Re: [PATCH v2 09/13] merge-ort: avoid accidental API mis-use

> From: Elijah Newren <newren@gmail.com>
> 
> Previously, callers of the merge-ort API could have passed an
> uninitialized value for struct merge_result *result.  However, we want
> to check result to see if it has cached renames from a previous merge
> that we can reuse; such values would be found behind result->priv.
> However, if result->priv is uninitialized, attempting to access behind
> it will give a segfault.  So, we need result->priv to be NULL (which
> will be the case if the caller does a memset(&result, 0)), or be written
> by a previous call to the merge-ort machinery.  Documenting this
> requirement may help, but despite being the person who introduced this
> requirement, I still missed it once and it did not fail in a very clear
> way and led to a long debugging session.

> Add a _properly_initialized field to merge_result; that value will be
> 0 if the caller zero'ed the merge_result, it will be set to a very
> specific value by a previous run by the merge-ort machinery, and if it's
> uninitialized it will most likely either be 0 or some value that does
> not match the specific one we'd expect allowing us to throw a much more
> meaningful error.

> +static unsigned RESULT_INITIALIZED = 0x1abe11ed; /* unlikely accidental value */
> +
>  struct traversal_callback_data {
>  	unsigned long mask;
>  	unsigned long dirmask;
> @@ -3746,6 +3748,10 @@ static void merge_start(struct merge_options *opt, struct merge_result *result)
>  	assert(opt->obuf.len == 0);
>  
>  	assert(opt->priv == NULL);
> +	if (result->_properly_initialized != 0 &&
> +	    result->_properly_initialized != RESULT_INITIALIZED)
> +		BUG("struct merge_result passed to merge_incore_*recursive() must be zeroed or filled with values from a previous run");
> +	assert(!!result->priv == !!result->_properly_initialized);
>  	if (result->priv) {
>  		opt->priv = result->priv;
>  		result->priv = NULL;
> @@ -3905,6 +3911,7 @@ static void merge_ort_nonrecursive_internal(struct merge_options *opt,
>  	result->clean &= strmap_empty(&opt->priv->conflicted);
>  	if (!opt->priv->call_depth) {
>  		result->priv = opt->priv;
> +		result->_properly_initialized = RESULT_INITIALIZED;
>  		opt->priv = NULL;
>  	}
>  }
> diff --git a/merge-ort.h b/merge-ort.h
> index d53a0a339f33..c011864ffeb1 100644
> --- a/merge-ort.h
> +++ b/merge-ort.h
> @@ -29,6 +29,8 @@ struct merge_result {
>  	 * !clean) and to print "CONFLICT" messages.  Not for external use.
>  	 */
>  	void *priv;
> +	/* Also private */
> +	unsigned _properly_initialized;
>  };