Fail streaming messages correctly on read failures

Fixes #18.

lib/Message.php

@@ -4,6 +4,7 @@ namespace Amp\ByteStream;
 
 use Amp\Coroutine;
 use Amp\Deferred;
+use Amp\Failure;
 use Amp\Promise;
 use Amp\Success;
 
@@ -49,6 +50,9 @@ class Message implements InputStream, Promise {
     /** @var bool True if the iterator has completed. */
     private $complete = false;
 
+    /** @var \Throwable Used to fail future reads on failure. */
+    private $error;
+
     /**
      * @param InputStream $source An iterator that only emits strings.
      */
@@ -95,6 +99,15 @@ class Message implements InputStream, Promise {
 
         if ($this->coroutine === null) {
             $this->coroutine = new Coroutine($this->consume());
+            $this->coroutine->onResolve(function ($error) {
+                if ($error) {
+                    $this->error = $error;
+                }
+            });
+        }
+
+        if ($this->error) {
+            return new Failure($this->error);
         }
 
         if ($this->buffer !== "") {

test/MessageTest.php

@@ -9,6 +9,7 @@ use Amp\ByteStream\PendingReadError;
 use Amp\Emitter;
 use Amp\Loop;
 use Amp\PHPUnit\TestCase;
+use Amp\PHPUnit\TestException;
 
 class MessageTest extends TestCase {
     public function testBufferingAll() {
@@ -118,7 +119,7 @@ class MessageTest extends TestCase {
 
     public function testFailingStream() {
         Loop::run(function () {
-            $exception = new \Exception;
+            $exception = new TestException;
             $value = "abc";
 
             $emitter = new Emitter;
@@ -127,12 +128,17 @@ class MessageTest extends TestCase {
             $emitter->emit($value);
             $emitter->fail($exception);
 
+            $callable = $this->createCallback(1);
+
             try {
                 while (($chunk = yield $stream->read()) !== null) {
                     $this->assertSame($value, $chunk);
                 }
-            } catch (\Exception $reason) {
+
+                $this->fail("No exception has been thrown");
+            } catch (TestException $reason) {
                 $this->assertSame($exception, $reason);
+                $callable(); // <-- ensure this point is reached
             }
         });
     }