How Can Go Middleware Handle Errors Returned by Request Handlers?

How to Combine Go Middleware Pattern with Error-Returning Request Handlers

The Go middleware pattern is a powerful tool for wrapping and pre-processing HTTP requests. However, traditional middleware handlers cannot return errors, which can complicate error handling.

Error-Returning Request Handlers

To enable handlers to return errors, we can define a custom handler type:

type errorHandler func(http.ResponseWriter, *http.Request) error

This type allows handlers to generate errors instead of using panic or logging internally.

Error Middleware

To handle these errors, we need a special middleware that can accept the error-returning handler type.

func errorHandler(h errorHandler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        err := h(w, r)
        if err != nil {
            // Handle error appropriately (e.g., log, send error response)
        }
    })
}

This middleware wraps the error-returning handler and deals with any errors that it generates.

Usage

To use a middleware chain with error-returning handlers, we simply wrap the final handler with the error middleware and chain it with other middleware.

// Create a middleware chain.
moreMiddleware(myMiddleware(errorHandler(myHandlerReturningError)))

Router Integration

For simplicity, consider creating your own router that supports this pattern and builds the middleware chain automatically.

Example Router

https://github.com/fragmenta/mux/blob/master/mux.go

This example router demonstrates how to build a middleware chain and handle errors efficiently. It showcases the benefits of using a custom handler type for error handling and simplifies middleware usage.

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