Fail only if not the first write in on-writable watcher

.travis.yml

@@ -20,7 +20,6 @@ jobs:
         - mv composer.phar /usr/local/bin/composer
   allow_failures:
     - php: nightly
-    - os: osx
   fast_finish: true
 
 env:

lib/ResourceOutputStream.php

@@ -60,6 +60,15 @@ public function __construct($stream, int $chunkSize = null)
         $resource = &$this->resource;
 
         $this->watcher = Loop::onWritable($stream, static function ($watcher, $stream) use ($writes, &$chunkSize, &$writable, &$resource) {
+            $firstWrite = true;
+
+            // Using error handler to verify that a write of zero bytes was not due an error.
+            // @see https://github.com/reactphp/stream/pull/150
+            $error = 0;
+            \set_error_handler(static function (int $errno) use (&$error) {
+                $error = $errno;
+            });
+
             try {
                 while (!$writes->isEmpty()) {
                     /** @var Deferred $deferred */
@@ -75,11 +84,6 @@ public function __construct($stream, int $chunkSize = null)
                         throw new ClosedException("The stream was closed by the peer");
                     }
 
-                    $error = null;
-                    \set_error_handler(static function (int $errno) use (&$error) {
-                        $error = $errno;
-                    });
-
                     // Error reporting suppressed since fwrite() emits E_WARNING if the pipe is broken or the buffer is full.
                     // Use conditional, because PHP doesn't like getting null passed
                     if ($chunkSize) {
@@ -88,8 +92,6 @@ public function __construct($stream, int $chunkSize = null)
                         $written = @\fwrite($stream, $data);
                     }
 
-                    \restore_error_handler();
-
                     \assert(
                         $written !== false || \PHP_VERSION_ID >= 70400, // PHP 7.4+ returns false on EPIPE.
                         "Trying to write on a previously fclose()'d resource. Do NOT manually fclose() resources the still referenced in the loop."
@@ -105,7 +107,8 @@ public function __construct($stream, int $chunkSize = null)
                     }
 
                     // Broken pipes between processes on macOS/FreeBSD do not detect EOF properly.
-                    if (($written === 0 || $written === false) && $error !== null) {
+                    // fwrite() may write zero bytes on subsequent calls due to the buffer filling again.
+                    if (($written === 0 || $written === false) && $error && $firstWrite) {
                         $message = "Failed to write to stream";
                         if ($error = \error_get_last()) {
                             $message .= \sprintf("; %s", $error["message"]);
@@ -120,6 +123,8 @@ public function __construct($stream, int $chunkSize = null)
                     }
 
                     $deferred->resolve($written + $previous);
+
+                    $firstWrite = false;
                 }
             } catch (\Throwable $exception) {
                 $resource = null;
@@ -137,6 +142,8 @@ public function __construct($stream, int $chunkSize = null)
                 if ($writes->isEmpty()) {
                     Loop::disable($watcher);
                 }
+
+                \restore_error_handler();
             }
         });
 

test/ResourceOutputStreamTest.php

@@ -78,6 +78,8 @@ public function testClosedRemoteSocket()
     }
 
     /**
+     * @requires PHPUnit >= 7
+     *
      * @see https://github.com/reactphp/stream/pull/150
      */
     public function testUploadBiggerBlockSecure()