On Fri, Feb 09, 2024 at 11:01:13AM -0500, John Cai wrote:
>
> Hi Patrick,
>
> On 1 Feb 2024, at 5:25, Patrick Steinhardt wrote:
>
> > 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.
> >
> > Signed-off-by: Patrick Steinhardt <ps@xxxxxx>
> > ---
> > 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) {
>
> what's the reason for moving the if statement that handles err down after
> table_iter_block_done?
Good question. Ultimately, it's a simplification because I just merge
the two blocks which checked for `err != 0` into a single block. There
is no need to mark the iterator as finished before calling
`table_iter_block_done()`.
So becaiuse `table_iter_block_done()` doesn't inspect `is_finished`,
these two implementations are in the end equivalent. Before:
```
if (err)
ti->is_finished = 1;
table_iter_block_done(ti);
if (err)
return err;
```
After:
```
table_iter_block_done(ti);
if (err) {
ti->is_finished = 1;
return err;
}
```
The latter is much easier to reason about I think. It's also more
efficient because there's one branch less.
Patrick
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