Why Doesn't Go's HTTP Context Detect Cancellation Signals with Request Bodies?

Go HTTP Context Failing to Detect Cancellation Signal with Request Body

In Go's HTTP server, context signals can provide a mechanism to handle client disconnections, allowing the server to clean up resources promptly. However, when a request includes a body, the context's Done channel stalls, preventing the server from detecting the client's departure.

Cause of Behavior

The reason for this behavior lies in how the HTTP server reads the connection. Until the request body is read, no checks for closed connections are performed. In the case of GET requests, no body is present, so the server actively monitors the connection. However, with POST requests, the body occupies a non-blocking reader, delaying the connection check.

Solution

To resolve this issue, explicitly read the request body within the handler function. This action triggers the server's connection monitoring, ensuring that it can detect client disconnections promptly.

Code Example

func handler(w http.ResponseWriter, r *http.Request) {
    go func(done <-chan struct{}) {
        <-done
        fmt.Println("message", "client connection has gone away, request got cancelled")
    }(r.Context().Done())

    // Explicitly read the body to trigger connection monitoring
    io.Copy(ioutil.Discard, r.Body)

    time.Sleep(30 * time.Second)
    fmt.Fprintf(w, "Hi there, I love %s!\n", r.URL.Path[1:])
}

This modification prompts the server to read the request body immediately, initiating the connection monitoring process. Consequently, the server can now effectively detect client disconnections, even in the presence of a request body.

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