From: wanghui <hui.wang@windriver.com>
Date: Wed, 07 Sep 2011 09:55:00 GMT
Subject: Re: [PATCH v2 2/5] sha1_file: remove a buggy value setting
Message-ID: <4E673F74.9060006@windriver.com>
In-Reply-To: <7vpqjdab76.fsf@alter.siamese.dyndns.org>

Junio C Hamano wrote:
> Wang Hui <Hui.Wang@windriver.com> writes:
>
>   
>> From: Hui Wang <Hui.Wang@windriver.com>
>>
>> The ent->base[] is a character array, it has pfxlen characters from
>> position 0 to (pfxlen-1) to contain an alt object dir name, the
>> position pfxlen should be the string terminating character '\0' and
>> is deliberately set to '\0' at the previous code line. The position
>> (pfxlen+1) is given to ent->name.
>>     
>
> Correct. Do you understand why?
>
> We temporarily NUL terminate the ent->base[] so that we can give it to
> is_directory() to see if that is a directory, but the invariants for a
> alternate_object_database instance after it is properly initialized by
> this function are to have:
>
>  - the directory name followed by a slash in the base[] array;
>  - the name pointer pointing at one byte beyond the slash;
>  - name[2] filled with a slash; and
>  - name[41] terminated with NUL.
>
> Later, has_loose_object_nonlocal() calls fill_sha1_path() with the name
> pointer to fill name[0..1, 3..40] with the hexadecimal representation of
> the object name, which would result in base[] array to have the pathname
> for a loose object found in that alternate. The same thing happens in
> open_sha1_file() to read from a loose object in an alternate.
>
> And you are breaking one of the above invariants by removing that slash
> after the directory name. These callers of fill_sha1_path() will see the
> directory name, your NUL, two hex, slash, and 38 hex in base[].
>
>   
Understand now, thanks for your explanation.
> How would the code even work with your patch?
>
>
>