Why Does Using Goroutines Cause 'Multiple response.WriteHeader Calls' in Go's HTTP Handlers?

Multiple WriteHeader Calls in Go's HTTP Handler

In a simple Go program featuring a net/http server, a developer encounters the "multiple response.WriteHeader calls" error when attempting to handle HTTP requests. The error occurs when the program incorporates a goroutine to execute the HandleIndex function, despite the fact that the handler function is designed to write a header once and provide a static body.

Explanation of the Error

The anonymous function responsible for handling incoming requests prints the URL, initiates the HandleIndex function in a new goroutine, and proceeds with execution. When a handler function omits setting the response status before the initial call to Write, Go automatically sets the status to 200 (HTTP OK). Additionally, if the handler function does not write any content to the response (and doesn't set the response status and completes normally), this is treated as successful request handling, resulting in a response status of 200.

In this case, the anonymous function does not set the response status and does not write anything to the response. Therefore, Go sets the response status to 200 HTTP OK. Since each request is handled in its own goroutine, if HandleIndex is called in a new goroutine, the original anonymous function will continue execution, including setting the response header. Concurrently, the newly started goroutine will also set the response header, causing the "multiple response.WriteHeader calls" error.

Solution: Removing the Goroutine

By eliminating the "go" keyword, HandleIndex will be invoked in the same goroutine as the handler function, ensuring that the response header is set before the handler function returns. This prevents the "net/http" package from attempting to set the response header again, resolving the error.

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