[PATCH] read-cache: fix index corruption with index v4

[Thomas Gummerer <t.gummerer@gmail.com>]

ce012deb98 ("read-cache: avoid allocating every ondisk entry when
writing", 2017-08-21) changed the way cache entries are written to the
index file.  While previously it wrote the name to an struct that was
allocated using xcalloc(), it now uses ce_write() directly.  Previously
ce_namelen - common bytes were written to the cache entry, which would
automatically make it nul terminated, as it was allocated using calloc.

Now we are writing ce_namelen - common + 1 bytes directly from the
ce->name to the index.  As ce->name is however not nul terminated, and
index-v4 needs the nul terminator to split between one index entry and
the next, this would end up in a corrupted index.

Fix that by only writing ce_namelen - common bytes directly from
ce->name to the index, and adding the nul terminator in an extra call to
ce_write.

This bug was turned up by setting TEST_GIT_INDEX_VERSION = 4 in
config.mak and running the test suite (t1700 specifically broke).

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
---

I unfortunately didn't have more time to dig so

> As ce->name is however not nul terminated

just comes from my memory and from the patch below actually fixing the
corruption, so it's really the most likely cause.  Would be great if
someone who can remember more about the index could confirm that this
is indeed the case.

 read-cache.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/read-cache.c b/read-cache.c
index 40da87ea71..80830ddcfc 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -2103,7 +2103,9 @@ static int ce_write_entry(git_SHA_CTX *c, int fd, struct cache_entry *ce,
 		if (!result)
 			result = ce_write(c, fd, to_remove_vi, prefix_size);
 		if (!result)
-			result = ce_write(c, fd, ce->name + common, ce_namelen(ce) - common + 1);
+			result = ce_write(c, fd, ce->name + common, ce_namelen(ce) - common);
+		if (!result)
+			result = ce_write(c, fd, "\0", 1);
 
 		strbuf_splice(previous_name, common, to_remove,
 			      ce->name + common, ce_namelen(ce) - common);
-- 
2.14.1.480.gb18f417b89