[PATCH] sparse-index: copy dir_hash in ensure_full_index()

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

From: Jeff Hostetler <jeffhost@microsoft.com>

Copy the 'index_state->dir_hash' back to the real istate after expanding
a sparse index.

A crash was observed in 'git status' during some hashmap lookups with
corrupted hashmap entries.  During an index expansion, new cache-entries
are added to the 'index_state->name_hash' and the 'dir_hash' in a
temporary 'index_state' variable 'full'.  However, only the 'name_hash'
hashmap from this temp variable was copied back into the real 'istate'
variable.  The original copy of the 'dir_hash' was incorrectly
preserved.  If the table in the 'full->dir_hash' hashmap were realloced,
the stale version (in 'istate') would be corrupted.

The test suite does not operate on index sizes sufficiently large to
trigger this reallocation, so they do not cover this behavior.
Increasing the test suite to cover such scale is fragile and likely
wasteful.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
    sparse-index: copy dir_hash in ensure_full_index()
    
    This fix is an issue we discovered in our first experimental release of
    the sparse index in the microsoft/git fork. We fixed it in the latest
    experimental release [1] and then I almost forgot about it until we
    started rebasing sparse-index work on top of the 2.33.0 release
    candidates.
    
    [1] https://github.com/microsoft/git/releases/tag/v2.32.0.vfs.0.102.exp
    
    This is a change that can be taken anywhere since 4300f8 (sparse-index:
    implement ensure_full_index(), 2021-03-30), but this version is based on
    v2.33.0-rc2.
    
    While the bug is alarming for users who hit it (seg fault) it requires
    sufficient scale and use of the optional sparse index feature. We are
    not recommending wide adoption of the sparse index yet because we don't
    have a sufficient density of integrated commands. For that reason, I
    don't think this should halt progress towards the full v2.33.0 release.
    I did want to send this as soon as possible so that could be at the
    discretion of the maintainer.
    
    Thanks, -Stolee

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1017%2Fderrickstolee%2Fsparse-index%2Ffix-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1017/derrickstolee/sparse-index/fix-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1017

 sparse-index.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/sparse-index.c b/sparse-index.c
index c6b4feec413..56eb65dc349 100644
--- a/sparse-index.c
+++ b/sparse-index.c
@@ -283,6 +283,7 @@ void ensure_full_index(struct index_state *istate)
 
 	/* Copy back into original index. */
 	memcpy(&istate->name_hash, &full->name_hash, sizeof(full->name_hash));
+	memcpy(&istate->dir_hash, &full->dir_hash, sizeof(full->dir_hash));
 	istate->sparse_index = 0;
 	free(istate->cache);
 	istate->cache = full->cache;

base-commit: 5d213e46bb7b880238ff5ea3914e940a50ae9369
-- 
gitgitgadget