x/net/route: parseInterfaceMessage broken on Dragonfly

Note: I think I know how to fix this, and hope to have a patch ready soon. I'm filing this bug first to organize my thoughts and in case it's helpful.

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

Git commit f21be2fdc.

falsifian angel-dfly go $ bin/go version
go version go1.16 dragonfly/amd64

Does this issue reproduce with the latest release?

I haven't tried, but it almost certainly will, since no Dragonfly implementation of parseInterfaceAddrMessage is there.

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

go env Output

falsifian angel-dfly go $ bin/go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/falsifian/.cache/go-build"
GOENV="/home/falsifian/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="dragonfly"
GOINSECURE=""
GOMODCACHE="/home/falsifian/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="dragonfly"
GOPATH="/home/falsifian/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/falsifian/src/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/falsifian/src/go/pkg/tool/dragonfly_amd64"
GOVCS=""
GOVERSION="go1.16"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build1287776679=/tmp/go-build -gno-record-gcc-switches"

What did you do?

route.ParseRIB can't parse messages of type RTM_IFINFO, and omits them instead.

For example, a call to net.InterfaceByName("tun0") (in the net package) returns an error, even if tun0 exists.

The list returned by route.ParseRIB omits all the routing messages of type RTM_IFINFO, which in turn is because the library is trying to use the generic parseInterfaceMessage implementation in interface_classic.go, which doesn't work since DragonflyBSD's RTM_VERSION got bumped to 7 in commit 43a37315. Related: #34368.

I guess the solution will be to implement a Dragonfly-specific parseInterfaceMessage; this is already done for FreeBSD and OpenBSD.

golang / go