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Discover how to log stack traces in Go when a `context.Context` is canceled by `http.TimeoutHandler`. This guide provides a step-by-step approach to improve your debugging process and handle timeouts effectively.
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How to Get a Stacktrace When context.Context is Canceled by http.TimeoutHandler

Debugging in web development can be particularly challenging, especially when dealing with cancellations and timeouts in your code. If you're using the Go programming language, understanding how to trace where your http.TimeoutHandler has interrupted a request can be essential for profiling and debugging.

This post tackles a common scenario where you might want more information on exactly where your http.TimeoutHandler wrapped handler canceled an active http.Request::Context(). We'll look into how to log stack traces when context cancellations occur, thereby providing invaluable insights into your application's behavior.

The Problem

While handling HTTP requests in Go, you may encounter situations where a request exceeds the set timeout. By default, nothing is logged when this occurs, leaving you with a 503 Service Unavailable response without any clues about what happened behind the scenes.

Example Setup

Consider the following example where you configure an HTTP server with a timeout:

[[See Video to Reveal this Text or Code Snippet]]

In this setup, your handler (handle_root) calls additional helper functions, and the http.Request context is passed through to SQL database queries. However, if the timeout is exceeded, you need a method to capture essential debugging information.

The Solution

To obtain a stack trace when your context is canceled due to a timeout, you can implement error checking and logging in your code wherever context-sensitive operations occur. Here’s how you can set this up:

Step 1: Implement Error Logging

In your handlers where errors might occur (such as during database queries), you need to log both the error message and a stack trace. This can be done using Go's runtime/debug package.

Here’s a revised version of your error handling:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Catch and Log Specific Errors

When your context is canceled, it usually results in a non-nil error. Capture these errors with specific messages:

context deadline exceeded

pq: canceling statement due to user request

http: Handler timeout

By logging these messages, you can pinpoint what part of your code triggered the context cancellation.

Step 3: Monitor Database Logs

In addition to logging in your application, consider that many database systems, like PostgreSQL, will log canceled statements by default. Check their logs for further insight. An example log entry might look like this:

[[See Video to Reveal this Text or Code Snippet]]

This context helps in understanding what SQL query was running at the time of the timeout.

Conclusion

Getting a stack trace when a context.Context is canceled can greatly improve your debugging and profiling process. By implementing error logging, capturing specific error messages, and monitoring your database logs, you can gain valuable insights into your application's behavior during timeouts. With these strategies, you can ensure that your Go applications are more resilient and easier to maintain.

By following these steps, you'll be better equipped to understand what’s happening under the hood when things don’t go as planned, making your development experience smoother and more efficient.