Convert SplFixedArray to Aggregate?

php.internals

Alex Dowad

6 years ago
Dear PHP Internals, I would like to propose a backwards-incompatible change to SplFixedArray which fixes the strange and almost certainly unintended behavior reported in Bug 79404 (https://bugs.php.net/bug.php?id=79404). In short: Because SplFixedArray is an Iterator, and stores its own iteration state, it cannot be used in nested foreach loops. If you try, the inner loop will overwrite the iteration position of the outer loop, so that the outer loop 'thinks' it is finished after the inner loop finishes. To illustrate: $spl = SplFixedArray::fromArray([0, 1]); foreach ($spl as $a) { foreach ($spl as $b) { echo "$a $b"; } } Only prints two lines: 0 0 0 1 The fix is to convert SplFixedArray to an IteratorAggregate, so each nested foreach loop operates on a different iterator object and thus nested loops don't interfere with each other. Now, it would be possible to do this while still defining key(), current(), next(), etc. methods on SplFixedArray. However, that would cause an even more insidious BC break, since these methods would no longer work as formerly when called in the body of a foreach loop. Therefore, I think it better to remove them. This may break a few users' code, but in a way which will be easier for them to debug than if the old methods were kept but subtly changed in behavior. Code to implement this change is here: https://github.com/php/php-src/pull/5384/files Your comments will be appreciated, Alex Dowad

Unnamed Person

6 years ago
On Fri, Jun 26, 2020 at 12:45 AM Alex <alexinbeijing@gmail.com> wrote:
> > Dear PHP Internals, > > I would like to propose a backwards-incompatible change to > SplFixedArray which fixes the strange and almost certainly unintended > behavior reported in Bug 79404 > (https://bugs.php.net/bug.php?id=79404). > > In short: Because SplFixedArray is an Iterator, and stores its own > iteration state, it cannot be used in nested foreach loops. If you > try, the inner loop will overwrite the iteration position of the outer > loop, so that the outer loop 'thinks' it is finished after the inner > loop finishes. > > To illustrate: > > $spl = SplFixedArray::fromArray([0, 1]); > foreach ($spl as $a) { > foreach ($spl as $b) { > echo "$a $b"; > } > } > > Only prints two lines: > > 0 0 > 0 1 > > The fix is to convert SplFixedArray to an IteratorAggregate, so each > nested foreach loop operates on a different iterator object and thus > nested loops don't interfere with each other. > > Now, it would be possible to do this while still defining key(), > current(), next(), etc. methods on SplFixedArray. However, that would > cause an even more insidious BC break, since these methods would no > longer work as formerly when called in the body of a foreach loop. > Therefore, I think it better to remove them. This may break a few > users' code, but in a way which will be easier for them to debug than > if the old methods were kept but subtly changed in behavior. > > Code to implement this change is here: > https://github.com/php/php-src/pull/5384/files > > Your comments will be appreciated, > Alex Dowad > > -- > PHP Internals - PHP Runtime Development Mailing List > To unsubscribe, visit: https://www.php.net/unsub.php >
As someone who has patches in the SPL, I support the idea of this change. One gotcha of aggregates is iterator invalidation; are you sure the patch takes care of it? If you have an array of size 3, get an iterator, iterate to offset 2, shrink the size with `setSize`, and use the iterator at offset 2, will everything be okay?

Alex Dowad

6 years ago
Dear Levi, I will add a test case to the PR to ensure that things work properly in the described situation. On Fri, Jun 26, 2020 at 3:57 PM Levi Morrison <levi.morrison@datadoghq.com> wrote:

Alex Dowad

6 years ago
Dear Levi Morrison, please see this added test case: https://github.com/php/php-src/pull/5384/files#diff-dc4d304baf106b4a30432f80dae1a538 The iteration behavior of the modified SplFixedArray is quite humane even in the face of changing array size.

Nikita Popov

6 years ago
On Fri, Jun 26, 2020 at 8:45 AM Alex <alexinbeijing@gmail.com> wrote:
> Dear PHP Internals, > > I would like to propose a backwards-incompatible change to > SplFixedArray which fixes the strange and almost certainly unintended > behavior reported in Bug 79404 > (https://bugs.php.net/bug.php?id=79404). > > In short: Because SplFixedArray is an Iterator, and stores its own > iteration state, it cannot be used in nested foreach loops. If you > try, the inner loop will overwrite the iteration position of the outer > loop, so that the outer loop 'thinks' it is finished after the inner > loop finishes. > > To illustrate: > > $spl = SplFixedArray::fromArray([0, 1]); > foreach ($spl as $a) { > foreach ($spl as $b) { > echo "$a $b"; > } > } > > Only prints two lines: > > 0 0 > 0 1 > > The fix is to convert SplFixedArray to an IteratorAggregate, so each > nested foreach loop operates on a different iterator object and thus > nested loops don't interfere with each other. > > Now, it would be possible to do this while still defining key(), > current(), next(), etc. methods on SplFixedArray. However, that would > cause an even more insidious BC break, since these methods would no > longer work as formerly when called in the body of a foreach loop. > Therefore, I think it better to remove them. This may break a few > users' code, but in a way which will be easier for them to debug than > if the old methods were kept but subtly changed in behavior. > > Code to implement this change is here: > https://github.com/php/php-src/pull/5384/files > > Your comments will be appreciated, > Alex Dowad >
I agree that we should make this change for PHP 8. The current behavior is pretty clearly a bug, and the practical backwards compatibility impact should be rather limited (and only annoying to work around if you extend SplFixedArray and override Iterator methods). Nikita