Re: [PATCH v2 7/7] reftable/reader: add comments to `table_iter_next()`

[Junio C Hamano <gitster@pobox.com>]

Patrick Steinhardt <ps@pks.im> writes:

> While working on the optimizations in the preceding patches I stumbled
> upon `table_iter_next()` multiple times. It is quite easy to miss the
> fact that we don't call `table_iter_next_in_block()` twice, but that the
> second call is in fact `table_iter_next_block()`.
>
> Add comments to explain what exactly is going on here to make things
> more obvious. While at it, touch up the code to conform to our code
> style better.
>
> Note that one of the refactorings merges two conditional blocks into
> one. Before, we had the following code:
>
> ```
> err = table_iter_next_block(&next, ti

");"???

> if (err != 0) {
> 	ti->is_finished = 1;
> }
> table_iter_block_done(ti);
> if (err != 0) {
> 	return err;
> }
> ```
>
> As `table_iter_block_done()` does not care about `is_finished`, the
> conditional blocks can be merged into one block:
>
> ```
> err = table_iter_next_block(&next, ti
> table_iter_block_done(ti);
> if (err != 0) {
> 	ti->is_finished = 1;
> 	return err;
> }
> ```
>
> This is both easier to reason about and more performant because we have
> one branch less.
>
> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> ---
>  reftable/reader.c | 26 +++++++++++++++++---------
>  1 file changed, 17 insertions(+), 9 deletions(-)
>
> diff --git a/reftable/reader.c b/reftable/reader.c
> index 64dc366fb1..add7d57f0b 100644
> --- a/reftable/reader.c
> +++ b/reftable/reader.c
> @@ -357,24 +357,32 @@ static int table_iter_next(struct table_iter *ti, struct reftable_record *rec)
>  
>  	while (1) {
>  		struct table_iter next = TABLE_ITER_INIT;
> -		int err = 0;
> -		if (ti->is_finished) {
> +		int err;
> +
> +		if (ti->is_finished)
>  			return 1;
> -		}
>  
> +		/*
> +		 * Check whether the current block still has more records. If
> +		 * so, return it. If the iterator returns positive then the
> +		 * current block has been exhausted.
> +		 */
>  		err = table_iter_next_in_block(ti, rec);
> -		if (err <= 0) {
> +		if (err <= 0)
>  			return err;
> -		}
>  
> +		/*
> +		 * Otherwise, we need to continue to the next block in the
> +		 * table and retry. If there are no more blocks then the
> +		 * iterator is drained.
> +		 */
>  		err = table_iter_next_block(&next, ti);
> -		if (err != 0) {
> -			ti->is_finished = 1;
> -		}
>  		table_iter_block_done(ti);
> -		if (err != 0) {
> +		if (err) {
> +			ti->is_finished = 1;
>  			return err;
>  		}
> +
>  		table_iter_copy_from(ti, &next);
>  		block_iter_close(&next.bi);
>  	}