Robust Process Spawning and IO Redirection in .NET
Many times something seems simple on the surface, but once you start you find it is a pain in the neck. Then, once in awhile, you find it is beyond a pain in the neck. For example, spawning a process and capturing stdout/stderr in .NET. Sure, it doesn't seem hard, create an instance of the Process class, fill in some members, kick it off and wait until it finishes. Works great! On to the next problem...
Until the day it locks up on you unexpectedly. Hmm.
The next step is then to ask the modern Oracle - Google, to see if others have this problem. Sure enough, you find out that if the child process' buffer becomes full, it blocks, and that is causing your deadlock. Even better, there are tons of helpful people on the Internet willing to share the solution to this problem, along with source code snippets. Score! Asynchronous reads for the win. Hook it up, and on to the next problem...
Until the day it locks up on you unexpectedly. Hmm.
I got to this point, and Google began to fail me. It seems either no one ran into this problem, or, the only solution people talk about is the method I'm already using. We write all our crunching tools using C/C++ as command line tools, and then a GUI in C# that spawns these tools. Most of the time, things ran fine for me - but, under heavy stress conditions, such as the child writing a lot of output, the whole thing would lock up. I even had a reproducible case, where it would lock up 100% of the time for a given command line tool with a certain set of arguments. We debugged our hearts out, trying every which way we could to keep things moving on, but we kept coming up short.
I can't say with 100% confidence, only 99%, that the issue was not mine, but Microsoft's. If you try to spawn processes that [may] write to both stdout and stderr (we'll ignore reading from stdin for now), you may find yourself in a world of hurt if one of those buffers fills up. This was a few months ago, and I'm having a hard time remembering the exact details, but it may have been related to the asynchronous call backs for the redirected stdout/stderr being called on the main thread, and, in my case, I was using threadpools for tasks that spawned the processes. My main thread of my application would wait until it got some sort of event from a worker thread that it was done, or there was an error. But, with Microsoft trying to invoke the callback on the main thread for the IO redirection, it never would call to process the buffer, which left the child process waiting for the buffer to clear, and my application's main thread waiting on the worker thread to finish...deadlock.
All I wanted to do was spawn a process and capture all of its stdout and all of its stderr. I did not have to interact with it via stdin at all. You would think this is a simple request. If I only wanted one or the other, I could have just used p.StandardOutput.ReadToEnd(); and just executed the process synchronously. You do not have that luxury if you want both stdout and stderr - you must use asynchronous reads, with the callbacks coming only on the main thread. Throw my need for threadpools into the mix and you find things don't work out so well.
All is not lost, I found a solution. The solution is to not use System.Diagnostics.Process and use raw Win32 API to get the same results. I could of just wrote a C/C++ DLL, or a C++/CLI assembly, but I hate having a mess of DLLs laying around. I wanted a solution that stayed in C#, and so P/Invoke to the rescue. First I got it all working using straight C/C++ and then moved it all to C# using P/Invoke calls to the Win32 API. I'll save you the work, you can find it here:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 | using System; using System.Collections.Generic; using System.Text; using System.Runtime.InteropServices; namespace ProcessAPI { #region WIN32 API class Win32 { [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)] public struct SECURITY_ATTRIBUTES { public Int32 nLength; public IntPtr lpSecurityDescriptor; public bool bInheritHandle; } [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)] public struct STARTUPINFO { public Int32 cb; public string lpReserved; public string lpDesktop; public string lpTitle; public Int32 dwX; public Int32 dwY; public Int32 dwXSize; public Int32 dwYSize; public Int32 dwXCountChars; public Int32 dwYCountChars; public Int32 dwFillAttribute; public Int32 dwFlags; public Int16 wShowWindow; public Int16 cbReserved2; public IntPtr lpReserved2; public IntPtr hStdInput; public IntPtr hStdOutput; public IntPtr hStdError; } [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)] public struct PROCESS_INFORMATION { public IntPtr hProcess; public IntPtr hThread; public Int32 dwProcessId; public Int32 dwThreadId; } [Flags] public enum DuplicateOptions : uint { DUPLICATE_CLOSE_SOURCE = (0x00000001),// Closes the source handle. This occurs regardless of any error status returned. DUPLICATE_SAME_ACCESS = (0x00000002), //Ignores the dwDesiredAccess parameter. The duplicate handle has the same access as the source handle. } [DllImport("kernel32.dll")] public static extern Boolean CreatePipe( out IntPtr hReadPipe, out IntPtr hWritePipe, ref SECURITY_ATTRIBUTES lpPipeAttributes, uint nSize); [DllImport("kernel32.dll")] public static extern IntPtr GetCurrentProcess(); [DllImport("kernel32.dll", SetLastError = true)] [return: MarshalAs(UnmanagedType.Bool)] public static extern Boolean DuplicateHandle( IntPtr hSourceProcessHandle, IntPtr hSourceHandle, IntPtr hTargetProcessHandle, out IntPtr lpTargetHandle, UInt32 dwDesiredAccess, [MarshalAs(UnmanagedType.Bool)] bool bInheritHandle, UInt32 dwOptions); [DllImport("kernel32.dll", SetLastError = true, CallingConvention = CallingConvention.Winapi, CharSet = CharSet.Auto)] public static extern IntPtr CreateEvent(IntPtr lpEventAttributes, [In, MarshalAs(UnmanagedType.Bool)] Boolean bManualReset, [In, MarshalAs(UnmanagedType.Bool)] Boolean bIntialState, [In, MarshalAs(UnmanagedType.BStr)] string lpName); [DllImport("kernel32.dll")] public static extern void SetLastError(Int32 dwErrCode); [DllImport("kernel32.dll")] public static extern bool SetEvent(IntPtr hEvent); [DllImport("kernel32.dll")] public static extern Boolean WriteFile( IntPtr hFile, Byte[] lpBuffer, UInt32 nNumberOfBytesToWrite, out UInt32 lpNumberOfBytesWritten, IntPtr lpOverlapped); [DllImport("kernel32.dll", SetLastError = true)] public static extern bool ReadFile( IntPtr hFile, [Out] Byte[] lpBuffer, UInt32 nNumberOfBytesToRead, out UInt32 lpNumberOfBytesRead, IntPtr lpOverlapped); [DllImport("kernel32.dll")] public static extern bool CreateProcess( string lpApplicationName, string lpCommandLine, IntPtr lpProcessAttributes, IntPtr lpThreadAttributes, Boolean bInheritHandles, UInt32 dwCreationFlags, IntPtr lpEnvironment, string lpCurrentDirectory, [In] ref STARTUPINFO lpStartupInfo, out PROCESS_INFORMATION lpProcessInformation); [DllImport("kernel32.dll", SetLastError = true)] [return: MarshalAs(UnmanagedType.Bool)] public static extern bool TerminateProcess(IntPtr hProcess, UInt32 uExitCode); [DllImport("kernel32.dll", SetLastError = true, CallingConvention = CallingConvention.Winapi, CharSet = CharSet.Auto)] [return: MarshalAs(UnmanagedType.Bool)] public static extern bool CloseHandle(IntPtr hObject); [DllImport("kernel32.dll", EntryPoint = "PeekNamedPipe", SetLastError = true)] public static extern bool PeekNamedPipe( IntPtr handle, Byte[] buffer, UInt32 nBufferSize, ref UInt32 bytesRead, ref UInt32 bytesAvail, ref UInt32 BytesLeftThisMessage); [DllImport("kernel32.dll", EntryPoint = "WaitForMultipleObjects", SetLastError = true)] public static extern int WaitForMultipleObjects( UInt32 nCount, IntPtr[] lpHandles, Boolean fWaitAll, UInt32 dwMilliseconds); [DllImport("kernel32.dll", SetLastError = true)] [return: MarshalAs(UnmanagedType.Bool)] public static extern Boolean GetExitCodeProcess(IntPtr hProcess, out Int32 lpExitCode); } #endregion public class ProcessRedirector : IDisposable { #region Internal Members private System.Threading.Thread m_hThread; // thread to receive the output of the child process private IntPtr m_hEvtStop; // event to notify the redir thread to exit private Int32 m_dwThreadId; // id of the redir thread private Int32 m_resultCode; // returned result code of the process private UInt32 m_dwWaitTime; // wait time to check the status of the child process protected IntPtr m_hStdinWrite; // write end of child's stdin pipe protected IntPtr m_hStdoutRead; // read end of child's stdout pipe protected IntPtr m_hStderrRead; // read end of child's stderr pipe protected IntPtr m_hChildProcess; #endregion #region Constructor public ProcessRedirector() { m_hStdinWrite = IntPtr.Zero; m_hStdoutRead = IntPtr.Zero; m_hStderrRead = IntPtr.Zero; m_hChildProcess = IntPtr.Zero; m_hThread = null; m_hEvtStop = IntPtr.Zero; m_dwThreadId = 0; m_dwWaitTime = 100; m_resultCode = -1; } #endregion #region Finalizer // NOTE: Leave out the finalizer altogether if this class doesn't // own unmanaged resources itself, but leave the other methods // exactly as they are. ~ProcessRedirector() { // Finalizer calls Dispose(false) Dispose(false); } #endregion #region Dispose public void Dispose() { Dispose(true); GC.SuppressFinalize(this); } // The bulk of the clean-up code is implemented in Dispose(bool) protected virtual void Dispose(bool disposing) { if (disposing) { // free managed resources } // free native resources if there are any. Close(true); } #endregion #region Property: ResultCode /// <summary> /// Access the return code from the spawned process /// </summary> public Int32 ResultCode { get { return m_resultCode; } } #endregion #region protected LaunchChild protected bool LaunchChild(string pathToApp, string workingDirectory, string args, IntPtr hStdOut, IntPtr hStdIn, IntPtr hStdErr) { const Int16 SW_HIDE = 0; const Int32 STARTF_USESTDHANDLES = 0x100; const Int32 STARTF_USESHOWWINDOW = 0x1; const uint CREATE_NEW_CONSOLE = 0x00000010; // Set up the start up info struct. Win32.STARTUPINFO si = new Win32.STARTUPINFO(); si.cb = Marshal.SizeOf(si); si.hStdOutput = hStdOut; si.hStdInput = hStdIn; si.hStdError = hStdErr; si.wShowWindow = SW_HIDE; si.dwFlags = STARTF_USESTDHANDLES | STARTF_USESHOWWINDOW; // Note that dwFlags must include STARTF_USESHOWWINDOW if we // use the wShowWindow flags. This also assumes that the // CreateProcess() call will use CREATE_NEW_CONSOLE. Win32.PROCESS_INFORMATION pi = new Win32.PROCESS_INFORMATION(); // Launch the child process. if (!Win32.CreateProcess(pathToApp, args, IntPtr.Zero, IntPtr.Zero, true, CREATE_NEW_CONSOLE, IntPtr.Zero, workingDirectory, ref si, out pi)) return false; m_hChildProcess = pi.hProcess; // Close any non-useful handles Win32.CloseHandle(pi.hThread); return true; } #endregion #region protected DestroyHandle protected void DestroyHandle(ref IntPtr rhObject) { if (rhObject == IntPtr.Zero) return; Win32.CloseHandle(rhObject); rhObject = IntPtr.Zero; } #endregion #region private InternalClose private void InternalClose(bool getExitCode) { if (getExitCode && m_hChildProcess != IntPtr.Zero) { // Snag the exit code before it's gone if (!Win32.GetExitCodeProcess(m_hChildProcess, out m_resultCode)) { m_resultCode = -1; } } DestroyHandle(ref m_hEvtStop); DestroyHandle(ref m_hChildProcess); DestroyHandle(ref m_hStdinWrite); DestroyHandle(ref m_hStdoutRead); DestroyHandle(ref m_hStderrRead); m_dwThreadId = 0; } #endregion #region Stdout/Stderr redirection processing delegate void RedirectDelegate(string msg); int InternalRedirect(IntPtr hPipeRead, RedirectDelegate del) { const int ERROR_BROKEN_PIPE = 109; const int ERROR_NO_DATA = 232; for (; ; ) { uint bytesRead = 0; uint dwAvail = 0; uint bytesLeft = 0; if (!Win32.PeekNamedPipe(hPipeRead, null, 0, ref bytesRead, ref dwAvail, ref bytesLeft)) break; // error if (dwAvail == 0) { // no data available return 1; } byte[] szOutput = new byte[dwAvail]; if (!Win32.ReadFile(hPipeRead, szOutput, dwAvail, out bytesRead, IntPtr.Zero) || bytesRead == 0) break; // error, the child might have ended del(ASCIIEncoding.ASCII.GetString(szOutput, 0, (int)bytesRead)); } int dwError = Marshal.GetLastWin32Error(); if (dwError == ERROR_BROKEN_PIPE || // pipe has been ended dwError == ERROR_NO_DATA) // pipe closing in progress { return 0; // child process ended } WriteStdError("Read stdout pipe error\r\n"); return -1; // os error } // redirect the child process's stdout: // return: 1: no more data, 0: child terminated, -1: os error protected int RedirectStdout() { return InternalRedirect(m_hStdoutRead, new RedirectDelegate(delegate(string msg) { WriteStdOut(msg); })); } // redirect the child process's stderr: // return: 1: no more data, 0: child terminated, -1: os error protected int RedirectStderr() { return InternalRedirect(m_hStderrRead, new RedirectDelegate(delegate(string msg) { WriteStdError(msg); })); } protected void OutputThread() { const int WAIT_OBJECT_0 = 0; IntPtr[] aHandles = new IntPtr[2]; aHandles[0] = m_hChildProcess; aHandles[1] = m_hEvtStop; bool exitNormally = false; for (; ; ) { // redirect stdout till there's no more data. int nRet; nRet = RedirectStdout(); if (nRet < 0) break; nRet = RedirectStderr(); if (nRet < 0) break; // check if the child process has terminated. int dwRc = Win32.WaitForMultipleObjects(2, aHandles, false, m_dwWaitTime); if (WAIT_OBJECT_0 == dwRc) // the child process ended { RedirectStdout(); RedirectStderr(); exitNormally = true; break; } if (WAIT_OBJECT_0 + 1 == dwRc) // m_hEvtStop was signalled { Win32.TerminateProcess(m_hChildProcess, 0xFFFFFFFF); break; } } // close handles InternalClose(exitNormally); } #endregion #region virtual WriteStdOut /// <summary> /// Override to handle processing data written to stdout by the process /// </summary> /// <param name="outputStr"></param> protected virtual void WriteStdOut(string outputStr) { Console.Out.Write(outputStr); } #endregion #region virtual WriteStdError /// <summary> /// Override to handle processing data written to stderr by the process /// </summary> /// <param name="errorStr"></param> protected virtual void WriteStdError(string errorStr) { Console.Error.Write(errorStr); } #endregion #region Open /// <summary> /// Initialize and spawn a process /// </summary> /// <param name="pathToApp">Absolute path to the executable to spawn</param> /// <param name="workingDirectory">The working directory for the process</param> /// <param name="args">The full set of command line arguments</param> /// <returns>True if everything went okay</returns> public bool Open(string pathToApp, string workingDirectory, string args) { Close(true); IntPtr hStdoutReadTmp = IntPtr.Zero; // parent stdout read handle IntPtr hStderrReadTmp = IntPtr.Zero; // parent stderr read handle IntPtr hStdoutWrite = IntPtr.Zero; // child stdout write handle IntPtr hStderrWrite = IntPtr.Zero; IntPtr hStdinWriteTmp = IntPtr.Zero; // parent stdin write handle IntPtr hStdinRead = IntPtr.Zero; // child stdin read handle // Set up the security attributes struct. Win32.SECURITY_ATTRIBUTES sa = new Win32.SECURITY_ATTRIBUTES(); sa.nLength = Marshal.SizeOf(sa); sa.lpSecurityDescriptor = IntPtr.Zero; sa.bInheritHandle = true; m_resultCode = -1; bool setupOk = false; try { // Create a child stdout pipe. if (!Win32.CreatePipe(out hStdoutReadTmp, out hStdoutWrite, ref sa, 0)) return false; // Create a child stderr pipe. if (!Win32.CreatePipe(out hStderrReadTmp, out hStderrWrite, ref sa, 0)) return false; // Create a child stdin pipe. if (!Win32.CreatePipe(out hStdinRead, out hStdinWriteTmp, ref sa, 0)) return false; // Create new stdout read handle, stderr read handle and the stdin write handle. // Set the inheritance properties to FALSE. Otherwise, the child // inherits the these handles; resulting in non-closeable // handles to the pipes being created. if (!Win32.DuplicateHandle(Win32.GetCurrentProcess(), hStdoutReadTmp, Win32.GetCurrentProcess(), out m_hStdoutRead, 0, false, (uint)Win32.DuplicateOptions.DUPLICATE_SAME_ACCESS)) return false; if (!Win32.DuplicateHandle(Win32.GetCurrentProcess(), hStderrReadTmp, Win32.GetCurrentProcess(), out m_hStderrRead, 0, false, (uint)Win32.DuplicateOptions.DUPLICATE_SAME_ACCESS)) return false; if (!Win32.DuplicateHandle(Win32.GetCurrentProcess(), hStdinWriteTmp, Win32.GetCurrentProcess(), out m_hStdinWrite, 0, false, (uint)Win32.DuplicateOptions.DUPLICATE_SAME_ACCESS)) return false; // Close inheritable copies of the handles we do not want to be inherited. DestroyHandle(ref hStdoutReadTmp); DestroyHandle(ref hStderrReadTmp); DestroyHandle(ref hStdinWriteTmp); // launch the child process string appFilename = System.IO.Path.GetFileName(pathToApp); string cmdArgs = "\"" + appFilename + "\" " + args; if (!LaunchChild(pathToApp, workingDirectory, cmdArgs, hStdoutWrite, hStdinRead, hStderrWrite)) return false; // Child is launched. Close the parents copy of those pipe // handles that only the child should have open. // Make sure that no handles to the write end of the stdout pipe // are maintained in this process or else the pipe will not // close when the child process exits and ReadFile will hang. DestroyHandle(ref hStdoutWrite); DestroyHandle(ref hStdinRead); DestroyHandle(ref hStderrWrite); // Launch a thread to receive output from the child process. m_hEvtStop = Win32.CreateEvent(IntPtr.Zero, true, false, null); try { m_hThread = new System.Threading.Thread(new System.Threading.ThreadStart(OutputThread)); m_hThread.Name = "StdOutErr Processor " + appFilename; } catch { return false; } m_hThread.Start(); m_dwThreadId = m_hThread.ManagedThreadId; setupOk = true; } finally { if (!setupOk) { Int32 dwOsErr = Marshal.GetLastWin32Error(); if (dwOsErr != 0) { WriteStdError("Redirect console error: " + dwOsErr.ToString("x8") + "\r\n"); } DestroyHandle(ref hStdoutReadTmp); DestroyHandle(ref hStderrReadTmp); DestroyHandle(ref hStdoutWrite); DestroyHandle(ref hStderrWrite); DestroyHandle(ref hStdinWriteTmp); DestroyHandle(ref hStdinRead); Close(true); Win32.SetLastError(dwOsErr); } } return true; } #endregion #region Close /// <summary> /// Close a process /// </summary> /// <param name="abort">If true, abort the processing (waiting up to /// 5 seconds before terminating), otherwise just wait until it exits</param> public virtual void Close(bool abort) { if (m_hThread != null) { if (abort) { // tell the thread to bail Win32.SetEvent(m_hEvtStop); if (!m_hThread.Join(5000)) { try { m_hThread.Abort(); } catch { } } } else { // wait until the thread exits m_hThread.Join(); } m_hThread = null; } InternalClose(false); } #endregion #region SendToStdIn /// <summary> /// Send the given string of data to the stdin of the spawned process /// </summary> /// <param name="str"></param> /// <returns></returns> public bool SendToStdIn(string str) { if (m_hStdinWrite == IntPtr.Zero) return false; byte[] strData = ASCIIEncoding.ASCII.GetBytes(str); uint dwWritten; return Win32.WriteFile(m_hStdinWrite, strData, (uint)strData.Length, out dwWritten, IntPtr.Zero); } #endregion #region SetWaitTime /// <summary> /// Adjust the waiting time between checks to see if the process /// has exited, this is also the time between checks to stdout and /// stderr processing. /// </summary> /// <param name="waitTime"></param> public void SetWaitTime(UInt32 waitTime) { m_dwWaitTime = waitTime; } #endregion } } |
Game Loops and Frametime
Kwasi Mensah wrote a blog on #altdevblogaday entitled Game Loops on IOS that I skimmed through. I'm currently not dealing with game loops, frametime, nor IOS right now, so this post is just a little reminder for my future self should I find myself tumbling down that wormhole.
Glenn Fiedler's article Fix Your Timestep is the oft-referenced article on the topic for good reason, and is always worth another read through.
The summary for Kwasi's article:
- He has a function that gets called when there was a vsync
- If a vsync happens while he is in the function, the function will get called again immediately after it exits
- Most of the time you should be hitting 60 fps (or 30 fps) and occasionally your frametime might go over
- What you don't want is to spiral out of control if you occasionally miss a frame. If you are consistently missing frames than you should drop your goal frametime to the next level (60 -> 30)
- Start by timing how long it took to run the "DoFrame" function, call this time elapsed.
- If elapsed took longer than your goal frametime, then change the value of elapsed to your goal frametime plus how much you overran the vsync: fmod(elapsed,frametime)
- Store elapsed in a persistent variable, called bank, for the next call to "DoFrame".
- At the start of "DoFrame" subtract your goal frametime from the persistent bank variable.
- If bank is higher than zero, that means your last processing through "DoFrame" lasted longer than your goal frametime, and therefore, you missed a vsync (or more). In this case he suggests just forgoing the processing of "DoFrame" for this frame and just wait until the next vsync (the article explains why). So exit out of "DoFrame" until next time.
- Otherwise bank is less than, or equal to, zero. Which means that the last processing through "DoFrame" was quicker than your goal frametime (or, just as long). Reset bank to zero because we are starting a new, and do the processing of "DoFrame".
So, occasionally you'll have to eat some time to wait for the next vsync should you miss a goal vsync. But the idea is, if you don't, another vsync will come along, and if you miss that (as you are already under a sub-vsync interval of time remaining due to the miss) than you are going to throw off things even more. I guess this will cause a, probably unnoticeable, slow down in time for when you miss a vsync.
I have to give some thought to all of this, but until then...