Why does `json.NewDecoder().Decode()` not return a context deadline error when `ioutil.ReadAll()` does within a context deadline in Go?

Unexpected Behavior of json.NewDecoder().Decode() Within Context Deadline

In Go programs with a context deadline set, the response body read by ioutil.ReadAll() is expected to return an error (context.DeadlineExceeded). However, this behavior isn't observed when reading the response body with json.NewDecoder(resp.Body).Decode(), which returns nil.

Let's delve deeper into the issue:

According to the responses, the net/http package may employ buffers to process requests. This entails that the incoming response body could be partially or entirely read and buffered prior to the time you do it. Therefore, an expiring context might not prevent you from completing the body read.

To illustrate this more clearly, we adjusted an example to launch a test HTTP server that deliberately delays the response partially:

<code class="go">ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    s := []byte(`{"ip":"12.34.56.78"}`)
    w.Write(s[:10])
    if f, ok := w.(http.Flusher); ok {
        f.Flush()
    }
    time.Sleep(time.Second * 6)
    w.Write(s[10:])
}))
defer ts.Close()
url = ts.URL

readDoesntFail()
readFails()</code>

This test server outputs a JSON object resembling ip.jsontest.com's response. In contrast to that, it only delivers 10 body bytes initially, then flushes them, and sleeps for 6 seconds intentionally before transmitting the remainder, giving the client time to expire.

When we execute readDoesntFail() with this server, we get:

before reading response body, context error is: context deadline exceeded
panic: Get "http://127.0.0.1:38230": context deadline exceeded

goroutine 1 [running]:
main.readDoesntFail()
    /tmp/sandbox721114198/prog.go:46 +0x2b4
main.main()
    /tmp/sandbox721114198/prog.go:28 +0x93

Now, with our updated example, json.Decoder.Decode() makes an effort to read from the connection because the information is not yet buffered, allowing the context expiration to prompt an error due to expired context.

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