stream, test: test _readableState.emittedReadable

Part of nodejs#10230, increase coverage of the internal
state machine of streams.

test/parallel/test-stream-readable-emittedReadable.js

@@ -0,0 +1,70 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const Readable = require('stream').Readable;
+
+const readable = new Readable({
+  read: () => {}
+});
+
+// Initialized to false.
+assert.strictEqual(readable._readableState.emittedReadable, false);
+
+readable.on('readable', common.mustCall(() => {
+  // emittedReadable should be true when the readable event is emitted
+  assert.strictEqual(readable._readableState.emittedReadable, true);
+  readable.read();
+  // emittedReadable is reset to false during read()
+  assert.strictEqual(readable._readableState.emittedReadable, false);
+}, 4));
+
+// When the first readable listener is just attached,
+// emittedReadable should be false
+assert.strictEqual(readable._readableState.emittedReadable, false);
+
+// Each one of these should trigger a readable event.
+process.nextTick(common.mustCall(() => {
+  readable.push('foo');
+}));
+process.nextTick(common.mustCall(() => {
+  readable.push('bar');
+}));
+process.nextTick(common.mustCall(() => {
+  readable.push('quo');
+}));
+process.nextTick(common.mustCall(() => {
+  readable.push(null);
+}));
+
+const noRead = new Readable({
+  read: () => {}
+});
+
+noRead.on('readable', common.mustCall(() => {
+  // emittedReadable should be true when the readable event is emitted
+  assert.strictEqual(noRead._readableState.emittedReadable, true);
+  noRead.read(0);
+  // emittedReadable is not reset during read(0)
+  assert.strictEqual(noRead._readableState.emittedReadable, true);
+}));
+
+noRead.push('foo');
+noRead.push(null);
+
+const flowing = new Readable({
+  read: () => {}
+});
+
+flowing.on('data', common.mustCall(() => {
+  // When in flowing mode, emittedReadable is always false.
+  assert.strictEqual(flowing._readableState.emittedReadable, false);
+  flowing.read();
+  assert.strictEqual(flowing._readableState.emittedReadable, false);
+}, 3));
+
+flowing.push('foooo');
+flowing.push('bar');
+flowing.push('quo');
+process.nextTick(common.mustCall(() => {
+  flowing.push(null);
+}));