Replace statefulMapConcat with statefulMap + mapConcat in codebase

[mdedetrich]

As the title suggests, statefulMapConcat is deprecated so it should replaced with statefulMap + mapConcat so that the 1.1.0 version of pekko-http isn't relying on deprecated pekko methods.

[laglangyue]

also found in pekko-connectors

[mdedetrich]

also found in pekko-connectors

Thanks, issue created at apache/pekko-connectors#307

[pjfanning]

#311 was merged already but a quick search indicates that we might still have 1 or 2 remaining uses.

[He-Pin]

I did a check seems there are no usage now. @pjfanning , the only lefts are in scaladoc and benchmarks

[jrudolph]

This might have performance implications (mapConcat had performance issues in the past) so make sure to benchmark before doing these changes.

[pjfanning]

This was deprecated in apache/pekko#601 - if we think deprecating the code was a mistake then we can reopen that discussion. If we think it was the right decision to deprecate statefulMapConcat, then we should start removing its usage.

[jrudolph]

I like the change from an API orthogonality perspective, just saying that it might not come for free. In the worst case, it could be possible to keep a copy in StreamUtils, but better if that is not needed.

[He-Pin]

I think this issue can be closed as the associated change has been one. in #311.

Or we can do some dedicated implementation just for pekko-http usage in StreamUtils, without any additional allocations.

[He-Pin]

The current statefulMap or mapAccumlate implementation will need a (S, A) , which is a kind of allocation, in JEP https://openjdk.org/jeps/461 @viktorklang @viktorklang-ora is using an object which reducing this allocation.

[He-Pin]

@jrudolph I will review the changes and try to come up with an allocation-free implementation in pekko-http.

[pjfanning]

I'm going to close this because we undeprecated statefulMapConcat. Feel free to reopen.