Why doesn\'t `json.NewDecoder().Decode()` respect HTTP Context Deadlines?

HTTP Context Deadline Disregarded by json.NewDecoder().Decode()

Question:

When using context.WithTimeout to establish a time limit for an HTTP request, the ioutil.ReadAll method interrupts the request and returns an appropriate error when the timeout is exceeded. However, json.NewDecoder(resp.Body).Decode appears to disregard the deadline, returning a nil error instead of context.DeadlineExceeded. Is this a bug?

Answer:

No, this is not a bug. The net/http package utilizes buffers for processing requests, resulting in the potential for partial or full reading of the response body into buffers before the program reads it. Consequently, an expiring context does not necessarily prevent the program from completing the read operation.

Example:

Consider a modified HTTP test server that intentionally delays responses. When using readDoesntFail, the server sends a partial response (10 bytes), flushes it, and waits 6 seconds before sending the remainder.

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()

In this scenario, json.Decoder.Decode() attempts to read from the connection because the data is not yet buffered. Once the context expires, further reading from the connection triggers a context.DeadlineExceeded error.

Conclusion:

json.Decoder.Decode() will respect the context deadline when data is not yet buffered. However, if data is partially or fully buffered, the context deadline may not prevent the program from completing the read operation. It is recommended to use ioutil.ReadAll or a similar approach for more consistent behavior with context deadlines.

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