x/tools/gopls: False alarm with unused cancel() of derived context

[bronger]

gopls version

Build info

golang.org/x/tools/gopls v0.16.1
golang.org/x/tools/gopls@v0.16.1 h1:1hO/dCeUvjEYx3V0rVvCtOkwnpEpqS29paE+Jw4dcAc=
github.com/BurntSushi/toml@v1.2.1 h1:9F2/+DoOYIOksmaJFPw1tGFy1eDnIJXg+UHjuD8lTak=
github.com/google/go-cmp@v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
golang.org/x/exp/typeparams@v0.0.0-20221212164502-fae10dda9338 h1:2O2DON6y3XMJiQRAS1UWU+54aec2uopH3x7MAiqGW6Y=
golang.org/x/mod@v0.18.0 h1:5+9lSbEzPSdWkH32vYPBwEpX8KwDbM52Ud9xBUvNlb0=
golang.org/x/sync@v0.7.0 h1:YsImfSBoP9QPYL0xyKJPq0gcaJdG3rInoqxTWbfQu9M=
golang.org/x/telemetry@v0.0.0-20240607193123-221703e18637 h1:3Wt8mZlbFwG8llny+t18kh7AXxyWePFycXMuVdHxnyM=
golang.org/x/text@v0.16.0 h1:a94ExnEXNtEwYLGJSIUxnWoxoRz/ZcCsV63ROupILh4=
golang.org/x/tools@v0.22.1-0.20240628205440-9c895dd76b34 h1:Kd+Z5Pm6uwYx3T2KEkeHMHUMZxDPb/q6b1m+zEcy62c=
golang.org/x/vuln@v1.0.4 h1:SP0mPeg2PmGCu03V+61EcQiOjmpri2XijexKdzv8Z1I=
honnef.co/go/tools@v0.4.7 h1:9MDAWxMoSnB6QoSqiVr7P5mtkT9pOc1kSxchzPCnqJs=
mvdan.cc/gofumpt@v0.6.0 h1:G3QvahNDmpD+Aek/bNOLrFR2XC6ZAdo62dZu65gmwGo=
mvdan.cc/xurls/v2@v2.5.0 h1:lyBNOm8Wo71UknhUs4QTFUNNMyxy2JEIaKKo0RWOh+8=
go: go1.23.0 X:rangefunc

go env

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOEXE=''
GOEXPERIMENT='rangefunc'
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPRIVATE=''
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.23.0'
GODEBUG=''
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
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 -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build2627358158=/tmp/go-build -gno-record-gcc-switches'

What did you do?

package main

import (
	"context"
	"fmt"
	"time"
)

func supi(ctx context.Context) {
	var (
		innerCtx context.Context
		cancel   context.CancelFunc
	)
	ticker := time.Tick(time.Second)
	fmt.Println(innerCtx, cancel)
	for {
		select {
		case <-ticker:
			innerCtx, cancel = context.WithCancel(ctx)
		case <-ctx.Done():
			// fmt.Println(cancel)
			return
		}
	}
}

func main() {
	ctx := context.Background()
	supi(ctx)
}

What did you see happen?

The return in line 22 received the following gopls warning: “this return statement may be reached without using the cancel var defined on line 19 [default]”

What did you expect to see?

No warning, as the return can only be reached when the context is already cancelled.

Editor and settings

No response

Logs

No response

[gabyhelp]

Related Issues and Documentation

• x/tools/go/analysis/passes/lostcancel: incorrect warning about context leak using defer #58850 (closed)
• x/tools/gopls: stop logging context cancellation errors #34103 (closed)
• x/tools/gopls: No warning "loop variable xx captured by func literal". #60016 (closed)
• x/tools/gopls: #52425 (closed)
• x/tools/gopls: error reporting type as `chan<- invalid type` #67637
• x/tools/gopls: error range for function with missing return is the entire function body #35139 (closed)
• x/tools/gopls: references doesn't work properly. #60676 (closed)
• x/tools/gopls: range over func without variable shows 'simplify range expression' warning #65799 (closed)
• x/tools/gopls: missing semantic tokens for function parameters in block #56257 (closed)
• x/tools/gopls: diagnostic doesn't go away #54777 (closed)

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

[adonovan]

the return can only be reached when the context is already cancelled.

That is not in fact the case [Edit: I was wrong]. If the context passed to supi is cancelled, the return statement will be reached; its cancellation is independent of the ticker. Even if the context is never cancelled (as is the case for Background), this program is still not correct: it leaks cancellation functions at the rate of the ticker. Each call to WithCancel must be paired with exactly one call to cancel.

The analyzer is working as intended.

[bronger]

Each call to WithCancel must be paired with exactly one call to cancel.

But the call to cancel happens when ctx is cancelled, doesn’t it?

[adonovan]

But the call to cancel happens when ctx is cancelled, doesn’t it?

Sorry, you're quite right; I had forgotten that cancellation of the parent does release the resources of its WithCancel child contexts.

Nonetheless, this program generates a potentially unbounded number of child contexts that will never be cancelled until the parent context is cancelled (which is never for Background), so this program will indeed leak contexts at the rate of one per tick, and the analyzer is right to point out the error.