x/sys/unix: Select signature on BSDs omits integer return value, unlike Linux etc

[ThomasHabets]

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

$ go version
go version go1.12.1 openbsd/amd64

Does this issue reproduce with the latest release?

Yes. It's still in the code in HEAD.

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

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/thompa/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="openbsd"
GOOS="openbsd"
GOPATH="/home/thompa/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/openbsd_amd64"
GCCGO="gccgo"
CC="cc"
CXX="c++"
CGO_ENABLED="1"
GOMOD=""
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=/tmp/go-build396939324=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Tried to build this on OpenBSD, which builds fine on Linux:

package main

import (
	"fmt"
	"golang.org/x/sys/unix"
	"syscall"
)

type s func(n int, r, w, e *syscall.FdSet, timeout *syscall.Timeval) (int, error)
type s2 func(n int, r, w, e *unix.FdSet, timeout *unix.Timeval) (int, error)

func main() {
	var t s
	t = syscall.Select

	var t2 s2
	t2 = unix.Select

	fmt.Println("Hello, playground", t, t2)

}

What did you expect to see?

Since OpenBSD has the same POSIX semantics for Select, as far as I know, as Linux, I expect the same function prototype and for the example above to successfully build.

What did you see instead?

On OpenBSD (and Mac, and all BSDs?) the above fails to build because syscall.Select and unix.Select only return error, not (int, error).

$ go build t.go
# command-line-arguments
./t.go:14:4: cannot use syscall.Select (type func(int, *syscall.FdSet, *syscall.FdSet, *syscall.FdSet, *syscall.Timeval) error) as type s in assignment
./t.go:17:5: cannot use unix.Select (type func(int, *unix.FdSet, *unix.FdSet, *unix.FdSet, *unix.Timeval) error) as type s2 in assignment

How can I make portable programs when Go removes my ability to call Select?

[bcmills]

CC @tklauser

This part of the syscall API appears to date to CL 29723, before Go's open-source release, and the first place it appears is in syscall_darwin.go.

Is it possible that the Darwin version of this syscall does not return the number of file descriptors from the kernel, and instead computes it on the caller side? What about the other *BSDs?

[ThomasHabets]

OpenBSD certainly claims to "select() returns the total number of ready descriptors in all the sets".

And looking at kernel code it looks like yes.

[tklauser]

OpenBSD certainly claims to "select() returns the total number of ready descriptors in all the sets".

The same seems to be true for Darwin (didn't find an online source for this, checked locally on macOS 10.14), FreeBSD, NetBSD and DragonflyBSD. Probably also Solaris and Illumos.

On Darwin I also checked that unix.Select returns the correct number of ready file descriptors (both with the Go1.12+ libSystem syscalls and the Go1.11 raw syscalls) using the following addition to TestSelect:

        rr, ww, err := os.Pipe()
        if err != nil {
                t.Fatal(err)
        }
        defer rr.Close()
        defer ww.Close()

        if _, err := ww.Write([]byte("HELLO GOPHER")); err != nil {
                t.Fatal(err)
        }

        rFdSet := &unix.FdSet{}
        fd := rr.Fd()
        // FD_SET(fd, rFdSet), this needs NFDBITS from sys/select.h as a generated const
        rFdSet.Bits[fd/unix.NFDBITS] |= (1 << (fd % unix.NFDBITS))

        n, err := unix.Select(int(fd+1), rFdSet, nil, nil, nil)
        if err != nil {
                t.Fatalf("Select: %v", err)
        }
        if n != 1 {
                t.Fatalf("Select: expected 1 ready file descriptors, got %v", n)
        }

How would we go about changing Select? I guess it should be no problem in x/sys/unix as it is using modules. I'm not sure about syscall.

/cc @bradfitz @ianlancetaylor

[bradfitz]

How can I make portable programs when Go removes my ability to call Select?

The default answer to that is:

"Syscall must always be guarded with build tags." -- https://go-proverbs.github.io

That said, this signature should probably match everywhere.

Let's just fix x/sys/unix but not syscall.

[gopherbot]

Change https://golang.org/cl/196802 mentions this issue: unix: fix Select to return number of ready fds on Darwin and *BSD