offsetof(struct cache_entry, ce_stat_data));
> + ce->ce_flags = ce_flags;
> + base->ce_flags = base_flags;
> + if (ret)
> + ce->ce_flags |= CE_UPDATE_IN_BASE;
> + }
> discard_cache_entry(base);
> si->base->cache[ce->index - 1] = ce;
> }
> for (i = 0; i < si->base->cache_nr; i++) {
> ce = si->base->cache[i];
> if ((ce->ce_flags & CE_REMOVE) ||
> --
> 2.19.0.361.gafc87ffe72
>