x/net: ReadBatch fails if conn passed to ipv4.New{Packet}Conn is not a net.{TCP/UDP}Conn

[marten-seemann]

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

$ go version
go version go1.15.3 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="on"
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/marten/Library/Caches/go-build"
GOENV="/Users/marten/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/marten/src/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/marten/src/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.15.3/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.15.3/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/dev/null"
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/q0/b5ynf00142l7bl9sp8y098zr0000gn/T/go-build393900320=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

When passing a wrapped connection to ipv4.NewPacketConn(), calls to ReadBatch() will fail with an invalid connection error:

type wrappedConn { *net.UDPConn }
conn := ipv4.NewPacketConn(&wrappedConn{...})
conn.ReadBatch() // will return the "invalid connection" error

This is unexpected, as NewPacketConn accepts a net.PacketConn, and does not require a net.UDPConn.
Analogously for NewConn with a net.TCPConn.

The problem here is that the constructor calls socket.NewConn, which does a type assertion: https://github.com/golang/net/blob/ff519b6c91021e6316e1df005bc19f266994ddda/internal/socket/rawconn.go#L21-L41, which will fail if the UPDConn is wrapped.

What did you expect to see?

I expected ReadBatch to work.

What did you see instead?

See above.

[marten-seemann]

While one could argue that ipv4.New{Packet}Conn should take a concrete type (net.TCPConn / net.UDPConn) instead of an interface, there's a solution that doesn't require a breaking API change, while at the same time allowing for wrapped connections to be used: In that case socket.NewConn() would have to do an interface assertion instead of a type assertion.

This is easy to accomplish, since TCPConn, UDPConn as well as IPConn each have methods that are unique to that type. For example, the UDPConn can be identified by its ReadMsgUDP method, the IPConn by its ReadMsgIP and the TCPConn by SetLinger. There are other methods that would also achieve the same result.

socket.NewConn would then look something like this:

func NewConn(c net.Conn) (*Conn, error) {
	var err error
	var cc Conn
	if tcpConn, ok := c.(interface {
		SyscallConn() (syscall.RawConn, error)
		SetLinger() error
	}); ok {
		cc.network = "tcp"
		cc.c, err = tcpConn.SyscallConn()
	} else if udpConn, ok := c.(interface {
		SyscallConn() (syscall.RawConn, error)
		ReadMsgUDP(b, oob []byte) (n, oobn, flags int, addr *net.UDPAddr, err error)
	}); ok {
		cc.network = "udp"
		cc.c, err = udpConn.SyscallConn()
	} else if ipConn, ok := := c.(interface {
		SyscallConn() (syscall.RawConn, error)
		ReadMsgIP(b, oob []byte) (n, oobn, flags int, addr *net.IPAddr, err error)
	}); ok {
		cc.network = "ip"
		cc.c, err = ipConn.SyscallConn()
	} else
		return nil, errors.New("unknown connection type")
	}
	if err != nil {
		return nil, err
	}
	return &cc, nil
}

[networkimprov]

cc @odeke-em @ianlancetaylor

[cagedmantis]

/cc @bradfitz @ianlancetaylor

[marten-seemann]

Suggested fix: https://go-review.googlesource.com/c/net/+/271927.

[gopherbot]

Change https://golang.org/cl/271927 mentions this issue: x/net: use interface assertions to determine the net.Conn type in socket.NewConn