Subject: Re: [PATCH 2/2] xdiff/xprepare: fix a memory leak

From: Junio C Hamano <[email-masked]>
Date: Fri, 04 Mar 2016 15:50:31 -0800

Ramsay Jones <[email-masked]> writes:

> The xdl_prepare_env() function may initialise an xdlclassifier_t
> data structure via xdl_init_classifier(), which allocates memory
> to several fields, for example 'rchash', 'rcrecs' and 'ncha'.
> If this function later exits due to the failure of xdl_optimize_ctxs(),
> then this xdlclassifier_t structure, and the memory allocated to it,
> is not cleaned up.
>
> In order to fix the memory leak, insert a call to xdl_free_classifier()
> before returning.
>
> Signed-off-by: Ramsay Jones <[email-masked]>
> ---
>  xdiff/xprepare.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/xdiff/xprepare.c b/xdiff/xprepare.c
> index 5ffcf99..13b55ab 100644
> --- a/xdiff/xprepare.c
> +++ b/xdiff/xprepare.c
> @@ -301,6 +301,7 @@ int xdl_prepare_env(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
>  
>  		xdl_free_ctx(&xe->xdf2);
>  		xdl_free_ctx(&xe->xdf1);
> +		xdl_free_classifier(&cf);
>  		return -1;
>  	}

This looks obviously correct from the pattern of prepare's and
free's in the code that this part follows.  This potential leak has
been that way since 3443546f (Use a *real* built-in diff generator,
2006-03-24), i.e. the very beginning.

I find it somewhat strange that the call to xdl_free_classifier() at
the end of this function is made conditional to XDF_HISTOGRAM_DIFF,
though.  I can half-buy the argument "that is because we do not call
init-classifier for XDF_HISTOGRAM_DIFF", but in the error path we
call free-classifier unconditionally, so the code clearly knows that
it is safe to call free-classifier on a classifier that is cleared
with the initial memset(&cf, 0, sizeof cf).

I think the latter funkiness came from 9f37c275.