net: make errCanceled and errTimeout be "errors.Is" context.Canceled and context.DeadlineExceeded

[gprossliner]

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

$ go version
go version go1.17.6 linux/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="/home/guenter.prossliner@world-direct.at/.cache/go-build"
GOENV="/home/guenter.prossliner@world-direct.at/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/guenter.prossliner@world-direct.at/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/guenter.prossliner@world-direct.at/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.17.6"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/guenter.prossliner@world-direct.at/proj/polly2/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build117018826=/tmp/go-build -gno-record-gcc-switches"

What did you do?

This is part of a bigger application, but here is a repo:
https://go.dev/play/p/mD-R6gerA4c

What did you expect to see?

I expect that the net.OpErr which is returned by net.dialer.DialContext included the wrapped original error, which is a context.Canceled error.

This may be the problematic function. There is also a TODO from @bradfitz

go/src/net/net.go

Lines 419 to 434 in 40e24a9

	// mapErr maps from the context errors to the historical internal net
	// error values.
	//
	// TODO(bradfitz): get rid of this after adjusting tests and making
	// context.DeadlineExceeded implement net.Error?
	func mapErr(err error) error {
	switch err {
	case context.Canceled:
	return errCanceled
	case context.DeadlineExceeded:
	return errTimeout
	default:
	return err
	}
	}

What did you see instead?

UNEXPECTED:dial tcp: operation was canceled
EXPECTED: Get "http://github.com": context canceled

[ianlancetaylor]

This can be confusing but I don't see how to return context.Canceled without breaking existing applications.

One thing we can do is add an Is method to net.errCanceled that makes it match context.Canceled. Then using errors.Is(err, context.Canceled) will return true for net.errCanceled, and I think that your program would work. Similarly for net.errTimeout. Does that seem useful?

[bcmills]

Another option might be to give net.errCanceled an Unwrap method that returns context.Canceled, even though the context.Canceled error text does not appear in the errCanceled text.

It's not obvious to me whether that is better or worse than adding the Is method.

[gprossliner]

Adding the Is function is probably the best option.

[ianlancetaylor]

OK, I turned this into a proposal for the behavior change.

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group

[rsc]

No change in consensus, so accepted. 🎉
This issue now tracks the work of implementing the proposal.
— rsc for the proposal review group