Do not crash server if we fail to determine language for non-mutating request

src/LanguageServer/Microsoft.CommonLanguageServerProtocol.Framework/AbstractLanguageServer.cs

@@ -7,6 +7,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Reflection;
 using System.Threading;
@@ -172,10 +173,11 @@ protected IRequestExecutionQueue<TRequestContext> GetRequestExecutionQueue()
         return _queue.Value;
     }
 
-    public virtual string GetLanguageForRequest(string methodName, object? serializedRequest)
+    public virtual bool TryGetLanguageForRequest(string methodName, object? serializedRequest, [NotNullWhen(true)] out string? language)
     {
         Logger.LogInformation($"Using default language handler for {methodName}");
-        return LanguageServerConstants.DefaultLanguageName;
+        language = LanguageServerConstants.DefaultLanguageName;
+        return true;
     }
 
     protected abstract DelegatingEntryPoint CreateDelegatingEntryPoint(string method);

src/LanguageServer/Microsoft.CommonLanguageServerProtocol.Framework/IQueueItem.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -37,6 +38,12 @@ internal interface IQueueItem<TRequestContext>
     /// </summary>
     Task<(TRequestContext, TRequest)?> CreateRequestContextAsync<TRequest>(IMethodHandler handler, RequestHandlerMetadata requestHandlerMetadata, AbstractLanguageServer<TRequestContext> languageServer, CancellationToken cancellationToken);
 
+    /// <summary>
+    /// Handles when the queue needs to manually fail a request before the
+    /// handler is invoked without shutting down the entire queue.
+    /// </summary>
+    void FailRequest(string message);
+
     /// <summary>
     /// Provides access to LSP services.
     /// </summary>

src/LanguageServer/Microsoft.CommonLanguageServerProtocol.Framework/QueueItem.cs

@@ -6,11 +6,7 @@
 #nullable enable
 
 using System;
-using System.Collections.Frozen;
 using System.Diagnostics.CodeAnalysis;
-using System.Diagnostics.Contracts;
-using System.Linq;
-using System.Reflection;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.VisualStudio.Threading;
@@ -30,6 +26,13 @@ internal class QueueItem<TRequestContext> : IQueueItem<TRequestContext>
     private readonly ILspLogger _logger;
     private readonly AbstractRequestScope? _requestTelemetryScope;
 
+    /// <summary>
+    /// True if this queue item has actually started handling the request
+    /// by delegating to the handler.  False while the item is still being
+    /// processed by the queue.
+    /// </summary>
+    private bool _requestHandlingStarted = false;
+
     /// <summary>
     /// A task completion source representing the result of this queue item's work.
     /// This is the task that the client is waiting on.
@@ -158,6 +161,7 @@ private bool TryDeserializeRequest<TRequest>(
     /// </summary>
     public async Task StartRequestAsync<TRequest, TResponse>(TRequest request, TRequestContext? context, IMethodHandler handler, string language, CancellationToken cancellationToken)
     {
+        _requestHandlingStarted = true;
         _logger.LogStartContext($"{MethodName}");
 
         try
@@ -240,4 +244,20 @@ public async Task StartRequestAsync<TRequest, TResponse>(TRequest request, TRequ
         // so it can decide how to handle the result / exception.
         await _completionSource.Task.ConfigureAwait(false);
     }
+
+    public void FailRequest(string message)
+    {
+        // This is not valid to call after StartRequestAsync starts as they both access the same state.
+        // StartRequestAsync handles any failures internally once it runs.
+        if (_requestHandlingStarted)
+        {
+            throw new InvalidOperationException("Cannot manually fail queue item after it has started");
+        }
+        var exception = new Exception(message);
+        _requestTelemetryScope?.RecordException(exception);
+        _logger.LogException(exception);
+
+        _completionSource.TrySetException(exception);
+        _requestTelemetryScope?.Dispose();
+    }
 }

src/LanguageServer/Microsoft.CommonLanguageServerProtocol.Framework/RequestExecutionQueue.cs

@@ -10,6 +10,7 @@
 using System.Collections.Frozen;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Reflection;
 using System.Threading;
@@ -245,10 +246,27 @@ private async Task ProcessQueueAsync()
                     // notifications have been completed by the time we attempt to determine the language, so we have the up to date map of URI to language.
                     // Since didOpen notifications are marked as mutating, the queue will not advance to the next request until the server has finished processing
                     // the didOpen, ensuring that this line will only run once all prior didOpens have completed.
-                    var language = _languageServer.GetLanguageForRequest(work.MethodName, work.SerializedRequest);
+                    var didGetLanguage = _languageServer.TryGetLanguageForRequest(work.MethodName, work.SerializedRequest, out var language);
 
                     // Now that we know the actual language, we can deserialize the request and start creating the request context.
-                    var (metadata, handler, methodInfo) = GetHandlerForRequest(work, language);
+                    var (metadata, handler, methodInfo) = GetHandlerForRequest(work, language ?? LanguageServerConstants.DefaultLanguageName);
+
+                    // We had an issue determining the language.  Generally this is very rare and only occurs
+                    // if there is a mis-behaving client that sends us requests for files where we haven't saved the languageId.
+                    // We should only crash if this was a mutating method, otherwise we should just fail the single request. 
+                    if (!didGetLanguage)
+                    {
+                        var message = $"Failed to get language for {work.MethodName}";
+                        if (handler.MutatesSolutionState)
+                        {
+                            throw new InvalidOperationException(message);
+                        }
+                        else
+                        {
+                            work.FailRequest(message);
+                            return;
+                        }
+                    }
 
                     // We now have the actual handler and language, so we can process the work item using the concrete types defined by the metadata.
                     await InvokeProcessCoreAsync(work, metadata, handler, methodInfo, concurrentlyExecutingTasks, currentWorkCts, cancellationToken).ConfigureAwait(false);

src/LanguageServer/Protocol/ILanguageInfoProvider.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
+using System.Diagnostics.CodeAnalysis;
 using Microsoft.CodeAnalysis.Features.Workspaces;
 
 namespace Microsoft.CodeAnalysis.LanguageServer
@@ -19,7 +20,6 @@ internal interface ILanguageInfoProvider : ILspService
         /// It is totally possible to not find language based on the file path (e.g. a newly created file that hasn't been saved to disk).
         /// In that case, we use the language Id that the LSP client gave us.
         /// </remarks>
-        /// <exception cref="InvalidOperationException">Thrown when the language information cannot be determined.</exception>
-        LanguageInformation GetLanguageInformation(Uri documentUri, string? lspLanguageId);
+        bool TryGetLanguageInformation(Uri uri, string? lspLanguageId, [NotNullWhen(true)] out LanguageInformation? languageInformation);
     }
 }

src/LanguageServer/Protocol/LanguageInfoProvider.cs

@@ -4,6 +4,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using Microsoft.CodeAnalysis.Features.Workspaces;
 
@@ -43,22 +44,22 @@ internal class LanguageInfoProvider : ILanguageInfoProvider
             { ".mts", s_typeScriptLanguageInformation },
         };
 
-        public LanguageInformation GetLanguageInformation(Uri uri, string? lspLanguageId)
+        public bool TryGetLanguageInformation(Uri uri, string? lspLanguageId, [NotNullWhen(true)] out LanguageInformation? languageInformation)
         {
             // First try to get language information from the URI path.
             // We can do this for File uris and absolute uris.  We use local path to get the value without any query parameters.
             if (uri.IsFile || uri.IsAbsoluteUri)
             {
                 var localPath = uri.LocalPath;
                 var extension = Path.GetExtension(localPath);
-                if (s_extensionToLanguageInformation.TryGetValue(extension, out var languageInformation))
+                if (s_extensionToLanguageInformation.TryGetValue(extension, out languageInformation))
                 {
-                    return languageInformation;
+                    return true;
                 }
             }
 
             // If the URI file path mapping failed, use the languageId from the LSP client (if any).
-            return lspLanguageId switch
+            languageInformation = lspLanguageId switch
             {
                 "csharp" => s_csharpLanguageInformation,
                 "fsharp" => s_fsharpLanguageInformation,
@@ -67,8 +68,10 @@ public LanguageInformation GetLanguageInformation(Uri uri, string? lspLanguageId
                 "xaml" => s_xamlLanguageInformation,
                 "typescript" => s_typeScriptLanguageInformation,
                 "javascript" => s_typeScriptLanguageInformation,
-                _ => throw new InvalidOperationException($"Unable to determine language for '{uri}' with LSP language id '{lspLanguageId}'")
+                _ => null,
             };
+
+            return languageInformation != null;
         }
     }
 }

src/LanguageServer/Protocol/RoslynLanguageServer.cs

@@ -6,6 +6,7 @@
 using System.Collections.Frozen;
 using System.Collections.Generic;
 using System.Collections.Immutable;
+using System.Diagnostics.CodeAnalysis;
 using System.Text.Json;
 using System.Threading;
 using System.Threading.Tasks;
@@ -164,12 +165,13 @@ public Task OnInitializedAsync(ClientCapabilities clientCapabilities, RequestCon
             return Task.CompletedTask;
         }
 
-        public override string GetLanguageForRequest(string methodName, object? serializedParameters)
+        public override bool TryGetLanguageForRequest(string methodName, object? serializedParameters, [NotNullWhen(true)] out string? language)
         {
             if (serializedParameters == null)
             {
                 Logger.LogInformation("No request parameters given, using default language handler");
-                return LanguageServerConstants.DefaultLanguageName;
+                language = LanguageServerConstants.DefaultLanguageName;
+                return true;
             }
 
             // We implement the STJ language server so this must be a JsonElement.
@@ -179,7 +181,8 @@ public override string GetLanguageForRequest(string methodName, object? serializ
             // as we do not want languages to be able to override them.
             if (ShouldUseDefaultLanguage(methodName))
             {
-                return LanguageServerConstants.DefaultLanguageName;
+                language = LanguageServerConstants.DefaultLanguageName;
+                return true;
             }
 
             var lspWorkspaceManager = GetLspServices().GetRequiredService<LspWorkspaceManager>();
@@ -188,34 +191,41 @@ public override string GetLanguageForRequest(string methodName, object? serializ
             // { "textDocument": { "uri": "<uri>" ... } ... }
             //
             // We can easily identify the URI for the request by looking for this structure
+            Uri? uri = null;
             if (parameters.TryGetProperty("textDocument", out var textDocumentToken) ||
                 parameters.TryGetProperty("_vs_textDocument", out textDocumentToken))
             {
                 var uriToken = textDocumentToken.GetProperty("uri");
-                var uri = JsonSerializer.Deserialize<Uri>(uriToken, ProtocolConversions.LspJsonSerializerOptions);
+                uri = JsonSerializer.Deserialize<Uri>(uriToken, ProtocolConversions.LspJsonSerializerOptions);
                 Contract.ThrowIfNull(uri, "Failed to deserialize uri property");
-                var language = lspWorkspaceManager.GetLanguageForUri(uri);
-                Logger.LogInformation($"Using {language} from request text document");
-                return language;
             }
-
-            // All the LSP resolve params have the following known json structure
-            // { "data": { "TextDocument": { "uri": "<uri>" ... } ... } ... }
-            //
-            // We can deserialize the data object using our unified DocumentResolveData.
-            //var dataToken = parameters["data"];
-            if (parameters.TryGetProperty("data", out var dataToken))
+            else if (parameters.TryGetProperty("data", out var dataToken))
             {
+                // All the LSP resolve params have the following known json structure
+                // { "data": { "TextDocument": { "uri": "<uri>" ... } ... } ... }
+                //
+                // We can deserialize the data object using our unified DocumentResolveData.
+                //var dataToken = parameters["data"];
                 var data = JsonSerializer.Deserialize<DocumentResolveData>(dataToken, ProtocolConversions.LspJsonSerializerOptions);
                 Contract.ThrowIfNull(data, "Failed to document resolve data object");
-                var language = lspWorkspaceManager.GetLanguageForUri(data.TextDocument.Uri);
-                Logger.LogInformation($"Using {language} from data text document");
-                return language;
+                uri = data.TextDocument.Uri;
+            }
+
+            if (uri == null)
+            {
+                // This request is not for a textDocument and is not a resolve request.
+                Logger.LogInformation("Request did not contain a textDocument, using default language handler");
+                language = LanguageServerConstants.DefaultLanguageName;
+                return true;
+            }
+
+            if (!lspWorkspaceManager.TryGetLanguageForUri(uri, out language))
+            {
+                Logger.LogError($"Failed to get language for {uri} with language {language}");
+                return false;
             }
 
-            // This request is not for a textDocument and is not a resolve request.
-            Logger.LogInformation("Request did not contain a textDocument, using default language handler");
-            return LanguageServerConstants.DefaultLanguageName;
+            return true;
 
             static bool ShouldUseDefaultLanguage(string methodName)
             {

src/LanguageServer/Protocol/Workspaces/LspMiscellaneousFilesWorkspace.cs

@@ -50,8 +50,7 @@ internal sealed class LspMiscellaneousFilesWorkspace(ILspServices lspServices, I
             }
 
             var languageInfoProvider = lspServices.GetRequiredService<ILanguageInfoProvider>();
-            var languageInformation = languageInfoProvider.GetLanguageInformation(uri, languageId);
-            if (languageInformation == null)
+            if (!languageInfoProvider.TryGetLanguageInformation(uri, languageId, out var languageInformation))
             {
                 // Only log here since throwing here could take down the LSP server.
                 logger.LogError($"Could not find language information for {uri} with absolute path {documentFilePath}");

src/LanguageServer/Protocol/Workspaces/LspWorkspaceManager.cs

@@ -5,6 +5,7 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
@@ -532,15 +533,22 @@ private static async ValueTask<bool> AreChecksumsEqualAsync(TextDocument documen
     /// <summary>
     /// Returns a Roslyn language name for the given URI.
     /// </summary>
-    internal string GetLanguageForUri(Uri uri)
+    internal bool TryGetLanguageForUri(Uri uri, [NotNullWhen(true)] out string? language)
     {
         string? languageId = null;
         if (_trackedDocuments.TryGetValue(uri, out var trackedDocument))
         {
             languageId = trackedDocument.LanguageId;
         }
 
-        return _languageInfoProvider.GetLanguageInformation(uri, languageId).LanguageName;
+        if (_languageInfoProvider.TryGetLanguageInformation(uri, languageId, out var languageInfo))
+        {
+            language = languageInfo.LanguageName;
+            return true;
+        }
+
+        language = null;
+        return false;
     }
 
     /// <summary>

src/LanguageServer/ProtocolUnitTests/HandlerTests.cs

@@ -17,6 +17,7 @@
 using Roslyn.Test.Utilities;
 using Xunit;
 using Xunit.Abstractions;
+using static Microsoft.CodeAnalysis.LanguageServer.UnitTests.LocaleTests;
 
 namespace Microsoft.CodeAnalysis.LanguageServer.UnitTests
 {
@@ -295,6 +296,24 @@ await Assert.ThrowsAnyAsync<Exception>(async ()
             Assert.False(didReport);
         }
 
+        [Theory, CombinatorialData]
+        public async Task TestMutatingHandlerCrashesIfUnableToDetermineLanguage(bool mutatingLspWorkspace)
+        {
+            await using var testLspServer = await CreateTestLspServerAsync(string.Empty, mutatingLspWorkspace, new InitializationOptions { ServerKind = WellKnownLspServerKinds.CSharpVisualBasicLspServer });
+
+            // Run a mutating request against a file which we have no saved languageId for
+            // and where the language cannot be determined from the URI.
+            // This should crash the server.
+            var looseFileUri = ProtocolConversions.CreateAbsoluteUri(@"untitled:untitledFile");
+            var request = new TestRequestTypeOne(new TextDocumentIdentifier
+            {
+                Uri = looseFileUri
+            });
+
+            await Assert.ThrowsAnyAsync<Exception>(async () => await testLspServer.ExecuteRequestAsync<TestRequestTypeOne, string>(TestDocumentHandler.MethodName, request, CancellationToken.None)).ConfigureAwait(false);
+            await testLspServer.AssertServerShuttingDownAsync();
+        }
+
         internal record TestRequestTypeOne([property: JsonPropertyName("textDocument"), JsonRequired] TextDocumentIdentifier TextDocumentIdentifier);
 
         internal record TestRequestTypeTwo([property: JsonPropertyName("textDocument"), JsonRequired] TextDocumentIdentifier TextDocumentIdentifier);