How to release http.Client in Go?

http.Client does not require any special way to free "used" resources. When it becomes unreachable, memory used by it will be reclaimed by the garbage collector.

http.Client does not store connection or state information. The documentation even states that http.Client should be reused:

The Client's Transport typically has internal state (cached TCP connections), so Clients should be reused instead of created as needed. Clients are safe for concurrent use by multiple goroutines.

If you build your own client using (e.g. embedding) http.Client and you allocate resources that must be released explicitly, provide your own Close() method on it and document that anyone who uses your own implementation must call Close() if it is not needed anymore.

Note:

What you might confuse it with is that if you use an http.Client to do HTTP operations (like Client.Do(), Client.Get(), Client.Post() etc.), they return a value of *http.Response, and that response does hold a connection, state and other resources, which does need to be freed, typically via Response.Body.Close(). Quoting from the package doc of http:

The client must close the response body when finished with it:

resp, err := http.Get("http://example.com/")
if err != nil {
  // handle error
}
defer resp.Body.Close()
body, err := ioutil.ReadAll(resp.Body)
// ...

It is also documented at Client.Get():

When err is nil, resp always contains a non-nil resp.Body. Caller should close resp.Body when done reading from it.