Cover the completion handler

tests/Settex.LanguageServer.Tests/CompletionHandlerTests.cs

@@ -0,0 +1,136 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using OmniSharp.Extensions.LanguageServer.Protocol.Models;
+
+namespace Settex.LanguageServer.Tests;
+
+/// <summary>
+/// The completion handler is the largest file in the language server and the most
+/// visible feature of both editor extensions, and it had no tests at all. These cover
+/// what a user actually sees at each cursor position.
+/// </summary>
+public sealed class CompletionHandlerTests
+{
+    [Test]
+    public async Task Completion_AfterADot_OffersThePropertiesOfThatObjectAsync()
+    {
+        const string source = """
+            settings {
+                Server {
+                    Host = "localhost"
+                    Port = 8080
+                }
+                Other = 1
+            }
+            env "Dev" {
+                settings {
+                    Server.Host = "dev"
+                }
+            }
+            """;
+
+        // Cursor immediately after "Server." on the overlay line (0-based line 9, col 15).
+        var items = await CompleteAsync(source, 9, 15);
+
+        await Assert.That(items.Any(i => i.Label == "Host")).IsTrue();
+        await Assert.That(items.Any(i => i.Label == "Port")).IsTrue();
+
+        // Only that object's properties — a sibling key is not one of them.
+        await Assert.That(items.Any(i => i.Label == "Other")).IsFalse();
+    }
+
+    [Test]
+    public async Task Completion_AtTopLevel_OffersTheLanguageKeywordsAsync()
+    {
+        var items = await CompleteAsync("settings {\n    A = 1\n}\n", 3, 0);
+
+        await Assert.That(items.Any(i => i.Kind == CompletionItemKind.Keyword)).IsTrue();
+    }
+
+    [Test]
+    public async Task Completion_OffersDeclaredVariablesAsync()
+    {
+        const string source = """
+            let basePort = 5000
+            settings {
+                Port = basePort
+            }
+            """;
+
+        // Cursor just after "Port = " (0-based line 2, column 11).
+        var items = await CompleteAsync(source, 2, 11);
+
+        await Assert.That(items.Any(i => i.Label == "basePort")).IsTrue();
+    }
+
+    /// <summary>
+    /// A limitation worth recording rather than hiding: properties and variables both
+    /// come from the evaluated AST, so while the file does not parse — which is most of
+    /// the time completion is wanted — only the static keyword list is offered. The
+    /// handler degrades instead of failing, but it does not help.
+    /// </summary>
+    [Test]
+    public async Task Completion_WhileTheFileDoesNotParse_OffersKeywordsOnlyAsync()
+    {
+        const string source = "let basePort = 5000\nsettings {\n    Port = \n";
+
+        var items = await CompleteAsync(source, 2, 11);
+
+        await Assert.That(items.Any(i => i.Kind == CompletionItemKind.Keyword)).IsTrue();
+        await Assert.That(items.Any(i => i.Label == "basePort")).IsFalse();
+    }
+
+    [Test]
+    public async Task Completion_OffersDeclaredEnvironmentsAsync()
+    {
+        const string source = """
+            settings { A = 1 }
+            env "Staging" { settings { A = 2 } }
+            env "Production" { settings { A = 3 } }
+            """;
+
+        var items = await CompleteAsync(source, 3, 0);
+
+        await Assert.That(items.Any(i => i.Label == "Staging")).IsTrue();
+        await Assert.That(items.Any(i => i.Label == "Production")).IsTrue();
+    }
+
+    [Test]
+    public async Task Completion_OnADocumentTheServerDoesNotKnow_ReturnsNothingAsync()
+    {
+        // The handler must degrade rather than throw when asked about a URI that was
+        // never opened — a race the client can genuinely produce.
+        var handler = new SettexCompletionHandler(new SettexWorkspace(), NullLogger<SettexCompletionHandler>.Instance);
+
+        var result = await handler.Handle(Request("untitled:never-opened", 0, 0), CancellationToken.None);
+
+        await Assert.That(result.Items.Count()).IsEqualTo(0);
+    }
+
+    [Test]
+    public async Task Completion_OnAnUnparsableDocument_StillAnswersAsync()
+    {
+        // Completion is most useful exactly while the file is half-written and does not
+        // parse. It must not fault.
+        var items = await CompleteAsync("settings {\n    A = \n", 1, 8);
+
+        await Assert.That(items).IsNotNull();
+    }
+
+    private static async Task<IReadOnlyList<CompletionItem>> CompleteAsync(string source, int line, int character)
+    {
+        var workspace = new SettexWorkspace();
+        const string uri = "untitled:completion-test";
+        workspace.DidOpen(uri, source);
+
+        var handler = new SettexCompletionHandler(workspace, NullLogger<SettexCompletionHandler>.Instance);
+        var result = await handler.Handle(Request(uri, line, character), CancellationToken.None);
+
+        return result.Items.ToList();
+    }
+
+    private static CompletionParams Request(string uri, int line, int character) => new()
+    {
+        TextDocument = new TextDocumentIdentifier { Uri = uri },
+        Position = new Position(line, character),
+    };
+}