Enhancement: S3 StreamWrapper error handling (aws#2372)

Improve error message for failed writes due to open file handlers

.changes/nextrelease/enhancement-streamwrapper.json

@@ -0,0 +1,7 @@
+[
+  {
+    "type": "enhancement",
+    "category": "S3",
+    "description": "Improved error handling for failed writes and appends on unclosed streams."
+  }
+]

src/S3/StreamWrapper.php

@@ -130,7 +130,10 @@ public static function register(
 
     public function stream_close()
     {
-        if ($this->body->getSize() === 0 && !($this->isFlushed)) {
+        if (!$this->isFlushed
+            && empty($this->body->getSize())
+            && $this->mode !== 'r'
+        ) {
             $this->stream_flush();
         }
         $this->body = $this->cache = null;
@@ -163,6 +166,14 @@ public function stream_eof()
 
     public function stream_flush()
     {
+        // Check if stream body size has been
+        // calculated via a flush or close
+        if($this->body->getSize() === null && $this->mode !== 'r') {
+            return $this->triggerError(
+                "Unable to determine stream size. Did you forget to close or flush the stream?"
+            );
+        }
+
         $this->isFlushed = true;
         if ($this->mode == 'r') {
             return false;

tests/S3/StreamWrapperTest.php

@@ -1026,4 +1026,37 @@ public function testStatDataIsClearedOnWriteUsingCustomProtocol()
 
         stream_wrapper_unregister('foo');
     }
+
+    public function contentProvider()
+    {
+        return [
+            ['foo'],
+            ['']
+        ];
+    }
+
+    /**
+     * @expectedException \PHPUnit\Framework\Error\Warning
+     * @expectedExceptionMessage Unable to determine stream size. Did you forget to close or flush the stream?
+     * @dataProvider contentProvider
+     */
+    public function testTriggersErrorOnNoFlushOrClose($content)
+    {
+        $stream = $this->getMockBuilder(Psr7\Stream::class)
+            ->disableOriginalConstructor()
+            ->getMock();
+        $stream->expects($this->any())
+            ->method('getSize')
+            ->willReturn(null);
+
+        $this->addMockResults(
+            $this->client,
+            [
+                new Result(['Body' => $stream]),
+            ]
+        );
+
+        $stream = fopen('s3://bucket/key', 'a');
+        fwrite($stream, $content);
+    }
 }