Preserving Backtrace When Wrapping Errors for Rollbar Reporting

When dealing with error handling in Ruby, especially in the context of integrating with error monitoring tools like Rollbar, it’s common to encounter scenarios where you need to wrap an existing error in a custom error class. However, a common issue arises when you do this: the original backtrace can be lost, which makes it harder to pinpoint where the error initially occurred.

Let’s walk through an example to illustrate this issue and discuss how to solve it.

Step 1: Defining a Custom Error Class

Imagine you start by creating a custom error class to encapsulate errors that might occur in your application. Here’s how you would typically define such a class:

a, b = nil
CustomError = Class.new(StandardError)

This custom class, CustomError, is a subclass of StandardError and is intended to wrap any errors that occur, providing more context or custom behavior. The challenge arises when you want to wrap an existing error with this class but still retain the original error’s backtrace. We also instantiate a and b variables to have them at hand later on.

Step 2: Simulating an Error and Wrapping It

Now, let’s simulate an error and try to wrap it in our CustomError class:

begin
  # Code that causes an error
  1 / 0
rescue StandardError => e
  # Assign the original error to 'a' and wrap it in CustomError
  a = e
  raise(b = CustomError.new(e))
end

In this example, dividing by zero (1 / 0) will raise a ZeroDivisionError, which is a subclass of StandardError. The error is caught in the rescue block, where we then wrap it in our CustomError class and re-raise it. However, there’s a problem with this approach: the backtrace that gets attached to b (the CustomError instance) will not reflect where the original error occurred.

Step 3: Inspecting the Backtrace

After raising the CustomError, let’s inspect the backtraces of both the original error (a) and the wrapped error (b):

a.backtrace
# => ["(irb):3:in `/'", "(irb):2:in `<top (required)>'", ...]

b.backtrace
# => ["(irb):4:in `rescue in <top (required)>'", "(irb):2:in `<top (required)>'", ...]

Notice the difference in backtraces:

a.backtrace: This shows the original backtrace, pointing directly to where the division by zero occurred.
b.backtrace: This backtrace points to where the CustomError was raised, which is not where the actual error happened.

This discrepancy can lead to confusion during debugging, as the CustomError backtrace doesn’t provide a clear path to the root cause of the error.

Step 4: Preserving the Original Backtrace

To address this issue, you can manually set the backtrace of the CustomError to match the original error’s backtrace:

b.set_backtrace(a.backtrace)

After calling set_backtrace, the CustomError instance (b) will now have the same backtrace as the original error (a):

b.backtrace
# => ["(irb):3:in `/'", "(irb):2:in `<top (required)>'", ...]

This solution ensures that the backtrace in the custom error accurately reflects where the error originally occurred. However, manually setting the backtrace every time you wrap an error can be tedious and error-prone.

Step 5: Automating Backtrace Preservation

To streamline this process, you can create a CustomError class that automates the backtrace preservation when wrapping errors. Here’s how you can implement this:

class CustomError < StandardError
  def initialize(err, context)
    super(err).set_backtrace(err.backtrace)
    @rollbar_context = context
  end
end

Now, instead of manually setting the backtrace, you can simply use the CustomError class:

begin
  # Code that causes an error
  1 / 0
rescue StandardError => e
  # Wrap and raise the custom error with preserved backtrace
  raise CustomError.new(e)
end

This approach ensures that every time you wrap an error in your custom class, the original backtrace is automatically preserved. This makes your error handling more reliable and your debugging process more straightforward, especially when reporting errors to Rollbar. And it can also be adjusted to some other reporting tool.

Conclusion

When wrapping errors in custom classes in Ruby, it’s essential to preserve the original backtrace to maintain clarity in your error reports. By using a custom error wrapper class, you can automate this process and ensure that your errors are always correctly traced back to their origin, making tools like Rollbar more effective in helping you debug and maintain your application.