Extract extra info from headers_sent

src/Emitter/SapiEmitterTrait.php

@@ -31,8 +31,10 @@ trait SapiEmitterTrait
      */
     private function assertNoPreviousOutput()
     {
-        if (headers_sent()) {
-            throw EmitterException::forHeadersSent();
+        $file = null;
+        $line = null;
+        if (headers_sent($file, $line)) {
+            throw EmitterException::forHeadersSent($file, $line);
         }
 
         if (ob_get_level() > 0 && ob_get_length() > 0) {

src/Exception/EmitterException.php

@@ -13,9 +13,15 @@
 
 class EmitterException extends RuntimeException implements ExceptionInterface
 {
-    public static function forHeadersSent() : self
+    public static function forHeadersSent(string $file, int $line) : self
     {
-        return new self('Unable to emit response; headers already sent');
+        return new self(
+            sprintf(
+                'Unable to emit response; headers already sent, output started at %s:%d',
+                $file,
+                $line
+            )
+        );
     }
 
     public static function forOutputSent() : self

test/Emitter/SapiStreamEmitterTest.php

@@ -58,6 +58,48 @@ public function testEmitCallbackStreamResponse()
         $this->assertSame('it works', ob_get_clean());
     }
 
+    public function testAssertNoPreviousOutput()
+    {
+        $stream = new CallbackStream(function () {
+            return 'it works';
+        });
+        $response = (new Response())
+            ->withStatus(200)
+            ->withBody($stream);
+        ob_start();
+        echo 'Unexpected Output';
+        try {
+            $this->emitter->emit($response);
+        } catch (\Throwable $e) {
+            $this->assertEquals(
+                'Output has been emitted previously; cannot emit response',
+                $e->getMessage()
+            );
+        } finally {
+            ob_end_clean();
+        }
+    }
+
+    public function testAssertNoPreviousOutputNoBuffer()
+    {
+        $stream = new CallbackStream(function () {
+            return 'it works';
+        });
+        $response = (new Response())
+            ->withStatus(200)
+            ->withBody($stream);
+        echo 'Unexpected Output';
+        try {
+            $this->emitter->emit($response);
+        } catch (\Throwable $e) {
+            $this->assertEquals($e->getMessage(),sprintf(
+                'Unable to emit response; headers already sent, output started at %s:%d',
+                 __FILE__,
+                 __LINE__ - 5
+            ));
+        }
+    }
+
     public function testDoesNotInjectContentLengthHeaderIfStreamSizeIsUnknown()
     {
         $stream = $this->prophesize(StreamInterface::class);