stream, test: add tests for _readableStream.awaitDrain

[mcollina]

Part of #8644.

The _readableState that needs to be tested is:

stream._readableState.awaitDrain

node/lib/_stream_readable.js

Lines 88 to 89 in 774146d

	// the number of writers that are awaiting a drain event in .pipe()s
	this.awaitDrain = 0;

.

See also #8683

cc @Fishrock123 @nodejs/streams

[shmuga]

I have made a proposal how this state variable can be tested in PR #8914 .
As we already have some tests for awaitDrain - I think it's better to add some asserts to them for covering real awairDrain value.
Also tests now cover reseting awaitDrain counter.
But I don't understand what this test is doing and can't rewrite it correctly. Can you someone help me with that?

[mcollina]

@shmuga yes, good job!

The test you mentioned is increasing the awaitDrain mechanism manually, by calling resume: https://github.com/nodejs/node/blob/master/test/parallel/test-stream-pipe-await-drain-manual-resume.js#L29-L34.
You need to check that awaitDrain gets increased.

On that test, you should basically check that awaitDrain goes to zero as describe in: https://github.com/nodejs/node/blob/master/test/parallel/test-stream-pipe-await-drain-manual-resume.js#L37-L40

[shmuga]

@mcollina great. will add some more tests for manual-resume. But I think this comment

// First pause, resume manually. The next write() to writable will still
// return false, because chunks are still being buffered, so it will increase
// the awaitDrain counter again.

is a bit strange at the place "increase again".
As we have only one writer here - the counter can't be increased more than one time.
So probably I will change that comment.

[mcollina]

@shmuga resume interacts with awaitDrain: https://github.com/nodejs/node/blob/master/lib/_stream_readable.js#L741

See also https://github.com/nodejs/node/blob/master/lib/_stream_readable.js#L556.

When was that test added? Who did it? It might be worth getting the original PR here, and discussing with the author.

[shmuga]

@mcollina I found nice description of this test in commit e2e615e

I will just follow those instructions and add cover tests for awaitDrain state variable.

[mcollina]

Closed by 21a077a