How Can I Inspect an HTTP Request Body Without Modifying the Request State?

Inspecting HTTP Request Body and Maintaining Request State

In HTTP handling implementations, inspecting request bodies is essential for processing certain requests, such as POST requests with form-encoded data. However, direct inspection using methods like req.ParseForm() can disrupt the request state, leading to errors when forwarding the request to reverse proxies.

To address this issue, it's possible to peek at the request body without leaving any trace in the original request object. This maintains the original state for subsequent consumers, such as reverse proxies.

Implementation

One effective approach involves reading the request body into an intermediate buffer and using that buffer to create multiple readers. The code snippet below demonstrates this:

// Read the request body into a buffer
buf, _ := io.ReadAll(r.Body)

// Create two readers from the buffer
rdr1 := io.NopCloser(bytes.NewBuffer(buf))
rdr2 := io.NopCloser(bytes.NewBuffer(buf))

// Perform desired operations on the first reader
doStuff(rdr1)

// Reset the request body with the second reader
r.Body = rdr2

In this code, the bytes.NewBuffer(buf) creates readers that wrap the stored buffer. The io.NopCloser ensures that the Close() method doesn't return an error, which is necessary for the reader to implement the io.ReadCloser interface.

Usage

The code snippet modifies the example provided in the question, where doStuff() is the function that processes the request body:

buf, _ := io.ReadAll(r.Body)
rdr1 := io.NopCloser(bytes.NewBuffer(buf))
rdr2 := io.NopCloser(bytes.NewBuffer(buf))

doStuff(rdr1)
r.Body = rdr2

With this modification, the request body can be inspected without affecting the original request state. The reverse proxy can then proceed to handle the request with the unmodified body.

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