move implementation to internal/ directory (#828)

internal/util/socketpair/socketpair.go

@@ -0,0 +1,39 @@
+package socketpair
+
+import (
+	"net"
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+func SocketPair() (a, b *net.UnixConn, err error) {
+	// don't use net.Pipe, as it doesn't implement things like lingering, which our code relies on
+	sockpair, err := unix.Socketpair(unix.AF_UNIX, unix.SOCK_STREAM, 0)
+	if err != nil {
+		return nil, nil, err
+	}
+	toConn := func(fd int) (*net.UnixConn, error) {
+		f := os.NewFile(uintptr(fd), "fileconn")
+		if f == nil {
+			panic(fd)
+		}
+		c, err := net.FileConn(f)
+		f.Close() // net.FileConn uses dup under the hood
+		if err != nil {
+			return nil, err
+		}
+		// strictly, the following type assertion is an implementation detail
+		// however, will be caught by test TestSocketPairWorks
+		fileConnIsUnixConn := c.(*net.UnixConn)
+		return fileConnIsUnixConn, nil
+	}
+	if a, err = toConn(sockpair[0]); err != nil { // shadowing
+		return nil, nil, err
+	}
+	if b, err = toConn(sockpair[1]); err != nil { // shadowing
+		a.Close()
+		return nil, nil, err
+	}
+	return a, b, nil
+}