slog: Wrapping default handler causes deadlock

[mkch]

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

$ go version
go version go1.21.0 darwin/amd64

Does this issue reproduce with the latest release?

Yes.

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

go env Output

$ go env
GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/Users/me/Library/Caches/go-build'
GOENV='/Users/me/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/me/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/me/go'
GOPRIVATE=''
GOPROXY='https://goproxy.cn,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/darwin_amd64'
GOVCS=''
GOVERSION='go1.21.0'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='1'
GOMOD='/dev/null'
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 x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/4v/g652j2zn37lg22crrdwmcqdm0000gn/T/go-build1896165565=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

https://go.dev/play/p/gu5St1Epba_9

What did you expect to see?

Log message like the following:
2023/09/02 10:24:00 INFO debug message

What did you see instead?

fatal error: all goroutines are asleep - deadlock!

[panjf2000]

Check out the comment in slog.SetDefault

go/src/log/slog/logger.go

Lines 29 to 36 in 83fe0e0

	func SetDefault(l *Logger) {
	defaultLogger.Store(l)
	// If the default's handler is a defaultHandler, then don't use a handleWriter,
	// or we'll deadlock as they both try to acquire the log default mutex.
	// The defaultHandler will use whatever the log default writer is currently
	// set to, which is correct.
	// This can occur with SetDefault(Default()).
	// See TestSetDefault.

[mkch]

Check out the comment in slog.SetDefault

go/src/log/slog/logger.go

Lines 29 to 36 in 83fe0e0

	func SetDefault(l *Logger) {
	defaultLogger.Store(l)
	// If the default's handler is a defaultHandler, then don't use a handleWriter,
	// or we'll deadlock as they both try to acquire the log default mutex.
	// The defaultHandler will use whatever the log default writer is currently
	// set to, which is correct.
	// This can occur with SetDefault(Default()).
	// See TestSetDefault.

Sorry, I can't see any relevance.

[panjf2000]

slog.SetDefault can only prevent the loggers that use the exact defaultHandler from deadlock, and it doesn't handle the case you provided (embed the default handler into a struct), I'm not sure whether we should fix this as it seems like a rare case.

/cc @jba

[mkch]

slog.SetDefault can only prevent the deadlock from the loggers that use the exact defaultHandler, and it doesn't handle the case you provided (embed the default handler into a struct), I'm not sure whether we should fix this as it seems like a rare case.

/cc @jba

Thanks for your clarification. Can I consider this a implementation limitation?
What I really want to do is changing the level of the default Handler. Something lile https://go.dev/play/p/bXoD9C49mNy

[panjf2000]

Looks like #62418 will work for you once it's accepted and implemented.

[seankhliao]

Duplicate of #61892