Avoid redirecting process output

src/Dotnet.Watch/Watch/Browser/BrowserLauncher.cs

@@ -16,10 +16,9 @@ internal sealed class BrowserLauncher(ILogger logger, IProcessOutputReporter pro
     private ImmutableHashSet<ProjectInstanceId> _browserLaunchAttempted = [];
 
     /// <summary>
-    /// Installs browser launch/reload trigger.
+    /// Returns an output observing action that triggers the launch of the browser, or null if the browser should not be launched.
     /// </summary>
-    public void InstallBrowserLaunchTrigger(
-        ProcessSpec processSpec,
+    public Action<OutputLine>? TryGetBrowserLaunchOutputObserver(
         ProjectGraphNode projectNode,
         ProjectOptions projectOptions,
         AbstractBrowserRefreshServer? server,
@@ -32,10 +31,10 @@ public void InstallBrowserLaunchTrigger(
                 logger.LogError("Test requires browser to launch");
             }
 
-            return;
+            return null;
         }
 
-        WebServerProcessStateObserver.Observe(projectNode, processSpec, url =>
+        return WebServerProcessStateObserver.GetObserver(projectNode, url =>
         {
             if (projectOptions.IsMainProject &&
                 ImmutableInterlocked.Update(ref _browserLaunchAttempted, static (set, key) => set.Add(key), projectNode.ProjectInstance.GetId()))

src/Dotnet.Watch/Watch/Process/ProcessSpec.cs

@@ -24,4 +24,25 @@ internal sealed class ProcessSpec
 
     public string GetArgumentsDisplay()
         => CommandLineUtilities.JoinArguments(Arguments ?? []);
+
+    /// <summary>
+    /// Stream output lines to the process output reporter when
+    /// - output observer is installed so that the output is also streamd to the console;
+    /// - testing to synchonize the output of the process with the logger output, so that the printed lines don't interleave;
+    /// unless the caller has already redirected the output (e.g. for Aspire child processes).
+    ///
+    /// Do not redirect output otherwise as it disables the ability of the process to use Console APIs.
+    /// </summary>
+    public void RedirectOutput(Action<OutputLine>? outputObserver, IProcessOutputReporter outputReporter, EnvironmentOptions environmentOptions, string projectDisplayName)
+    {
+        if (environmentOptions.RunningAsTest || outputObserver != null)
+        {
+            OnOutput ??= line =>
+            {
+                outputReporter.ReportOutput(outputReporter.PrefixProcessOutput ? line with { Content = $"[{projectDisplayName}] {line.Content}" } : line);
+            };
+
+            OnOutput += outputObserver;
+        }
+    }
 }

src/Dotnet.Watch/Watch/Process/ProjectLauncher.cs

@@ -60,15 +60,6 @@ public CompilationHandler CompilationHandler
             OnExit = onExit,
         };
 
-        // Stream output lines to the process output reporter.
-        // The reporter synchronizes the output of the process with the logger output,
-        // so that the printed lines don't interleave.
-        // Only send the output to the reporter if no custom output handler was provided (e.g. for Aspire child processes).
-        processSpec.OnOutput ??= line =>
-        {
-            context.ProcessOutputReporter.ReportOutput(context.ProcessOutputReporter.PrefixProcessOutput ? line with { Content = $"[{projectDisplayName}] {line.Content}" } : line);
-        };
-
         var environmentBuilder = new Dictionary<string, string>();
 
         // initialize with project settings:
@@ -91,9 +82,10 @@ public CompilationHandler CompilationHandler
 
         processSpec.Arguments = GetProcessArguments(projectOptions, environmentBuilder);
 
-        // Attach trigger to the process that detects when the web server reports to the output that it's listening.
-        // Launches browser on the URL found in the process output for root projects.
-        context.BrowserLauncher.InstallBrowserLaunchTrigger(processSpec, projectNode, projectOptions, clients.BrowserRefreshServer, cancellationToken);
+        // Observes main project process output and launches browser when the URL is found in the output.
+        var outputObserver = context.BrowserLauncher.TryGetBrowserLaunchOutputObserver(projectNode, projectOptions, clients.BrowserRefreshServer, cancellationToken);
+
+        processSpec.RedirectOutput(outputObserver, context.ProcessOutputReporter, context.EnvironmentOptions, projectDisplayName);
 
         return await compilationHandler.TrackRunningProjectAsync(
             projectNode,

src/Dotnet.Watch/Watch/Process/WebServerProcessStateObserver.cs

@@ -21,7 +21,7 @@ internal static partial class WebServerProcessStateObserver
     [GeneratedRegex(@"Login to the dashboard at (?<url>.*)\s*$", RegexOptions.Compiled)]
     private static partial Regex GetAspireDashboardUrlRegex();
 
-    public static void Observe(ProjectGraphNode serverProject, ProcessSpec serverProcessSpec, Action<string> onServerListening)
+    public static Action<OutputLine> GetObserver(ProjectGraphNode serverProject, Action<string> onServerListening)
     {
         // Workaround for Aspire dashboard launching: scan for "Login to the dashboard at " prefix in the output and use the URL.
         // TODO: https://github.com/dotnet/sdk/issues/9038
@@ -30,7 +30,7 @@ public static void Observe(ProjectGraphNode serverProject, ProcessSpec serverPro
 
         var _notified = false;
 
-        serverProcessSpec.OnOutput += line =>
+        return line =>
         {
             if (_notified)
             {

src/Dotnet.Watch/dotnet-watch/Watch/DotNetWatcher.cs

@@ -58,8 +58,7 @@ public static async Task WatchAsync(DotNetWatchContext context, CancellationToke
                 {
                     [EnvironmentVariables.Names.DotnetWatch] = "1",
                     [EnvironmentVariables.Names.DotnetWatchIteration] = (iteration + 1).ToString(CultureInfo.InvariantCulture),
-                },
-                OnOutput = line => context.ProcessOutputReporter.ReportOutput(line)
+                }
             };
 
             var browserRefreshServer = projectRootNode != null && HotReloadAppModel.InferFromProject(context, projectRootNode) is WebApplicationAppModel webAppModel
@@ -68,15 +67,18 @@ public static async Task WatchAsync(DotNetWatchContext context, CancellationToke
 
             browserRefreshServer?.ConfigureLaunchEnvironment(environmentBuilder, enableHotReload: false);
 
-            foreach (var (name, value) in environmentBuilder)
+            Action<OutputLine>? outputObserver = null;
+            if (projectRootNode != null)
             {
-                processSpec.EnvironmentVariables.Add(name, value);
+                Debug.Assert(context.MainProjectOptions != null);
+                outputObserver = context.BrowserLauncher.TryGetBrowserLaunchOutputObserver(projectRootNode, context.MainProjectOptions, browserRefreshServer, shutdownCancellationToken);
             }
 
-            if (projectRootNode != null)
+            processSpec.RedirectOutput(outputObserver, context.ProcessOutputReporter, context.EnvironmentOptions, projectRootNode?.GetDisplayName() ?? "");
+
+            foreach (var (name, value) in environmentBuilder)
             {
-                Debug.Assert(context.MainProjectOptions != null);
-                context.BrowserLauncher.InstallBrowserLaunchTrigger(processSpec, projectRootNode, context.MainProjectOptions, browserRefreshServer, shutdownCancellationToken);
+                processSpec.EnvironmentVariables.Add(name, value);
             }
 
             // Reset for next run