tls: Improve TLS flow control

Fixes #1775.

lib/tls.js

@@ -52,6 +52,7 @@ function CryptoStream(pair) {
   this.readable = this.writable = true;
 
   this._paused = false;
+  this._needDrain = false;
   this._pending = [];
   this._pendingCallbacks = [];
   this._pendingBytes = 0;
@@ -86,7 +87,7 @@ CryptoStream.prototype.write = function(data /* , encoding, cb */) {
     data = new Buffer(data, encoding);
   }
 
-  debug('clearIn data');
+  debug((this === this.pair.cleartext ? 'clear' : 'encrypted') + 'In data');
 
   this._pending.push(data);
   this._pendingCallbacks.push(cb);
@@ -95,7 +96,26 @@ CryptoStream.prototype.write = function(data /* , encoding, cb */) {
   this.pair._writeCalled = true;
   this.pair.cycle();
 
-  return this._pendingBytes < 128 * 1024;
+  // In the following cases, write() should return a false,
+  // then this stream should eventually emit 'drain' event.
+  //
+  // 1. There are pending data more than 128k bytes.
+  // 2. A forward stream shown below is paused.
+  //    A) EncryptedStream for CleartextStream.write().
+  //    B) CleartextStream for EncryptedStream.write().
+  //
+  if (!this._needDrain) {
+    if (this._pendingBytes >= 128 * 1024) {
+      this._needDrain = true;
+    } else {
+      if (this === this.pair.cleartext) {
+        this._needDrain = this.pair.encrypted._paused;
+      } else {
+        this._needDrain = this.pair.cleartext._paused;
+      }
+    }
+  }
+  return !this._needDrain;
 };
 
 
@@ -380,12 +400,26 @@ CryptoStream.prototype._pull = function() {
     assert(rv === tmp.length);
   }
 
-  // If we've cleared all of incoming encrypted data, emit drain.
-  if (havePending && this._pending.length === 0) {
+  // If pending data has cleared, 'drain' event should be emitted
+  // after write() returns a false.
+  // Except when a forward stream shown below is paused.
+  //   A) EncryptedStream for CleartextStream._pull().
+  //   B) CleartextStream for EncryptedStream._pull().
+  //
+  if (this._needDrain && this._pending.length === 0) {
+    var paused;
+    if (this === this.pair.cleartext) {
+      paused = this.pair.encrypted._paused;
+    } else {
+      paused = this.pair.cleartext._paused;
+    }
+    if (!paused) {
       debug('drain');
-    this.emit('drain');
+      process.nextTick(this.emit.bind(this, 'drain'));
+      this._needDrain = false;
       if (this.__destroyOnDrain) this.end();
     }
+  }
 };