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)

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[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.

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