日期输出从json.Encoder哪里来？

Question

At the end of the JSON and Go blog post you'll find this sample program:

package main

import (
    "encoding/json"
    "log"
    "os"
)

func main() {
    dec := json.NewDecoder(os.Stdin)
    enc := json.NewEncoder(os.Stdout)
    for {
        var v map[string]interface{}
        if err := dec.Decode(&v); err != nil {
            log.Println(err)
            return
        }
        for k := range v {
            if k != "Name" {
                delete(v, k)
            }
        }
        if err := enc.Encode(&v); err != nil {
            log.Println(err)
        }
    }
}

I compiled this with go build json_decoder.go and then ran the program like so in bash:

echo '{"Name":"Wednesday","Age":6,"Parents":["Gomez","Morticia"]}' | json_decoder

and received this output:

{"Name":"Wednesday"}
2019/08/17 22:09:20 EOF

The first line of output is exactly what I'd expect. But where is the line 2019/08/17 22:09:20 EOF coming from?

Answer 1

When EOF is reached the decoder returns io.EOF which is then is being output by the logger log.Println(err) with a timestamp prepended.

You can check for EOF when decoding like this for example

if err := dec.Decode(&v); err != nil {
    if err != io.EOF {
        log.Println(err)
    }
    return
}

Output:

➜ echo '{"Name":"Wednesday","Age":6,"Parents":["Gomez","Morticia"]}' | ./json_decoder
{"Name":"Wednesday"}