Subject: Re: [PATCH] strbuf: allocate enough space when strbuf_setlen() is called first time

From: René Scharfe <[email-masked]>
Date: Tue, 26 Apr 2011 18:54:43 +0200

Am 26.04.2011 17:32, schrieb Nguyen Thai Ngoc Duy:
> 2011/4/26 Junio C Hamano<[email-masked]>:
>> Nguyễn Thái Ngọc Duy<[email-masked]>  writes:
>>
>>> strbuf_grow(sb, 0) may allocate less than requested len and violate the
>>> next assertion.
>>>
>>> Signed-off-by: Nguyễn Thái Ngọc Duy<[email-masked]>
>>> ---
>>>   strbuf.h |    2 +-
>>>   1 files changed, 1 insertions(+), 1 deletions(-)
>>>
>>> diff --git a/strbuf.h b/strbuf.h
>>> index 07060ce..ab213da 100644
>>> --- a/strbuf.h
>>> +++ b/strbuf.h
>>> @@ -34,7 +34,7 @@ extern void strbuf_grow(struct strbuf *, size_t);
>>>
>>>   static inline void strbuf_setlen(struct strbuf *sb, size_t len) {
>>>        if (!sb->alloc)
>>> -             strbuf_grow(sb, 0);
>>> +             strbuf_grow(sb, len);
>>>        assert(len<  sb->alloc);
>>
>> This looks so obviously correct that it is scary.
>>
>> How could 60 callsites of this function manage to have run without crashes
>> so far?  They all happen to use the function on a buffer that already has
>> something on it?
>
> I guess no current call site does _setlen right after initialization.
> It's new code that triggers it.

Documentation/technical/api-strbuf.txt says that you can't use 
strbuf_setlen() to allocate more space for a strbuf, i.e. you can use it 
only to shorten a string or set it to its current length.  If the strbuf 
in question hasn't allocated any buffer at all (sb->alloc == 0) then the 
only valid len is zero.

Where is it used to extend strbufs, i.e. which is the new code you 
mentioned?

René