stream, test: Add test for _readableState.needReadable and _readableState.emittedReadable

[mcollina]

• Version: all
• Platform: all
• Subsystem: stream

Part of #8644.

State variable definition: https://github.com/nodejs/node/blob/master/lib/_stream_readable.js#L72-L77

See also #10214

cc @addaleax @Fishrock123 @nodejs/streams

[mcollina]

This is really a good issue if someone wants to dig into how streams work. I'm happy to help.

[joyeecheung]

Hi, I am interested in the internals of streams since folks in my company have bumped into some perf issues related to it. I would love to spend some time digging into it and learning how it works :)

[mcollina]

@joyeecheung please go ahead and prep a PR!

[joyeecheung]

I have attempted a PR for needReadable(#10241), please take a look.

Since there is #10214 involved I might need a little bit more time to get my head around emittedReadable.

[mcollina]

@joyeecheung I would like to have a test for emittedReadable before a fix for #10214 happen. I would like to track possible regression, so write a test for what it is doing now.

[joyeecheung]

@mcollina OK, working on it :)

[italoacasas]

@mcollina this is duplicated ? #8683