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authorJunio C Hamano <junkio@cox.net>2005-12-20 22:18:47 (GMT)
committerJunio C Hamano <junkio@cox.net>2005-12-20 22:18:47 (GMT)
commit4b3511b0f8422fd2c5b1b37c9655ae2ce904bca5 (patch)
tree823033e53c3caf6805f160bedea1075e464a0bdd /read-cache.c
parent407c8eb0d09d4b84bbfda9e04895a35c8fd6fef6 (diff)
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ce_smudge_racily_clean_entry: explain why it works.
This is a tricky code and warrants extra commenting. I wasted 30 minutes trying to break it until I realized why it works. Signed-off-by: Junio C Hamano <junkio@cox.net>
Diffstat (limited to 'read-cache.c')
-rw-r--r--read-cache.c26
1 files changed, 25 insertions, 1 deletions
diff --git a/read-cache.c b/read-cache.c
index afdc2b0..c5474d4 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -616,7 +616,31 @@ static void ce_smudge_racily_clean_entry(struct cache_entry *ce)
if (ce_match_stat_basic(ce, &st))
return;
if (ce_modified_check_fs(ce, &st)) {
- /* This is "racily clean"; smudge it */
+ /* This is "racily clean"; smudge it. Note that this
+ * is a tricky code. At first glance, it may appear
+ * that it can break with this sequence:
+ *
+ * $ echo xyzzy >frotz
+ * $ git-update-index --add frotz
+ * $ : >frotz
+ * $ sleep 3
+ * $ echo filfre >nitfol
+ * $ git-update-index --add nitfol
+ *
+ * but it does not. Whe the second update-index runs,
+ * it notices that the entry "frotz" has the same timestamp
+ * as index, and if we were to smudge it by resetting its
+ * size to zero here, then the object name recorded
+ * in index is the 6-byte file but the cached stat information
+ * becomes zero --- which would then match what we would
+ * obtain from the filesystem next time we stat("frotz").
+ *
+ * However, the second update-index, before calling
+ * this function, notices that the cached size is 6
+ * bytes and what is on the filesystem is an empty
+ * file, and never calls us, so the cached size information
+ * for "frotz" stays 6 which does not match the filesystem.
+ */
ce->ce_size = htonl(0);
}
}