Wait for project changes when unchanged project doesn't build (#45167)

Author: tmat

src/BuiltInTools/dotnet-watch/HotReloadDotNetWatcher.cs

@@ -122,8 +122,7 @@ public override async Task WatchAsync(CancellationToken shutdownCancellationToke
                     if (!await BuildProjectAsync(rootProjectOptions.ProjectPath, rootProjectOptions.BuildArguments, iterationCancellationToken))
                     {
                         // error has been reported:
-                        waitForFileChangeBeforeRestarting = false;
-                        return;
+                        continue;
                     }
 
                     rootRunningProject = await projectLauncher.TryLaunchProcessAsync(

test/TestAssets/TestProjects/WatchNoDepsApp/WatchNoDepsApp.csproj

@@ -21,4 +21,8 @@
     <Warning Text="The value of property is '$(TestProperty)'" />
   </Target>
 
+  <ItemGroup>
+    <!-- add item -->
+  </ItemGroup>
+
 </Project>

test/dotnet-watch.Tests/HotReload/ApplyDeltaTests.cs

@@ -65,6 +65,75 @@ public static void Print()
             await App.AssertOutputLineStartsWith("Changed!");
         }
 
+        /// <summary>
+        /// Unchanged project doesn't build. Wait for source change and rebuild.
+        /// </summary>
+        [Fact]
+        public async Task BaselineCompilationError()
+        {
+            var testAsset = TestAssets.CopyTestAsset("WatchNoDepsApp")
+                .WithSource();
+
+            var programPath = Path.Combine(testAsset.Path, "Program.cs");
+            File.WriteAllText(programPath,
+                """
+                Console.Write
+                """);
+
+            App.Start(testAsset, []);
+
+            await App.AssertOutputLineStartsWith(MessageDescriptor.WaitingForFileChangeBeforeRestarting, failure: _ => false);
+
+            UpdateSourceFile(programPath, """
+                System.Console.WriteLine("<Updated>");
+                """);
+
+            await App.AssertOutputLineStartsWith("<Updated>");
+        }
+
+        /// <summary>
+        /// We currently do not support applying project changes.
+        /// The workaround is to restart via Ctrl+R.
+        /// </summary>
+        [Fact]
+        public async Task ProjectChangeAndRestart()
+        {
+            var testAsset = TestAssets.CopyTestAsset("WatchNoDepsApp")
+                .WithSource();
+
+            var programPath = Path.Combine(testAsset.Path, "Program.cs");
+            var projectPath = Path.Combine(testAsset.Path, "WatchNoDepsApp.csproj");
+
+            App.Start(testAsset, ["--no-exit"], testFlags: TestFlags.ReadKeyFromStdin);
+
+            await App.AssertOutputLineStartsWith(MessageDescriptor.WaitingForChanges);
+
+            // missing System.Linq import:
+            UpdateSourceFile(programPath, content => content.Replace("""
+                Console.WriteLine("Started");
+                """,
+                """
+                Console.WriteLine($">>> {typeof(Enumerable)}");
+                """));
+
+            await App.AssertOutputLineStartsWith("dotnet watch ⌚ Unable to apply hot reload due to compilation errors.", failure: _ => false);
+
+            UpdateSourceFile(projectPath, content => content.Replace("""
+                <!-- add item -->
+                """,
+                """
+                <Using Include="System.Linq" />
+                """));
+
+            // project change not applied:
+            await App.AssertOutputLineStartsWith("dotnet watch ⌚ Unable to apply hot reload due to compilation errors.", failure: _ => false);
+
+            // Ctlr+R rebuilds and restarts:
+            App.SendControlR();
+
+            await App.AssertOutputLineStartsWith(">>> System.Linq.Enumerable", failure: _ => false);
+        }
+
         [Fact]
         public async Task ChangeFileInFSharpProject()
         {

test/dotnet-watch.Tests/Utilities/TestReporter.cs

@@ -24,7 +24,7 @@ public bool IsVerbose
 
         public void ReportProcessOutput(OutputLine line)
         {
-            output.WriteLine(line.Content);
+            WriteTestOutput(line.Content);
             ProcessOutput.Add(line.Content);
 
             OnProcessOutput?.Invoke(line);
@@ -34,7 +34,7 @@ public void ReportProcessOutput(ProjectGraphNode project, OutputLine line)
         {
             var content = $"[{project.GetDisplayName()}]: {line.Content}";
 
-            output.WriteLine(content);
+            WriteTestOutput(content);
             ProcessOutput.Add(content);
 
             OnProjectProcessOutput?.Invoke(project.ProjectInstance.FullPath, line);
@@ -67,7 +67,7 @@ public void Report(MessageDescriptor descriptor, string prefix, object?[] args)
         {
             if (descriptor.TryGetMessage(prefix, args, out var message))
             {
-                output.WriteLine($"{ToString(descriptor.Severity)} {descriptor.Emoji} {message}");
+                WriteTestOutput($"{ToString(descriptor.Severity)} {descriptor.Emoji} {message}");
             }
 
             if (descriptor.Id.HasValue && _actions.TryGetValue(descriptor.Id.Value, out var action))
@@ -76,6 +76,18 @@ public void Report(MessageDescriptor descriptor, string prefix, object?[] args)
             }
         }
 
+        private void WriteTestOutput(string message)
+        {
+            try
+            {
+                output.WriteLine(message);
+            }
+            catch (InvalidOperationException)
+            {
+                // May happen when a test is aborted and no longer running.
+            }
+        }
+
         private static string ToString(MessageSeverity severity)
             => severity switch
             {