log/slog: LogValuer implementation panics on nil value

[denpeshkov]

Go version

go1.23 darwin/arm64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='arm64'
GOBIN=''
GOCACHE='/Users/denpeshkov/Library/Caches/go-build'
GOENV='/Users/denpeshkov/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/denpeshkov/go/pkg/mod'
GONOPROXY='gitlab.wildberries.ru/sender'
GONOSUMDB='gitlab.wildberries.ru/sender'
GOOS='darwin'
GOPATH='/Users/denpeshkov/go'
GOPRIVATE='gitlab.wildberries.ru/sender'
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/opt/homebrew/Cellar/go/1.23.2/libexec'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/opt/homebrew/Cellar/go/1.23.2/libexec/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.23.2'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/denpeshkov/Library/Application Support/go/telemetry'
GCCGO='gccgo'
GOARM64='v8.0'
AR='ar'
CC='cc'
CXX='c++'
CGO_ENABLED='1'
GOMOD='/Users/denpeshkov/src/gitlab.wildberries.ru/sender/sender-template/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/xx/y0wxwyw553j3s155ywc95mqw0000gn/T/go-build141366395=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

Go Playground link

What did you see happen?

2009/11/10 23:00:00 INFO Message text foo="LogValue panicked\ncalled from runtime.panicwrap (/usr/local/go-faketime/src/runtime/error.go:355)\ncalled from main.(*foo).LogValue (:1)\ncalled from log/slog.Value.Resolve (/usr/local/go-faketime/src/log/slog/value.go:512)\ncalled from log/slog.(*handleState).appendAttr (/usr/local/go-faketime/src/log/slog/handler.go:468)\ncalled from log/slog.(*handleState).appendNonBuiltIns.func1 (/usr/local/go-faketime/src/log/slog/handler.go:342)\n(rest of stack elided)\n"

What did you expect to see?

2009/11/10 23:00:00 INFO Message text foo=

[gabyhelp]

Related Issues and Documentation

• x/exp/slog: TextHandler panics on nil arg #58652 (closed)
• x/tools/go/analysis/passes/slog: nil panic in `isAttr` via `analysisutil.IsNamed` #61228 (closed)
• log/slog: panic: bad kind: 9 in JSONHandler Handler().WithAttrs(LogValuer{}) #59727 (closed)
• log/slog: nil pointer dereference when printing nil regexp #61648 (closed)
• log/slog: catch panics in LogValue
• x/tools/go/packages: nil pointer dereference #69524 (closed)
• cmd/go: panic when building a `main` package in GOPATH mode when a go.work file exists #58767 (closed)
• go/internal/gcimporter: untyped interface type parameter causes "panic: nil underlying" during testing #57015 (closed)
• cmd/compile/internal/types2: unify.go:449: assertion failed, panic #67872 (closed)
• Unexpected execution flow when overshadowing error-type with *Error (both nil) #26000 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[seankhliao]

this looks working as intended for slog, the issue is in your code

[denpeshkov]

Thanks for your reply! Just to confirm, do I need to handle nil values explicitly even if the method receiver is defined on a value rather than a pointer?

Even without accessing struct fields, I'm encountering a panic: https://go.dev/play/p/vott61McxDf?v=gotip

[seankhliao]

given a pointer, Go needs to dereference it to pass to the method. That's where your nil pointer dereference is coming from. It also panics if you call f.LogValue() directly, without slog.

Unlike many projects, the Go project does not use GitHub Issues for general discussion or asking questions. GitHub Issues are used for tracking bugs and proposals only.

For questions please refer to https://github.com/golang/go/wiki/Questions

[denpeshkov]

I was just thinking it would be helpful if this behaved similarly to fmt.Stringer when handling nil values. It explicitly handles nil in panic:

// If it's a nil pointer, output "<nil>". This approach could handle cases 
// where a Stringer might not check for nil or where a nil pointer is used 
// with a value receiver. In both scenarios, "<nil>" seems like a reasonable default.
if v := reflect.ValueOf(arg); v.Kind() == reflect.Pointer && v.IsNil() {
    p.buf.writeString(nilAngleString)
    return
}