Remove Worker.Core assembly from DOTNET_STARTUP_HOOKS at startup

release_notes.md

@@ -9,5 +9,6 @@
 - Update Microsoft.Azure.Functions.Worker.Grpc dependency to 1.10.1
 ### Microsoft.Azure.Functions.Worker.Core 1.12.1
 - Minor documentation updates (no functional changes)
+- Fixed issue spawning child process while debugging due to "DOTNET_STARTUP_HOOKS" always containing "Microsoft.Azure.Functions.Worker.Core". (#1539)
 ### Microsoft.Azure.Functions.Worker.Grpc 1.10.1
 - Fixed an issue causing throughput degradation and for synchronous functions, blocked the execution pipeline. (#1516)

src/DotNetWorker.Core/StartupHook.cs

@@ -2,7 +2,10 @@
 // Licensed under the MIT License. See License.txt in the project root for license information.
 
 using System;
+using System.Collections.Generic;
 using System.Diagnostics;
+using System.Linq;
+using System.Runtime.InteropServices;
 using System.Threading;
 
 /// <summary>
@@ -16,12 +19,17 @@
 /// </summary>
 internal class StartupHook
 {
+    const string StartupHooksEnvVar = "DOTNET_STARTUP_HOOKS";
+    private static readonly char s_startupSeparator = RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ? ';' : ':';
+    private static readonly string? s_assemblyName = typeof(StartupHook).Assembly.GetName().Name;
+
     public static void Initialize()
     {
         // Time to wait between checks, in ms.
         const int SleepTime = 500;
         const int MaxWaitCycles = (60 * 1000) / SleepTime;
 
+        RemoveSelfFromStartupHooks();
         string? debuggerWaitEnabled = Environment.GetEnvironmentVariable("FUNCTIONS_ENABLE_DEBUGGER_WAIT");
         string? jsonOutputEnabled = Environment.GetEnvironmentVariable("FUNCTIONS_ENABLE_JSON_OUTPUT");
 #if NET5_0_OR_GREATER
@@ -62,4 +70,22 @@ static bool WaitOnDebugger(int cycle)
             }
         }
     }
+
+    internal static void RemoveSelfFromStartupHooks()
+    {
+        string? startupHooks = Environment.GetEnvironmentVariable(StartupHooksEnvVar);
+        if (string.IsNullOrEmpty(startupHooks))
+        {
+            // If this call happened, we are clearly part of this environment variable.
+            // This is mostly to make strict-nulls happy.
+            return;
+        }
+
+        // netstandard2.0 has no StringSplitOptions overload.
+        IEnumerable<string> parts = startupHooks.Split(s_startupSeparator).Where(x => !string.IsNullOrEmpty(x));
+        string newValue = string.Join(
+            s_startupSeparator.ToString(), // netstandard2.0 only accepts string here.
+            parts.Where(x => !x.Equals(s_assemblyName, StringComparison.Ordinal)));
+        Environment.SetEnvironmentVariable(StartupHooksEnvVar, newValue);
+    }
 }

test/DotNetWorkerTests/StartupHookTests.cs

@@ -2,7 +2,10 @@
 // Licensed under the MIT License. See License.txt in the project root for license information.
 
 using System;
+using System.Collections.Generic;
+using System.Linq;
 using System.Reflection;
+using System.Runtime.InteropServices;
 using Xunit;
 
 namespace Microsoft.Azure.Functions.Worker.Tests
@@ -24,5 +27,37 @@ public void ValidadeHookSetup()
             Assert.Empty(initializeMethod.GetParameters());
             Assert.Equal(typeof(void), initializeMethod.ReturnType);
         }
+
+        [Theory]
+        [InlineData(null)]
+        [InlineData("Microsoft.Azure.Functions.Worker.Core")]
+        [InlineData("Some.Other.Assembly")]
+        [InlineData("Microsoft.Azure.Functions.Worker.Core", "Some.Other.Assembly")]
+        [InlineData("Some.Other.Assembly", "Microsoft.Azure.Functions.Worker.Core")]
+        [InlineData("Some.Other.Assembly", "Microsoft.Azure.Functions.Worker.Core", "Another.Assembly")]
+        public void ValidateHookSetup_HookVariableRemoved(params string[] hooks)
+        {
+            hooks ??= Array.Empty<string>();
+            char separator = RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ? ';' : ':';
+            string envVar = "DOTNET_STARTUP_HOOKS";
+            string original = Environment.GetEnvironmentVariable(envVar);
+
+            try
+            {
+                IEnumerable<string> expected = hooks.Where(
+                    x => x != typeof(StartupHook).Assembly.GetName().Name);
+                Environment.SetEnvironmentVariable(envVar, string.Join(separator, hooks));
+                StartupHook.RemoveSelfFromStartupHooks();
+
+                string value = Environment.GetEnvironmentVariable(envVar);
+                IEnumerable<string> actual = string.IsNullOrEmpty(value)
+                    ? Enumerable.Empty<string>() : value.Split(separator);
+                Assert.Equal(expected, actual);
+            }
+            finally
+            {
+                Environment.SetEnvironmentVariable(envVar, original);
+            }
+        }
     }
 }