Does `json.NewDecoder().Decode()` Ignore Context Deadlines in Go HTTP Requests?-Golang-php.cn

Does `json.NewDecoder().Decode()` Ignore Context Deadlines in Go HTTP Requests?

Mary-Kate Olsen

Release： 2024-10-29 04:41:29

Original

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Does `json.NewDecoder().Decode()` Ignore Context Deadlines in Go HTTP Requests?

Go json.NewDecoder().Decode() Disregards Context Deadline?

Problem:

In a Go program with a context deadline, reading a response body using ioutil.ReadAll() results in the expected deadline exceeded error. However, using json.NewDecoder(resp.Body).Decode() returns nil instead.

Code Example:

<code class="go">package main

import (
    "context"
    "encoding/json"
    "fmt"
    "io/ioutil"
    "net/http"
    "time"
)

var url string = "http://ip.jsontest.com/"

func main() {
    readDoesntFail()
    readFails()
}

type IpResponse struct {
    Ip string
}

func readDoesntFail() {
    ctx, _ := context.WithTimeout(context.Background(), time.Second*5)

    req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
    if err != nil {
        panic(err)
    }
    resp, err := http.DefaultClient.Do(req)
    if err != nil {
        panic(err)
    }

    ipResponse := new(IpResponse)
    time.Sleep(time.Second * 6)
    fmt.Println("before reading response body, context error is:", ctx.Err())
    err = json.NewDecoder(resp.Body).Decode(ipResponse)
    if err != nil {
        panic(err)
    }
    fmt.Println("Expected panic but there was none")
}

func readFails() {
    ctx, _ := context.WithTimeout(context.Background(), time.Second*5)
    req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
    if err != nil {
        panic(err)
    }
    resp, err := http.DefaultClient.Do(req)
    if err != nil {
        panic(err)
    }

    time.Sleep(time.Second * 6)
    fmt.Println("before reading response body, context error is:", ctx.Err())
    _, err = ioutil.ReadAll(resp.Body)
    if err != nil {
        fmt.Println("received expected error", err)
    }
}</code>

Copy after login

Answer:

In the net/http package, buffers may be used to process requests. Consequently, the incoming response body may be partially or entirely read and buffered prior to your attempt to read it. As a result, an expiring context may not obstruct you from completing the reading of the body. This is precisely what occurs in this situation.

To better understand, let's alter the code to create a test HTTP server that intentionally postpones the response:

<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>

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The modified example sends a JSON object similar to the response of ip.jsontest.com, but transmits only the first 10 bytes of the body before flushing it. It then suspends the transmission for 6 seconds, giving the client an opportunity to time out.

When we execute readDoesntFail(), we observe the following behavior:

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

Copy after login

In this scenario, json.Decoder.Decode() attempts to read from the connection because the data hasn't been buffered yet. Once the context expires, further reading from the connection results in a deadline exceeded error. However, in the original example, json.Decoder.Decode() is reading already buffered data, rendering the expired context irrelevant.

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