[stream] Make _readableState and _writableState part of the public API

[mcollina]

Following up from #7077 (comment), and other discussion.

One of the problem that has lead to breaking userland stream modules is related to the fact that they rely on functionality available in _readableState and _writableState that is not documented, and that is the only possible way for solving some issues, like knowing if a stream is in objectMode: true, clearing up the leftover write callbacks, and many other functionalities.

I propose we document, standardize and test a piece of _readableState  and _writableState.

I think we should have been able to spot mafintosh/duplexify#6 before releasing, sigh. @calvinmetcalf @thealphanerd are we setting readable-stream in passthrough mode for citgm run?

I am opening this discussion here rather than in readable-stream because I would like all possible feedback. As @mikeal said, "So, what are we gonna do about streams?" openjs-foundation/summit#16 (comment).

@nodejs/streams we might want probably put this at in our next meeting.

cc @ronkorving @mafintosh

[vsemozhetbyt]

Are the '_readableStream' and '_writableStream' typos in the title?

[mcollina]

@vsemozhetbyt good catch!

[mscdex]

Related: #6799

[mscdex]

I personally think that having a single, non-underscore-prefixed property (to minimize impact) to house all state would be ideal (e.g. stream.streamState with stream.streamState._readable, etc. as necessary).

[mafintosh]

Adding any properties by now will just make things more complicated in userland though as you'd have yet another thing to test for.

I'm 👍 on just making _readableState and _writableState part of the public readonly api - this is how they are used right now anyway.

[mscdex]

Well technically it'd be trading two existing properties for one ;-)

I just don't like setting a precedent by documenting an underscore-prefixed property. It's already bad enough that _readableState is already publicly "documented" and its use is being encouraged....

[mcollina]

@mscdex Technically a single property is not correct, as you can have a Transform that is in objectMode only on one side.

[vkurchatkin]

_readableState and _writableState should remain private. If some functionality is missing, it should be added to stream instances instead

[mafintosh]

adding the properties directly on the stream instances will likely break some modules as inheritance from streams is encouraged. it'll also make it complicated for module authors to determine if a property was set by the stream impl or the userland subclass as you dont know which version of streams an instance was created with