net: can't unwrap DNSError

[budougumi0617]

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

$ go version
go version go1.21.1 darwin/arm64

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='arm64'
GOBIN=''
GOCACHE='/Users/budougumi0617/Library/Caches/go-build'
GOENV='/Users/budougumi0617/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/budougumi0617/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/budougumi0617/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/opt/homebrew/Cellar/go/1.21.1/libexec'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/opt/homebrew/Cellar/go/1.21.1/libexec/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.21.1'
GCCGO='gccgo'
AR='ar'
CC='cc'
CXX='c++'
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 arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/ll/hpp009q97nvdhy4j49dmw96h0000gn/T/go-build623399656=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

The release notes for Go 1.19 mention the following specification about the "operation was canceled" error in the net package:

When a net package function returns an "operation was canceled" error, the error will now satisfy errors.Is(err, context.Canceled).

However, when I execute errors.Is(err, context.Canceled) against the "operation was canceled" error that returns from methods like *Dialer.DialContext, it does not evaluate to true. Is this in accordance with the specification?

https://go.dev/play/p/Os-LQrrlUx6

	_, err := (&net.Dialer{}).DialContext(ctx, "tcp", "localhost:12345")

	t.Logf("error was %q\n", err) // error was "dial tcp: lookup localhost: operation was canceled"
	
	if !errors.Is(err, context.Canceled) {
		t.Errorf("%T, was not context.Canceled", err) // *net.OpError, was not context.Canceled
	}

Looking at the execution result, it appears that the *net.DNSError within the *net.OpError has not implemented the Unwrap method. If the Err field in the DNSError struct retained its value as an error type, I believe the Is method of errCanceled (of type canceledError) from the net package would be called, and errors.Is(err, context.Canceled) would return true.

https://pkg.go.dev/net#DNSError

type DNSError struct {
	Err         string // description of the error
	Name        string // name looked for
	Server      string // server used
	IsTimeout   bool   // if true, timed out; not all timeouts set this
	IsTemporary bool   // if true, error is temporary; not all errors set this
	IsNotFound  bool   // if true, host could not be found
}

https://github.com/golang/go/blob/go1.21.1/src/net/net.go#L411-L422

	// For both read and write operations.
	errCanceled         = canceledError{}
	ErrWriteToConnected = errors.New("use of WriteTo with pre-connected connection")
)

// canceledError lets us return the same error string we have always
// returned, while still being Is context.Canceled.
type canceledError struct{}

func (canceledError) Error() string { return "operation was canceled" }

func (canceledError) Is(err error) bool { return err == context.Canceled }

• ref: net: make errCanceled and errTimeout be "errors.Is" context.Canceled and context.DeadlineExceeded #51428
• ref: net: expose context Canceled/DeadlineExceeded error #36208

What did you expect to see?

When a net package function returns an "operation was canceled" error, errors.Is(err, context.Canceled) returns true.

What did you see instead?

When a net package function returns an "operation was canceled" error, errors.Is(err, context.Canceled) returns false.

[ianlancetaylor]

The problem, as your test shows, is that net.DNSError does not support unwrapping to an underlying error. I don't know how easy that will be to fix while retaining backward compatibility.

[mateusz834]

I've made a proposal to fix this issue #63116.

@ianlancetaylor It should retain backward compatibility.

[bcmills]

Duplicate of #63116

[budougumi0617]

I appreciate your quick triage and proposal creation!