From 4cd6590b5f4a9deeed7959039018aa52ff391803 Mon Sep 17 00:00:00 2001
From: Tom Longhurst <30480171+thomhurst@users.noreply.github.com>
Date: Sun, 5 Apr 2026 18:47:05 +0100
Subject: [PATCH] fix: prevent StringBuilder race in console interceptor during
 parallel tests
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Remove StringBuilder overrides from OptimizedConsoleInterceptor. The overrides
called ToString() which traverses the entire internal chunk linked list — if the
caller's StringBuilder is mutated concurrently, ToString() throws
ArgumentOutOfRangeException(chunkLength). The base TextWriter implementation
iterates chunks via GetChunks() and copies each one independently via
Write(ReadOnlySpan<char>), which the interceptor already overrides.

Also move StringBuilder.Length checks inside the ReaderWriterLockSlim in
Context.GetStandardOutput/GetErrorOutput — Length is not thread-safe.

Fixes #5411
---
 TUnit.Core/Context.cs                          | 18 ++++++------------
 .../Logging/OptimizedConsoleInterceptor.cs     |  9 +++------
 2 files changed, 9 insertions(+), 18 deletions(-)

diff --git a/TUnit.Core/Context.cs b/TUnit.Core/Context.cs
index 6721a283a4..dd5b916d00 100644
--- a/TUnit.Core/Context.cs
+++ b/TUnit.Core/Context.cs
@@ -82,16 +82,13 @@ public void AddAsyncLocalValues()
 
     public virtual string GetStandardOutput()
     {
-        if (_outputBuilder.Length == 0)
-        {
-            return string.Empty;
-        }
-
         _outputLock.EnterReadLock();
 
         try
         {
-            return _outputBuilder.ToString();
+            return _outputBuilder.Length == 0
+                ? string.Empty
+                : _outputBuilder.ToString();
         }
         finally
         {
@@ -101,16 +98,13 @@ public virtual string GetStandardOutput()
 
     public virtual string GetErrorOutput()
     {
-        if (_errorOutputBuilder.Length == 0)
-        {
-            return string.Empty;
-        }
-
         _errorOutputLock.EnterReadLock();
 
         try
         {
-            return _errorOutputBuilder.ToString();
+            return _errorOutputBuilder.Length == 0
+                ? string.Empty
+                : _errorOutputBuilder.ToString();
         }
         finally
         {
diff --git a/TUnit.Engine/Logging/OptimizedConsoleInterceptor.cs b/TUnit.Engine/Logging/OptimizedConsoleInterceptor.cs
index e8c4cbb428..fef11518c4 100644
--- a/TUnit.Engine/Logging/OptimizedConsoleInterceptor.cs
+++ b/TUnit.Engine/Logging/OptimizedConsoleInterceptor.cs
@@ -164,18 +164,15 @@ public override async Task WriteLineAsync(string? value)
     }
 
 #if NET
+    // StringBuilder overrides intentionally omitted — base TextWriter iterates chunks
+    // via GetChunks() and calls Write(ReadOnlySpan<char>) per chunk, avoiding ToString()
+    // which races with concurrent StringBuilder mutation (see #5411).
     public override void Write(ReadOnlySpan<char> buffer) => Write(new string(buffer));
-    public override void Write(StringBuilder? value) => Write(value?.ToString() ?? string.Empty);
     public override Task WriteAsync(ReadOnlyMemory<char> buffer, CancellationToken cancellationToken = new())
         => WriteAsync(new string(buffer.Span));
-    public override Task WriteAsync(StringBuilder? value, CancellationToken cancellationToken = new())
-        => WriteAsync(value?.ToString() ?? string.Empty);
     public override void WriteLine(ReadOnlySpan<char> buffer) => WriteLine(new string(buffer));
-    public override void WriteLine(StringBuilder? value) => WriteLine(value?.ToString() ?? string.Empty);
     public override Task WriteLineAsync(ReadOnlyMemory<char> buffer, CancellationToken cancellationToken = new())
         => WriteLineAsync(new string(buffer.Span));
-    public override Task WriteLineAsync(StringBuilder? value, CancellationToken cancellationToken = new())
-        => WriteLineAsync(value?.ToString() ?? string.Empty);
 #endif
 
     public override IFormatProvider FormatProvider => GetOriginalOut().FormatProvider;