Fix semantic highlighting cancellation during rapid typing

src/PowerShellEditorServices/Server/PsesLanguageServer.cs

@@ -14,7 +14,6 @@
 using Microsoft.PowerShell.EditorServices.Services.Extension;
 using Microsoft.PowerShell.EditorServices.Services.PowerShell.Host;
 using Newtonsoft.Json.Linq;
-using OmniSharp.Extensions.JsonRpc;
 using OmniSharp.Extensions.LanguageServer.Protocol.General;
 using OmniSharp.Extensions.LanguageServer.Protocol.Server;
 using OmniSharp.Extensions.LanguageServer.Server;
@@ -107,14 +106,7 @@ public async Task StartAsync()
                     .WithHandler<PsesCodeLensHandlers>()
                     .WithHandler<PsesCodeActionHandler>()
                     .WithHandler<InvokeExtensionCommandHandler>()
-                    // If PsesCompletionHandler is not marked as serial, then DidChangeTextDocument
-                    // notifications will end up cancelling completion. So quickly typing `Get-`
-                    // would result in no completions.
-                    //
-                    // This also lets completion requests interrupt time consuming background tasks
-                    // like the references code lens.
-                    .WithHandler<PsesCompletionHandler>(
-                        new JsonRpcHandlerOptions() { RequestProcessType = RequestProcessType.Serial })
+                    .WithHandler<PsesCompletionHandler>()
                     .WithHandler<PsesHoverHandler>()
                     .WithHandler<PsesSignatureHelpHandler>()
                     .WithHandler<PsesDefinitionHandler>()

src/PowerShellEditorServices/Services/TextDocument/Handlers/CompletionHandler.cs

@@ -161,22 +161,30 @@ public override async Task<CompletionItem> Handle(CompletionItem request, Cancel
                 return request;
             }
 
-            // Get the documentation for the function
-            CommandInfo commandInfo = await CommandHelpers.GetCommandInfoAsync(
-                request.Label,
-                _runspaceContext.CurrentRunspace,
-                _executionService,
-                cancellationToken).ConfigureAwait(false);
-
-            if (commandInfo is not null)
+            try
             {
-                return request with
+                // Get the documentation for the function
+                CommandInfo commandInfo = await CommandHelpers.GetCommandInfoAsync(
+                    request.Label,
+                    _runspaceContext.CurrentRunspace,
+                    _executionService,
+                    cancellationToken).ConfigureAwait(false);
+
+                if (commandInfo is not null)
                 {
-                    Documentation = await CommandHelpers.GetCommandSynopsisAsync(
-                        commandInfo,
-                        _executionService,
-                        cancellationToken).ConfigureAwait(false)
-                };
+                    return request with
+                    {
+                        Documentation = await CommandHelpers.GetCommandSynopsisAsync(
+                            commandInfo,
+                            _executionService,
+                            cancellationToken).ConfigureAwait(false)
+                    };
+                }
+            }
+            // Ignore canceled requests (logging will pollute the output).
+            catch (TaskCanceledException)
+            {
+                return request;
             }
 
             return request;

test/PowerShellEditorServices.Test/Language/CompletionHandlerTests.cs

@@ -126,6 +126,27 @@ public async Task CompletesFilePath()
             Assert.All(results, r => Assert.True(r.Kind is CompletionItemKind.File or CompletionItemKind.Folder));
         }
 
+        [Fact]
+        public async Task CompletionResolveHandlesTaskCanceledException()
+        {
+            using CancellationTokenSource cancellationTokenSource = new();
+#if NET5_0_OR_GREATER
+            await cancellationTokenSource.CancelAsync();
+#else
+            cancellationTokenSource.Cancel();
+#endif
+            CompletionItem request = new()
+            {
+                Kind = CompletionItemKind.Function,
+                Label = "Get-ChildItem",
+                Detail = "Microsoft.PowerShell.Management"
+            };
+
+            CompletionItem actual = await completionHandler.Handle(request, cancellationTokenSource.Token);
+
+            Assert.Equal(request, actual);
+        }
+
         // TODO: These should be an integration tests at a higher level if/when https://github.com/PowerShell/PowerShell/pull/25108 is merged. As of today, we can't actually test this in the PS engine currently.
         [Fact]
         public void CanExtractTypeAndDescriptionFromTooltip()