c# - Service not fully stopped after ServiceController.Stop()

Question

ServiceController serviceController = new ServiceController(someService);
serviceController.Stop();
serviceController.WaitForStopped();
DoSomething();

SomeService works on a sqlserver file. DoSomething() wants to copy that SQL file. If SomeService isn't closed fully it will throw an error because the database file is still locked. In the aforementioned code, I get past the WaitForStopped() method and yet the service doesn't release the database file until after DoSomething(), thus I get an error.

Doing some more investigation, I find that before the DoSomething method call I see that the service controller status shows a stopped and yet looking at some ProcMon logs the service releases the database file after I'm thrown an error from DoSomething.

Also, if I put a Thread.Sleep between the WaitForStopped and the DoSomething method for say... 5 seconds, the database file is released and all is well. Not the solution of surety I'm looking for however.

Any ideas?

Answer 1

Windows Services are a layer on top of processes; in order to be a service, an application must connect to the Service Control Manager and announce which services are available. This connection is handled within the ADVAPI32.DLL library. Once this connection is established, the library maintains a thread waiting for commands from the Service Control Manager, which can then start and stop services arbitrarily. I don't believe the process is required to exit when the last service in it terminates. Though that is what typically happens, the end of the link with the Service Control Manager, which occurs after the last service enters the "Stopped" state, can occur significantly before the process actually terminates, releasing any resources it hasn't already explicitly released.

The Windows Service API includes functionality that lets you obtain the Process ID of the process that is hosting the service. It is possible for a single process to host many services, and so the process might not actually exit when the service you are interested in has terminated, but you should be safe with SQL Server. Unfortunately, the .NET Framework does not expose this functionality. It does, however, expose the handle to the service that it uses internally for API calls, and you can use it to make your own API calls. With a bit of P/Invoke, then, you can obtain the process ID of the Windows Service process, and from there, provided you have the necessary permission, you can open a handle to the process that can be used to wait for it to exit.

Something like this:

[DllImport("advapi32")]
static extern bool QueryServiceStatusEx(IntPtr hService, int InfoLevel, ref SERVICE_STATUS_PROCESS lpBuffer, int cbBufSize, out int pcbBytesNeeded);

const int SC_STATUS_PROCESS_INFO = 0;

[StructLayout(LayoutKind.Sequential)]
struct SERVICE_STATUS_PROCESS
{
  public int dwServiceType;
  public int dwCurrentState;
  public int dwControlsAccepted;
  public int dwWin32ExitCode;
  public int dwServiceSpecificExitCode;
  public int dwCheckPoint;
  public int dwWaitHint;
  public int dwProcessId;
  public int dwServiceFlags;
}

const int SERVICE_WIN32_OWN_PROCESS = 0x00000010;
const int SERVICE_INTERACTIVE_PROCESS = 0x00000100;

const int SERVICE_RUNS_IN_SYSTEM_PROCESS = 0x00000001;

public static void StopServiceAndWaitForExit(string serviceName)
{
  using (ServiceController controller = new ServiceController(serviceName))
  {
    SERVICE_STATUS_PROCESS ssp = new SERVICE_STATUS_PROCESS();
    int ignored;

    // Obtain information about the service, and specifically its hosting process,
    // from the Service Control Manager.
    if (!QueryServiceStatusEx(controller.ServiceHandle.DangerousGetHandle(), SC_STATUS_PROCESS_INFO, ref ssp, Marshal.SizeOf(ssp), out ignored))
      throw new Exception("Couldn't obtain service process information.");

    // A few quick sanity checks that what the caller wants is *possible*.
    if ((ssp.dwServiceType & ~SERVICE_INTERACTIVE_PROCESS) != SERVICE_WIN32_OWN_PROCESS)
      throw new Exception("Can't wait for the service's hosting process to exit because there may be multiple services in the process (dwServiceType is not SERVICE_WIN32_OWN_PROCESS");

    if ((ssp.dwServiceFlags & SERVICE_RUNS_IN_SYSTEM_PROCESS) != 0)
      throw new Exception("Can't wait for the service's hosting process to exit because the hosting process is a critical system process that will not exit (SERVICE_RUNS_IN_SYSTEM_PROCESS flag set)");

    if (ssp.dwProcessId == 0)
      throw new Exception("Can't wait for the service's hosting process to exit because the process ID is not known.");

    // Note: It is possible for the next line to throw an ArgumentException if the
    // Service Control Manager's information is out-of-date (e.g. due to the process
    // having *just* been terminated in Task Manager) and the process does not really
    // exist. This is a race condition. The exception is the desirable result in this
    // case.
    using (Process process = Process.GetProcessById(ssp.dwProcessId))
    {
      // EDIT: There is no need for waiting in a separate thread, because MSDN says "The handles are valid until closed, even after the process or thread they represent has been terminated." ( http://msdn.microsoft.com/en-us/library/windows/desktop/ms684868%28v=vs.85%29.aspx ), so to keep things in the same thread, the process HANDLE should be opened from the process id before the service is stopped, and the Wait should be done after that.

      // Response to EDIT: What you report is true, but the problem is that the handle isn't actually opened by Process.GetProcessById. It's only opened within the .WaitForExit method, which won't return until the wait is complete. Thus, if we try the wait on the current therad, we can't actually do anything until it's done, and if we defer the check until after the process has completed, it won't be possible to obtain a handle to it any more.

      // The actual wait, using process.WaitForExit, opens a handle with the SYNCHRONIZE
      // permission only and closes the handle before returning. As long as that handle
      // is open, the process can be monitored for termination, but if the process exits
      // before the handle is opened, it is no longer possible to open a handle to the
      // original process and, worse, though it exists only as a technicality, there is
      // a race condition in that another process could pop up with the same process ID.
      // As such, we definitely want the handle to be opened before we ask the service
      // to close, but since the handle's lifetime is only that of the call to WaitForExit
      // and while WaitForExit is blocking the thread we can't make calls into the SCM,
      // it would appear to be necessary to perform the wait on a separate thread.
      ProcessWaitForExitData threadData = new ProcessWaitForExitData();

      threadData.Process = process;

      Thread processWaitForExitThread = new Thread(ProcessWaitForExitThreadProc);

      processWaitForExitThread.IsBackground = Thread.CurrentThread.IsBackground;
      processWaitForExitThread.Start(threadData);

      // Now we ask the service to exit.
      controller.Stop();

      // Instead of waiting until the *service* is in the "stopped" state, here we
      // wait for its hosting process to go away. Of course, it's really that other
      // thread waiting for the process to go away, and then we wait for the thread
      // to go away.
      lock (threadData.Sync)
        while (!threadData.HasExited)
          Monitor.Wait(threadData.Sync);
    }
  }
}

class ProcessWaitForExitData
{
  public Process Process;
  public volatile bool HasExited;
  public object Sync = new object();
}

static void ProcessWaitForExitThreadProc(object state)
{
  ProcessWaitForExitData threadData = (ProcessWaitForExitData)state;

  try
  {
    threadData.Process.WaitForExit();
  }
  catch {}
  finally
  {
    lock (threadData.Sync)
    {
      threadData.HasExited = true;
      Monitor.PulseAll(threadData.Sync);
    }
  }
}