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Re: [PATCH v2] strbuf doc: reuse after strbuf_release is fine




Jonathan Nieder <jrnieder@xxxxxxxxx> writes:

> strbuf_release leaves the strbuf in a valid, initialized state, so
> there is no need to call strbuf_init after it.
>
> Moreover, this is not likely to change in the future: strbuf_release
> leaving the strbuf in a valid state has been easy to maintain and has
> been very helpful for Git's robustness and simplicity (e.g.,
> preventing use-after-free vulnerabilities).
>
> Document the semantics so the next generation of Git developers can
> become familiar with them without reading the implementation.  It is
> still not advisable to call strbuf_release too often because it is
> wasteful, so add a note pointing to strbuf_reset for that.
>
> The same semantics apply to strbuf_detach.  Add a similar note to its
> docstring to make that clear.
>
> Improved-by: Jeff King <peff@xxxxxxxx>
> Signed-off-by: Jonathan Nieder <jrnieder@xxxxxxxxx>
> ---
> Jeff King wrote:
>
>> I think it's actually OK to use the string buffer after this function.
>> It's just an empty string.
>>
>> Perhaps we should be more explicit: this releases any resources and
>> resets to a pristine, empty state. I suspect strbuf_detach() probably
>> should make the same claim.
>
> Like this?

Looks good to me.


>
> Thanks,
> Jonathan
>
>  strbuf.h | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/strbuf.h b/strbuf.h
> index 7496cb8ec5..249df86711 100644
> --- a/strbuf.h
> +++ b/strbuf.h
> @@ -82,8 +82,12 @@ extern char strbuf_slopbuf[];
>  extern void strbuf_init(struct strbuf *, size_t);
>  
>  /**
> - * Release a string buffer and the memory it used. You should not use the
> - * string buffer after using this function, unless you initialize it again.
> + * Release a string buffer and the memory it used. After this call, the
> + * strbuf points to an empty string that does not need to be free()ed, as
> + * if it had been set to `STRBUF_INIT` and never modified.
> + *
> + * To clear a strbuf in preparation for further use without the overhead
> + * of free()ing and malloc()ing again, use strbuf_reset() instead.
>   */
>  extern void strbuf_release(struct strbuf *);
>  
> @@ -91,6 +95,9 @@ extern void strbuf_release(struct strbuf *);
>   * Detach the string from the strbuf and returns it; you now own the
>   * storage the string occupies and it is your responsibility from then on
>   * to release it with `free(3)` when you are done with it.
> + *
> + * The strbuf that previously held the string is reset to `STRBUF_INIT` so
> + * it can be reused after calling this function.
>   */
>  extern char *strbuf_detach(struct strbuf *, size_t *);