Fixing Lost Timeouts In Nested Application.Run() Calls
Have you ever encountered a situation where timeouts scheduled using IApplication.AddTimeout() fail to trigger correctly when a nested modal dialog is displayed using Application.Run()? This issue can lead to timeouts being lost, especially when using MessageBox or other dialogs. In this article, we'll dive deep into this problem, understand its root cause, and explore a robust solution.
Understanding the Problem: Timeouts in Nested Run Loops
The core issue lies in how timeouts are managed within nested Application.Run() calls. To illustrate this, let's consider a scenario where you schedule a timeout in your main application loop, and then, within that loop, you display a modal dialog using Application.Run(). Ideally, the timeout should still fire even when the modal dialog is active. However, due to the way TimedEvents.RunTimersImpl() is implemented, this doesn't always happen.
Timeouts are essential for creating responsive and well-behaved applications. They allow you to perform actions after a certain period, such as closing a dialog, updating the UI, or even preventing an application from hanging indefinitely. When timeouts are lost, it can lead to unexpected behavior and a poor user experience. Therefore, understanding and addressing this issue is crucial for building robust applications.
Reproducing the Issue: Minimal Code Examples
To better grasp the problem, let's examine a couple of code examples that highlight the issue.
[Fact]
public void Timeout_Fires_With_Single_Session ()
{
// Arrange
using IApplication? app = Application.Create (example: false);
app.Init ("FakeDriver");
// Create a simple window for the main run loop
var mainWindow = new Window { Title = "Main Window" };
// Schedule a timeout that will ensure the app quits
var requestStopTimeoutFired = false;
app.AddTimeout (
TimeSpan.FromMilliseconds (100),
() =>
{
output.WriteLine ({{content}}quot;RequestStop Timeout fired!");
requestStopTimeoutFired = true;
app.RequestStop ();
return false;
}
);
// Act - Start the main run loop
app.Run (mainWindow);
// Assert
Assert.True (requestStopTimeoutFired, "RequestStop Timeout should have fired");
mainWindow.Dispose ();
}
In the Timeout_Fires_With_Single_Session test, a timeout is scheduled to fire after 100 milliseconds, which then triggers app.RequestStop(). This test passes successfully because it operates within a single application run loop.
However, the next example demonstrates the problem:
[Fact]
public void Timeout_Fires_In_Nested_Run ()
{
// Arrange
using IApplication? app = Application.Create (example: false);
app.Init ("FakeDriver");
var timeoutFired = false;
var nestedRunStarted = false;
var nestedRunEnded = false;
// Create a simple window for the main run loop
var mainWindow = new Window { Title = "Main Window" };
// Create a dialog for the nested run loop
var dialog = new Dialog { Title = "Nested Dialog", Buttons = [new Button { Text = "Ok" }] };
// Schedule a safety timeout that will ensure the app quits if test hangs
var requestStopTimeoutFired = false;
app.AddTimeout (
TimeSpan.FromMilliseconds (5000),
() =>
{
output.WriteLine ({{content}}quot;SAFETY: RequestStop Timeout fired - test took too long!");
requestStopTimeoutFired = true;
app.RequestStop ();
return false;
}
);
// Schedule a timeout that will fire AFTER the nested run starts and stop the dialog
app.AddTimeout (
TimeSpan.FromMilliseconds (200),
() =>
{
output.WriteLine ({{content}}quot;DialogRequestStop Timeout fired! TopRunnable: {app.TopRunnableView?.Title ?? "null"}");
timeoutFired = true;
// Close the dialog when timeout fires
if (app.TopRunnableView == dialog)
{
app.RequestStop (dialog);
}
return false;
}
);
// After 100ms, start the nested run loop
app.AddTimeout (
TimeSpan.FromMilliseconds (100),
() =>
{
output.WriteLine ("Starting nested run...");
nestedRunStarted = true;
// This blocks until the dialog is closed (by the timeout at 200ms)
app.Run (dialog);
output.WriteLine ("Nested run ended");
nestedRunEnded = true;
// Stop the main window after nested run completes
app.RequestStop ();
return false;
}
);
// Act - Start the main run loop
app.Run (mainWindow);
// Assert
Assert.True (nestedRunStarted, "Nested run should have started");
Assert.True (timeoutFired, "Timeout should have fired during nested run");
Assert.True (nestedRunEnded, "Nested run should have ended");
Assert.False (requestStopTimeoutFired, "Safety timeout should NOT have fired");
dialog.Dispose ();
mainWindow.Dispose ();
}
In Timeout_Fires_In_Nested_Run, we introduce a nested Application.Run() call to display a dialog. A timeout is scheduled to close this dialog after 200 milliseconds. However, this test often hangs and fails because the timeout within the nested run loop is not correctly triggered. This highlights the core problem we're addressing.
Root Cause Analysis: Diving into TimedEvents.RunTimersImpl()
To understand why timeouts are lost in nested Application.Run() calls, we need to examine the implementation of TimedEvents.RunTimersImpl(). This method is responsible for managing and executing timeouts within the application.
Here’s the problematic code snippet:
private void RunTimersImpl()
{
long now = GetTimestampTicks();
SortedList<long, Timeout> copy;
lock (_timeoutsLockToken)
{
copy = _timeouts; // ← Copy ALL timeouts
_timeouts = new(); // ← Clear the queue
}
foreach ((long k, Timeout timeout) in copy)
{
if (k < now)
{
if (timeout.Callback!()) // ← This can block for a long time
{
AddTimeout(timeout.Span, timeout);
}
}
else
{
lock (_timeoutsLockToken)
{
_timeouts.Add(NudgeToUniqueKey(k), timeout);
}
}
}
}
Let's break down the issues step by step:
- All timeouts are removed from the queue: At the beginning of
RunTimersImpl(), all scheduled timeouts are copied from the_timeoutsqueue into a local variablecopy, and then the original queue is cleared. This means the main timeout queue is emptied before any timeouts are processed. - Callbacks are executed sequentially: The method then iterates through the
copylist and executes each timeout callback in aforeachloop. This sequential execution is a key bottleneck. - Blocking Callbacks: The crucial issue arises when a callback blocks, such as when
app.Run(dialog)is called to display a modal dialog. This call blocks the execution of the entireRunTimersImpl()method. - Timeouts Stuck in Local Variable: While the callback is blocking, any future timeouts remain stuck in the local
copyvariable. They are inaccessible to the nested run loop because the nested run loop has its own instance ofTimedEvents. - Empty Timeout Queue in Nested Run: Consequently, the nested dialog's
RunTimers()calls see an empty timeout queue. Timeouts scheduled before the nested run never fire during the nested run, leading to the lost timeout issue. - Stale Time: Additionally,
now = GetTimestampTicks()is captured only once at the start. If a callback takes a long time,nowbecomes stale, and the time evaluationk < nowuses outdated information.
In essence, the problem stems from the method's design of batch-processing timeouts and the blocking nature of certain callbacks. This design flaw prevents timeouts from being processed correctly in nested Application.Run() scenarios.
The Solution: A More Efficient RunTimersImpl()
To address this issue, we need to rewrite TimedEvents.RunTimersImpl() to process timeouts one at a time instead of batching them. This approach ensures that timeouts are not blocked by long-running callbacks and that nested run loops can access the timeout queue.
Here’s the proposed solution:
private void RunTimersImpl()
{
long now = GetTimestampTicks();
// Process due timeouts one at a time, without blocking the entire queue
while (true)
{
Timeout? timeoutToExecute = null;
long scheduledTime = 0;
// Find the next due timeout
lock (_timeoutsLockToken)
{
if (_timeouts.Count == 0)
{
break; // No more timeouts
}
// Re-evaluate current time for each iteration
now = GetTimestampTicks();
// Check if the earliest timeout is due
scheduledTime = _timeouts.Keys[0];
if (scheduledTime >= now)
{
// Earliest timeout is not yet due, we're done
break;
}
// This timeout is due - remove it from the queue
timeoutToExecute = _timeouts.Values[0];
_timeouts.RemoveAt(0);
}
// Execute the callback outside the lock
// This allows nested Run() calls to access the timeout queue
if (timeoutToExecute != null)
{
bool repeat = timeoutToExecute.Callback!();
if (repeat)
{
AddTimeout(timeoutToExecute.Span, timeoutToExecute);
}
}
}
}
Key Improvements Explained
- Lock → Check → Remove → Unlock → Execute Pattern: This pattern is crucial for ensuring thread safety while minimizing the time the lock is held. We first acquire the lock, check if there are any timeouts due, remove the earliest one, and then release the lock before executing the callback.
- Process One Timeout at a Time: Instead of copying all timeouts and processing them in a batch, we now process only one timeout that is currently due. This prevents long-running callbacks from blocking the entire queue.
- Execute Callbacks Outside the Lock: The most significant change is executing the timeout callback outside the lock. This allows nested
Run()calls to access the timeout queue and process their timeouts concurrently. - Timeouts Remain in Queue: Future timeouts remain in the
_timeoutsqueue, making them accessible to nestedRun()calls. This ensures that timeouts scheduled before the nested run can still fire during the nested run. - Re-evaluate Current Time: The current time (
now) is re-evaluated on each iteration of the loop. This addresses the issue of stale time, ensuring that timeouts are evaluated against the most current timestamp, even if a callback takes a long time to execute.
By implementing these changes, we ensure that timeouts are processed efficiently and correctly, even in complex scenarios involving nested Application.Run() calls. This leads to a more robust and predictable application behavior.
Conclusion
In this article, we've explored a common issue with timeouts in nested Application.Run() calls, delved into the root cause by examining TimedEvents.RunTimersImpl(), and presented a comprehensive solution. By processing timeouts one at a time and executing callbacks outside the lock, we can ensure that timeouts are handled correctly, even in complex scenarios.
Understanding these intricacies is vital for building robust and responsive applications. By implementing the proposed solution, you can avoid the pitfalls of lost timeouts and create a better user experience.
For more information on related topics, you might find it helpful to explore Microsoft's official documentation on multithreading in C#.
