read-cache.c: fix writing "link" index ext with null base oid

Since commit 7db118303a (unpack_trees: fix breakage when o->src_index !=
o->dst_index - 2018-04-23) and changes in merge code to use separate
index_state for source and destination, when doing a merge with split
index activated, we may run into this line in unpack_trees():

o->result.split_index = init_split_index(&o->result);

This is by itself not wrong. But this split index information is not
fully populated (and it's only so when move_cache_to_base_index() is
called, aka force splitting the index, or loading index_state from a
file). Both "base_oid" and "base" in this case remain null.

So when writing the main index down, we link to this index with null
oid (default value after init_split_index()), which also means "no split
index" internally. This triggers an incorrect base index refresh:

warning: could not freshen shared index '.../sharedindex.0{40}'

This patch makes sure we will not refresh null base_oid (because the
file is never there). It also makes sure not to write "link" extension
with null base_oid in the first place (no point having it at
all). Read code already has protection against null base_oid.

There is also another side fix in remove_split_index() that causes a
crash when doing "git update-index --no-split-index" when base_oid in
the index file is null. In this case we will not load
istate->split_index->base but we dereference it anyway and are rewarded
with a segfault. This should not happen anymore, but it's still wrong to
dereference a potential NULL pointer, especially when we do check for
NULL pointer in the next code.

Reported-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/read-cache.c b/read-cache.c
index 8f644f68b432e173dc31d5739944bbdfe9c4ad27..d140b44f8f29adb90c2bbcb74e6f55b06a9dcb47 100644 (file)
--- a/read-cache.c
+++ b/read-cache.c
@@ -2520,7 +2520,8 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
                return err;
 
        /* Write extension data here */
-       if (!strip_extensions && istate->split_index) {
+       if (!strip_extensions && istate->split_index &&
+           !is_null_oid(&istate->split_index->base_oid)) {
                struct strbuf sb = STRBUF_INIT;
 
                err = write_link_extension(&sb, istate) < 0 ||
@@ -2794,7 +2795,7 @@ int write_locked_index(struct index_state *istate, struct lock_file *lock,
        ret = write_split_index(istate, lock, flags);
 
        /* Freshen the shared index only if the split-index was written */
-       if (!ret && !new_shared_index) {
+       if (!ret && !new_shared_index && !is_null_oid(&si->base_oid)) {
                const char *shared_index = git_path("sharedindex.%s",
                                                    oid_to_hex(&si->base_oid));
                freshen_shared_index(shared_index, 1);

diff --git a/split-index.c b/split-index.c
index 5820412dc5203032d1247575d79c6107a163295a..a9d13611a432d7bceb041877348a9125111b3176 100644 (file)
--- a/split-index.c
+++ b/split-index.c
@@ -440,24 +440,26 @@ void add_split_index(struct index_state *istate)
 void remove_split_index(struct index_state *istate)
 {
        if (istate->split_index) {
-               /*
-                * When removing the split index, we need to move
-                * ownership of the mem_pool associated with the
-                * base index to the main index. There may be cache entries
-                * allocated from the base's memory pool that are shared with
-                * the_index.cache[].
-                */
-               mem_pool_combine(istate->ce_mem_pool, istate->split_index->base->ce_mem_pool);
+               if (istate->split_index->base) {
+                       /*
+                        * When removing the split index, we need to move
+                        * ownership of the mem_pool associated with the
+                        * base index to the main index. There may be cache entries
+                        * allocated from the base's memory pool that are shared with
+                        * the_index.cache[].
+                        */
+                       mem_pool_combine(istate->ce_mem_pool,
+                                        istate->split_index->base->ce_mem_pool);
 
-               /*
-                * The split index no longer owns the mem_pool backing
-                * its cache array. As we are discarding this index,
-                * mark the index as having no cache entries, so it
-                * will not attempt to clean up the cache entries or
-                * validate them.
-                */
-               if (istate->split_index->base)
+                       /*
+                        * The split index no longer owns the mem_pool backing
+                        * its cache array. As we are discarding this index,
+                        * mark the index as having no cache entries, so it
+                        * will not attempt to clean up the cache entries or
+                        * validate them.
+                        */
                        istate->split_index->base->cache_nr = 0;
+               }
 
                /*
                 * We can discard the split index because its

diff --git a/t/t1700-split-index.sh b/t/t1700-split-index.sh
index f053bf83eb9f716c3dc99a3bdf1a8e3b49802163..ea5181aff964e831378f3ae2d312dabd6858d4fe 100755 (executable)
--- a/t/t1700-split-index.sh
+++ b/t/t1700-split-index.sh
@@ -447,4 +447,22 @@ test_expect_success 'writing split index with null sha1 does not write cache tre
        test_line_count = 0 cache-tree.out
 '
 
+test_expect_success 'do not refresh null base index' '
+       test_create_repo merge &&
+       (
+               cd merge &&
+               test_commit initial &&
+               git checkout -b side-branch &&
+               test_commit extra &&
+               git checkout master &&
+               git update-index --split-index &&
+               test_commit more &&
+               # must not write a new shareindex, or we wont catch the problem
+               git -c splitIndex.maxPercentChange=100 merge --no-edit side-branch 2>err &&
+               # i.e. do not expect warnings like
+               # could not freshen shared index .../shareindex.00000...
+               test_must_be_empty err
+       )
+'
+
 test_done