log/slog: JSONHandler generates invalid JSON with empty Attr

[jaloren]

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

1.21.0

Does this issue reproduce with the latest release?

yes

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

This also reproduces on the go playground.

go env Output

GO111MODULE=''
GOARCH='arm64'
GOBIN='/Users/jlorenzini/go/bin'
GOCACHE='/Users/jlorenzini/Library/Caches/go-build'
GOENV='/Users/jlorenzini/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/jlorenzini/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/jlorenzini/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.21.0'
GCCGO='gccgo'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='1'
GOMOD='/Users/jlorenzini/repos/foo/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/62/n3p69dvx45l3c3wgng9c01sw0000gn/T/go-build1832919248=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

I executed this code in a main function. This can also be reproduced in the go playground: https://go.dev/play/p/i7VtMtNK4eV

var buf bytes.Buffer
	f := func(groups []string, a slog.Attr) slog.Attr {
		return slog.Attr{}
	}
	h := slog.NewJSONHandler(&buf, &slog.HandlerOptions{

		ReplaceAttr: f})

	results := func() []map[string]any {
		var ms []map[string]any
		for _, line := range bytes.Split(buf.Bytes(), []byte{'\n'}) {
			if len(line) == 0 {
				continue
			}
			var m map[string]any
			if err := json.Unmarshal(line, &m); err != nil {
				panic(err) // In a real test, use t.Fatal.
			}
			ms = append(ms, m)
		}
		return ms
	}
	err := slogtest.TestHandler(h, results)
	if err != nil {
		log.Fatal(err)
	}

What did you expect to see?

The program run successfully without an error.

What did you see instead?

The program panicked with the following error when json unmarshal happened.

panic: invalid character ',' looking for beginning of object key string

[dmitshur]

CC @jba.

[gopherbot]

Change https://go.dev/cl/521135 mentions this issue: log/slog: generate valid JSON string with empty attributes in Groups

[panjf2000]

This is a bug and it can occur in scenarios where users try to replace some attributes in embedded groups with empty strings. I believe these cases could be common for users.

[panjf2000]

@gopherbot please consider this for backport to 1.21, it could be a common case and has no workaround.

[gopherbot]

Backport issue(s) opened: #62256 (for 1.21).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.

[jba]

I don't think this requires a backport. It's probably very rare, and you can work around it by just not removing every attr (or adding a dummy one, if you have to remove them all for some security reason).