Re: [PATCH 3/5] optimize parse_sha1_header() a little by detecting object type
- From
- Liu Yubao <yubao.liu@gmail.com>
- Date
- Dec 3, 2008, 04:06 UTC
- Message-ID
- <493605BB.8020705@gmail.com>
- In-Reply-To
- <20081202155300.GL23984@spearce.org>
Shawn O. Pearce wrote:
Show 22 quoted lines
> Liu Yubao <yubao.liu@gmail.com> wrote:
>> diff --git a/sha1_file.c b/sha1_file.c
>> index dccc455..79062f0 100644
>> --- a/sha1_file.c
>> +++ b/sha1_file.c
>> @@ -1099,7 +1099,8 @@ static void *map_sha1_file(const unsigned char *sha1, unsigned long *size)
>>
>> if (!fstat(fd, &st)) {
>> *size = xsize_t(st.st_size);
>> - map = xmmap(NULL, *size, PROT_READ, MAP_PRIVATE, fd, 0);
>> + if (*size > 0)
>> + map = xmmap(NULL, *size, PROT_READ, MAP_PRIVATE, fd, 0);
>> }
>> close(fd);
>> }
>
> This has nothing to do with this change description. Why are we
> returning NULL from map_sha1_file when the file length is 0 bytes?
> No loose object should ever be an empty file, there must always be
> some sort of type header present. So it probably is an error to
> have a 0 length file here. But that bug is a different change.
> Also a defensive programming for uncompressed loose object because the mapped memory will be passed to parse_sha1_header() directly without being checked by inflateInit().
In fact unpack_sha1_header() in current code calls legacy_loose_object() without checking mapsize first. If it encounters (very very unlikely) a corrupted empty loose object, it will crash.
Show 16 quoted lines
>> @@ -1257,6 +1258,8 @@ static int parse_sha1_header(const char *hdr, unsigned long length, unsigned lon
>> * terminating '\0' that we add), and is followed by
>> * a space, at least one byte for size, and a '\0'.
>> */
>> + if ('b' != *hdr && 'c' != *hdr && 't' != *hdr) /* blob/commit/tag/tree */
>> + return -1;
>> i = 0;
>> while (hdr < hdr_end - 2) {
>> char c = *hdr++;
>
> Oh. I wouldn't do that. Its a cute trick and it works to quickly
> determine if the header is an uncompressed header vs. a zlib header
> vs. a new-style loose object header (which git cannot write anymore,
> but it still can read). But its just asking for trouble when/if a
> new object type was ever added to the type table.
> I can't agree any more, it's just a trick. I considered adding a function seems_likely_uncompressed_loose_object(), but I didn't because this patch series are just my first try, I don't know whether the idea to support uncompressed loose object is attractive enough.
Show 7 quoted lines
> Given that we know that no type name can be more than 10 bytes and > if you use my patch from earlier today you can be certain hdr has a > '\0' terminator, so you could write a function to test for the type > against the hdr, stopping on either ' ' or '\0'. Or find the first > ' ' in the first 10 bytes (which is what this loop does anyway) and > then test that against the type name table. >