From 5805bc76205541b6254dec382c31b0d1d1b29803 Mon Sep 17 00:00:00 2001
From: DualFroz <me@dualfroz.com>
Date: Fri, 2 Oct 2026 16:11:20 +0200
Subject: [PATCH] Stop output buffer cleanup at buffers that cannot be removed

zlib.output_compression makes its buffer non-removable once it has sent
compressed output, so ob_end_clean() raised a notice that replaced the
original exception. Fixes #704.
---
 src/Whoops/Run.php                     |  6 ++--
 src/Whoops/Util/SystemFacade.php       | 10 +++++++
 tests/Whoops/RunTest.php               | 40 ++++++++++++++++++++++++++
 tests/Whoops/Util/SystemFacadeTest.php | 19 ++++++++++++
 4 files changed, 73 insertions(+), 2 deletions(-)

diff --git a/src/Whoops/Run.php b/src/Whoops/Run.php
index b6eee28..73d021f 100644
--- a/src/Whoops/Run.php
+++ b/src/Whoops/Run.php
@@ -425,8 +425,10 @@ final class Run implements RunInterface
         if ($this->writeToOutput()) {
             // @todo Might be able to clean this up a bit better
             if ($willQuit) {
-                // Cleanup all other output buffers before sending our output:
-                while ($this->system->getOutputBufferLevel() > 0) {
+                // Cleanup all other output buffers before sending our output.
+                // Stop at a buffer that cannot be removed, e.g. zlib output
+                // compression once it has started sending compressed output:
+                while ($this->system->getOutputBufferLevel() > 0 && $this->system->isOutputBufferRemovable()) {
                     $this->system->endOutputBuffering();
                 }
 
diff --git a/src/Whoops/Util/SystemFacade.php b/src/Whoops/Util/SystemFacade.php
index 9eb0acf..7025cb3 100644
--- a/src/Whoops/Util/SystemFacade.php
+++ b/src/Whoops/Util/SystemFacade.php
@@ -85,6 +85,16 @@ class SystemFacade
         return ob_get_level();
     }
 
+    /**
+     * @return bool
+     */
+    public function isOutputBufferRemovable()
+    {
+        $status = ob_get_status();
+
+        return isset($status['flags']) && ($status['flags'] & PHP_OUTPUT_HANDLER_REMOVABLE);
+    }
+
     /**
      * @return bool
      */
diff --git a/tests/Whoops/RunTest.php b/tests/Whoops/RunTest.php
index 708fa21..6c3c844 100755
--- a/tests/Whoops/RunTest.php
+++ b/tests/Whoops/RunTest.php
@@ -479,6 +479,46 @@ class RunTest extends TestCase
         $this->assertEquals("", ob_get_clean());
     }
 
+    /**
+     * @covers Whoops\Run::handleException
+     */
+    public function testOutputBufferCleanupStopsAtNonRemovableBuffer()
+    {
+        // A removable buffer on top of one that cannot be removed, like
+        // zlib.output_compression once it has sent compressed output.
+        $removable = [false, true];
+        $test = $this;
+
+        $system = m::mock('Whoops\Util\SystemFacade')->makePartial();
+        $system->shouldReceive('stopExecution');
+        $system->shouldReceive('getOutputBufferLevel')
+            ->andReturnUsing(function () use (&$removable) {
+                return count($removable);
+            });
+        $system->shouldReceive('isOutputBufferRemovable')
+            ->andReturnUsing(function () use (&$removable) {
+                return end($removable);
+            });
+        $system->shouldReceive('endOutputBuffering')
+            ->andReturnUsing(function () use ($test, &$removable) {
+                if (!array_pop($removable)) {
+                    $test->fail('Tried to end an output buffer that cannot be removed');
+                }
+                return true;
+            });
+
+        $run = new Run($system);
+        $run->pushHandler(function () {
+            echo "hello there";
+            return Handler::QUIT;
+        });
+
+        ob_start();
+        $run->handleException(new RuntimeException());
+        $this->assertEquals("hello there", ob_get_clean());
+        $this->assertEquals([false], $removable);
+    }
+
     /**
      * @covers Whoops\Run::sendHttpCode
      */
diff --git a/tests/Whoops/Util/SystemFacadeTest.php b/tests/Whoops/Util/SystemFacadeTest.php
index 554bbbf..c4d5827 100644
--- a/tests/Whoops/Util/SystemFacadeTest.php
+++ b/tests/Whoops/Util/SystemFacadeTest.php
@@ -68,6 +68,20 @@ class SystemFacadeTest extends TestCase
         $this->facade->endOutputBuffering();
     }
 
+    public function test_it_reports_a_removable_output_buffer()
+    {
+        self::$runtime->shouldReceive('ob_get_status')->once()->andReturn(['flags' => PHP_OUTPUT_HANDLER_STDFLAGS]);
+
+        $this->assertTrue($this->facade->isOutputBufferRemovable());
+    }
+
+    public function test_it_reports_a_non_removable_output_buffer()
+    {
+        self::$runtime->shouldReceive('ob_get_status')->once()->andReturn(['flags' => PHP_OUTPUT_HANDLER_STDFLAGS & ~PHP_OUTPUT_HANDLER_REMOVABLE]);
+
+        $this->assertFalse($this->facade->isOutputBufferRemovable());
+    }
+
     public function test_it_delegates_flushing_the_current_buffer_to_the_native_implementation()
     {
         self::$runtime->shouldReceive('flush')->once();
@@ -161,6 +175,11 @@ function ob_end_clean()
     return SystemFacadeTest::delegate('ob_end_clean');
 }
 
+function ob_get_status()
+{
+    return SystemFacadeTest::delegate('ob_get_status');
+}
+
 function flush()
 {
     return SystemFacadeTest::delegate('flush');