stream, test: test _readableState.needReadable

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

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

@@ -0,0 +1,96 @@
+'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.needReadable, false);
+
+readable.on('readable', common.mustCall(() => {
+  // When the readable event fires, needReadable is reset.
+  assert.strictEqual(readable._readableState.needReadable, false);
+  readable.read();
+}));
+
+// If a readable listener is attached, then a readable event is needed.
+assert.strictEqual(readable._readableState.needReadable, true);
+
+readable.push('foo');
+readable.push(null);
+
+readable.on('end', common.mustCall(() => {
+  // No need to emit readable anymore when the stream ends.
+  assert.strictEqual(readable._readableState.needReadable, false);
+}));
+
+const asyncReadable = new Readable({
+  read: () => {}
+});
+
+asyncReadable.on('readable', common.mustCall(() => {
+  if (asyncReadable.read() !== null) {
+    // After each read(), the buffer is empty.
+    // If the stream doesn't end now,
+    // then we need to notify the reader on future changes.
+    assert.strictEqual(asyncReadable._readableState.needReadable, true);
+  }
+}, 3));
+
+process.nextTick(common.mustCall(() => {
+  asyncReadable.push('foooo');
+}));
+process.nextTick(common.mustCall(() => {
+  asyncReadable.push('bar');
+}));
+process.nextTick(common.mustCall(() => {
+  asyncReadable.push(null);
+}));
+
+const flowing = new Readable({
+  read: () => {}
+});
+
+// Notice this must be above the on('data') call.
+flowing.push('foooo');
+flowing.push('bar');
+flowing.push('quo');
+process.nextTick(common.mustCall(() => {
+  flowing.push(null);
+}));
+
+// When the buffer already has enough data, and the stream is
+// in flowing mode, there is no need for the readable event.
+flowing.on('data', common.mustCall(function(data) {
+  assert.strictEqual(flowing._readableState.needReadable, false);
+}, 3));
+
+const slowProducer = new Readable({
+  read: () => {}
+});
+
+slowProducer.on('readable', common.mustCall(() => {
+  if (slowProducer.read(8) === null) {
+    // The buffer doesn't have enough data, and the stream is not ened,
+    // we need to notify the reader when data arrives.
+    assert.strictEqual(slowProducer._readableState.needReadable, true);
+  } else {
+    assert.strictEqual(slowProducer._readableState.needReadable, false);
+  }
+}, 4));
+
+process.nextTick(common.mustCall(() => {
+  slowProducer.push('foo');
+}));
+process.nextTick(common.mustCall(() => {
+  slowProducer.push('foo');
+}));
+process.nextTick(common.mustCall(() => {
+  slowProducer.push('foo');
+}));
+process.nextTick(common.mustCall(() => {
+  slowProducer.push(null);
+}));