Improve compiler server/client logging

src/Compilers/Core/MSBuildTask/Directory.Build.props

@@ -47,6 +47,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)PropertyDictionary.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)RCWForCurrentContext.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)ShowMessageForImplicitlySkipAnalyzers.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)TaskCompilerServerLogger.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Utilities.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)ValidateBootstrap.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Vbc.cs" />

src/Compilers/Core/MSBuildTask/ManagedCompiler.cs

@@ -506,7 +506,8 @@ public string GeneratePathToTool()
 
         protected override int ExecuteTool(string pathToTool, string responseFileCommands, string commandLineCommands)
         {
-            using var logger = new CompilerServerLogger($"MSBuild {Process.GetCurrentProcess().Id}");
+            using var innerLogger = new CompilerServerLogger($"MSBuild {Process.GetCurrentProcess().Id}");
+            var logger = new TaskCompilerServerLogger(Log, innerLogger);
             return ExecuteTool(pathToTool, responseFileCommands, commandLineCommands, logger);
         }
 
@@ -818,7 +819,6 @@ private void LogCompilationMessage(ICompilerServerLogger logger, string requestI
             else
             {
                 logger.Log(message);
-                Log.LogMessage(message);
             }
         }
 

src/Compilers/Core/MSBuildTask/TaskCompilerServerLogger.cs

@@ -0,0 +1,28 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using Microsoft.Build.Utilities;
+using Microsoft.CodeAnalysis.CommandLine;
+
+namespace Microsoft.CodeAnalysis.BuildTasks;
+
+/// <summary>
+/// Logs to both the MSBuild task's output and the inner compiler server logger.
+/// </summary>
+internal sealed class TaskCompilerServerLogger(
+    TaskLoggingHelper taskLogger,
+    ICompilerServerLogger inner)
+    : ICompilerServerLogger
+{
+    private readonly TaskLoggingHelper _taskLogger = taskLogger;
+    private readonly ICompilerServerLogger _inner = inner;
+
+    public bool IsLogging => true;
+
+    public void Log(string message)
+    {
+        _inner.Log(message);
+        _taskLogger.LogMessage(message);
+    }
+}

src/Compilers/Shared/CompilerServerLogger.cs

@@ -100,7 +100,6 @@ internal sealed class CompilerServerLogger : ICompilerServerLogger, IDisposable
     {
         // Environment variable, if set, to enable logging and set the file to log to.
         internal const string EnvironmentVariableName = "RoslynCommandLineLogFile";
-        internal const string LoggingPrefix = "---";
 
         private Stream? _loggingStream;
         private readonly string _identifier;
@@ -152,7 +151,7 @@ public void Log(string message)
             if (_loggingStream is object)
             {
                 var threadId = Environment.CurrentManagedThreadId;
-                var prefix = $"ID={_identifier} TID={threadId}: ";
+                var prefix = $"[{DateTimeOffset.Now:o}] ID={_identifier} TID={threadId}: ";
                 string output = prefix + message + Environment.NewLine;
                 byte[] bytes = Encoding.UTF8.GetBytes(output);