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expvar: Func does not implement Var #42705

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rhysh opened this issue Nov 18, 2020 · 2 comments
Open

expvar: Func does not implement Var #42705

rhysh opened this issue Nov 18, 2020 · 2 comments

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@rhysh
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@rhysh rhysh commented Nov 18, 2020

What version of Go are you using (go version)?

Go playground, at version go1.15.5.

Does this issue reproduce with the latest release?

Yes, this is present in go1.15.3, and in the playground at go1.15.5.

What operating system and processor architecture are you using (go env)?

Go playground, but first seen on linux/amd64.

What did you do?

Used expvar.Publish with expvar.Func, with a function that returns a struct which encoding/json cannot serialize.

https://play.golang.org/p/9wpvZMJtDmw

package main

import (
	"bytes"
	"encoding/json"
	"expvar"
	"log"
	"net/http"
	"net/http/httptest"
)

func main() {
	try := func() {
		r, err := http.NewRequest("GET", "/debug/vars", nil)
		if err != nil {
			log.Fatal(err)
		}
		w := &httptest.ResponseRecorder{Body: new(bytes.Buffer)}
		expvar.Handler().ServeHTTP(w, r)

		err = json.Unmarshal(w.Body.Bytes(), new(interface{}))
		if err != nil {
			log.Fatalf("Invalid JSON: %v\n%s\n", err, w.Body)
		}
		log.Printf("Valid JSON")
	}

	try()

	log.Printf("Publishing new value")
	expvar.Publish("config", expvar.Func(func() interface{} {
		return struct {
			Fn func()
		}{}
	}))

	try()
}

What did you expect to see?

I expected expvar.Func to implement expvar.Var, meaning it would always return valid JSON. I expected that an HTTP request to /debug/vars would return a valid JSON object. I expected that expvar.Func would recover from an error by returning a boring but valid value, such as null.

What did you see instead?

The String method of expvar.Func ignores the error return value from encoding/json.Marshal, so will return an empty string when the func returns a value that cannot be serialized to JSON.

https://github.com/golang/go/blob/go1.15.5/src/expvar/expvar.go#L261-L264

2009/11/10 23:00:00 Valid JSON
2009/11/10 23:00:00 Publishing new value
2009/11/10 23:00:00 Invalid JSON: invalid character ',' looking for beginning of value
{
"cmdline": ["/tmpfs/play"],
"config": ,
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}
@cagedmantis cagedmantis added this to the Backlog milestone Dec 7, 2020
@cagedmantis
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@cagedmantis cagedmantis commented Dec 7, 2020

/cc @bradfitz

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@gopherbot
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@gopherbot gopherbot commented Jan 31, 2021

Change https://golang.org/cl/288294 mentions this issue: expvar: ensure Func returns valid JSON

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