Bring back debugging capabilities on the agent (#4926)

* Reapply "Allow running agent in debugging mode for specific node task (#4838)" (#4898)

This reverts commit c660a56.

* Fix of the production issue

* unset debug

---------

Co-authored-by: Grzegorz Gurgul (from Dev Box) <[email protected]>

src/Agent.Listener/Agent.cs

@@ -216,6 +216,20 @@ public async Task<int> ExecuteCommand(CommandSettings command)
                 Trace.Info($"Set agent startup type - {startType}");
                 HostContext.StartupType = startType;
 
+                bool debugModeEnabled = command.GetDebugMode();
+
+                if (debugModeEnabled)
+                {
+                    Trace.Warning("Agent is running in debug mode, don't use it in production");
+                    settings.DebugMode = true;
+                    store.SaveSettings(settings);
+                }
+                else if (settings.DebugMode && !debugModeEnabled)
+                {
+                    settings.DebugMode = false;
+                    store.SaveSettings(settings);
+                }
+
                 if (PlatformUtil.RunningOnWindows)
                 {
                     if (store.IsAutoLogonConfigured())

src/Agent.Listener/CommandLine/RunAgent.cs

@@ -18,5 +18,8 @@ public class RunAgent : BaseCommand
 
         [Option(Constants.Agent.CommandLine.Args.StartupType)]
         public string StartupType { get; set; }
+
+        [Option(Constants.Agent.CommandLine.Flags.DebugMode)]
+        public bool DebugMode { get; set; }
     }
 }

src/Agent.Listener/CommandSettings.cs

@@ -550,6 +550,11 @@ public bool GetRunOnce()
                    TestFlag(Run?.RunOnce, Constants.Agent.CommandLine.Flags.Once);
         }
 
+        public bool GetDebugMode()
+        {
+            return TestFlag(Run?.DebugMode, Constants.Agent.CommandLine.Flags.DebugMode);
+        }
+
         public bool GetDeploymentPool()
         {
             return TestFlag(Configure?.DeploymentPool, Constants.Agent.CommandLine.Flags.DeploymentPool);

src/Agent.Sdk/Knob/AgentKnobs.cs

@@ -213,6 +213,12 @@ public class AgentKnobs
             new EnvironmentKnobSource("VSTSAGENT_TRACE"),
             new BuiltInDefaultKnobSource(string.Empty));
 
+        public static readonly Knob DebugTask = new Knob(
+            nameof(DebugTask),
+            "If the agent executes a task which ID or name matches the value provided, it will run the task so that it will wait for debugger to attach",
+            new EnvironmentKnobSource("VSTSAGENT_DEBUG_TASK"),
+            new BuiltInDefaultKnobSource(string.Empty));
+
         public static readonly Knob DumpJobEventLogs = new Knob(
             nameof(DumpJobEventLogs),
             "If true, dump event viewer logs",

src/Agent.Worker/Handlers/NodeHandler.cs

@@ -224,6 +224,21 @@ public async Task RunAsync()
             var sigtermTimeout = TimeSpan.FromMilliseconds(AgentKnobs.ProccessSigtermTimeout.GetValue(ExecutionContext).AsInt());
             var useGracefulShutdown = AgentKnobs.UseGracefulProcessShutdown.GetValue(ExecutionContext).AsBoolean();
 
+            var configStore = HostContext.GetService<IConfigurationStore>();
+            var agentSettings = configStore.GetSettings();
+            if (agentSettings.DebugMode)
+            {
+                var debugTask = AgentKnobs.DebugTask.GetValue(ExecutionContext).AsString();
+                if (!string.IsNullOrEmpty(debugTask))
+                {
+                    if (string.Equals(Task?.Id.ToString("D"), debugTask, StringComparison.OrdinalIgnoreCase) || string.Equals(Task?.Name, debugTask, StringComparison.OrdinalIgnoreCase))
+                    {
+                        arguments = $"--inspect-brk {arguments}";
+                    }
+                }
+            }
+
+
             try
             {
                 // Execute the process. Exit code 0 should always be returned.

src/Microsoft.VisualStudio.Services.Agent/ConfigurationStore.cs

@@ -135,6 +135,9 @@ public string Fingerprint
 
         [DataMember(EmitDefaultValue = false)]
         public int MaxDedupParallelism { get; set; }
+
+        [DataMember(EmitDefaultValue = false)]
+        public bool DebugMode { get; set; }
     }
 
     [DataContract]

src/Microsoft.VisualStudio.Services.Agent/Constants.cs

@@ -226,6 +226,7 @@ public static class Flags
                     public const string NoRestart = "norestart";
                     public const string LaunchBrowser = "launchbrowser";
                     public const string Once = "once";
+                    public const string DebugMode = "debug";
                     public const string RunAsAutoLogon = "runasautologon";
                     public const string RunAsService = "runasservice";
                     public const string PreventServiceStart = "preventservicestart";