[PATCH] Prevent git from rehashing 4GBi files

["Jason D. Hatton" <jason.hatton@gmail.com>]

Git cache stores file sizes using uint32_t. This causes any file
that is a multiple of 2^32 to have a cached file size of zero.
Zero is a special value used by racily clean. This causes git to
rehash every file that is a multiple of 2^32 every time git status
or git commit is run.

This patch mitigates the problem by making all files that are a
multiple of 2^32 appear to have a size of 1 instead of zero.

Signed-off-by: Jason D. Hatton <jhatton@globalfinishing.com>
---
 read-cache.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/read-cache.c b/read-cache.c
index 4df97e185e..d80c80ef90 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -163,6 +163,22 @@ void rename_index_entry_at(struct index_state *istate, int nr, const char *new_n
 		add_index_entry(istate, new_entry, ADD_CACHE_OK_TO_ADD|ADD_CACHE_OK_TO_REPLACE);
 }
 
+/*
+ * stat_data only stores file sizes as an unsigned int. Any file that is an
+ * exact multiple of 4GiB will get a cached file size of zero. Unfortunately,
+ * this is a special flag used by the racy update logic. Substitute a new file
+ * size if a non-zero sized file would would be cached as zero. 1U<<31 is used
+ * as the substitute because it is the furthest away from 0 and 4GiB.
+ */
+static inline unsigned int munge_st_size(off_t st_size) {
+	unsigned int sd_size = (unsigned int) st_size;
+
+	if (!sd_size && st_size)
+		return 1U<<31;
+	else
+		return sd_size;
+}
+
 void fill_stat_data(struct stat_data *sd, struct stat *st)
 {
 	sd->sd_ctime.sec = (unsigned int)st->st_ctime;
@@ -173,7 +189,7 @@ void fill_stat_data(struct stat_data *sd, struct stat *st)
 	sd->sd_ino = st->st_ino;
 	sd->sd_uid = st->st_uid;
 	sd->sd_gid = st->st_gid;
-	sd->sd_size = st->st_size;
+	sd->sd_size = munge_st_size(st->st_size);
 }
 
 int match_stat_data(const struct stat_data *sd, struct stat *st)
@@ -212,7 +228,7 @@ int match_stat_data(const struct stat_data *sd, struct stat *st)
 			changed |= INODE_CHANGED;
 #endif
 
-	if (sd->sd_size != (unsigned int) st->st_size)
+	if (sd->sd_size != munge_st_size(st->st_size))
 		changed |= DATA_CHANGED;
 
 	return changed;
-- 
2.36.0.3