Re: git bugs

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Jun 10, 2008, 17:44 UTC
Message-ID: <alpine.LFD.1.10.0806101028040.3101@woody.linux-foundation.org>
In-Reply-To: <832adb090806100141n69c086a2v2f59fe94b2f4ead3@mail.gmail.com>

On Tue, 10 Jun 2008, Ben Lynn wrote:

> 
> Unfortunately, the solution isn't perfect. Try this:

Heh.

That's just because our "smudge_racily_clean_entry()" uses 0 as the magic smudging size.

You can fix this multiple ways. One would be to pick another size that is simply less likely (eg ~0 instead), which leaves the theoretical race, and just makes it practically impossible to hit (not that I think it's very practical to hit already).

The other approach is to know that an empty blob always has a very specific SHA1. Here's an trial patch.

		Linus

---
 read-cache.c |   16 ++++++++++++++++
 1 files changed, 16 insertions(+), 0 deletions(-)

diff --git a/read-cache.c b/read-cache.c
index 8e5fbb6..f83de8c 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -138,6 +138,16 @@ static int ce_modified_check_fs(struct cache_entry *ce, struct stat *st)
 	return 0;
 }
 
+static int is_empty_blob_sha1(const unsigned char *sha1)
+{
+	static const unsigned char empty_blob_sha1[20] = {
+		0xe6,0x9d,0xe2,0x9b,0xb2,0xd1,0xd6,0x43,0x4b,0x8b,
+		0x29,0xae,0x77,0x5a,0xd8,0xc2,0xe4,0x8c,0x53,0x91
+	};
+
+	return !hashcmp(sha1, empty_blob_sha1);
+}
+
 static int ce_match_stat_basic(struct cache_entry *ce, struct stat *st)
 {
 	unsigned int changed = 0;
@@ -193,6 +203,12 @@ static int ce_match_stat_basic(struct cache_entry *ce, struct stat *st)
 	if (ce->ce_size != (unsigned int) st->st_size)
 		changed |= DATA_CHANGED;
 
+	/* Racily smudged entry? */
+	if (!ce->ce_size) {
+		if (!is_empty_blob_sha1(ce->sha1))
+			changed |= DATA_CHANGED;
+	}
+
 	return changed;
 }