Revert "net: remove fallback path in sysSocket"

This reverts CL 40364.

Reason for revert: Fallback path is still required on Solaris.

For #45964
For #59359

Change-Id: I4b8c8af77ee987cad6617221793b90c9a8829c3e
Reviewed-on: https://go-review.googlesource.com/c/go/+/501276
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/net/sock_cloexec.go

@@ -10,6 +10,7 @@
 package net
 
 import (
+	"internal/poll"
 	"os"
 	"syscall"
 )
@@ -18,8 +19,32 @@ import (
 // descriptor as nonblocking and close-on-exec.
 func sysSocket(family, sotype, proto int) (int, error) {
 	s, err := socketFunc(family, sotype|syscall.SOCK_NONBLOCK|syscall.SOCK_CLOEXEC, proto)
+	// On Linux the SOCK_NONBLOCK and SOCK_CLOEXEC flags were
+	// introduced in 2.6.27 kernel and on FreeBSD both flags were
+	// introduced in 10 kernel. If we get an EINVAL error on Linux
+	// or EPROTONOSUPPORT error on FreeBSD, fall back to using
+	// socket without them.
+	switch err {
+	case nil:
+		return s, nil
+	default:
+		return -1, os.NewSyscallError("socket", err)
+	case syscall.EPROTONOSUPPORT, syscall.EINVAL:
+	}
+
+	// See ../syscall/exec_unix.go for description of ForkLock.
+	syscall.ForkLock.RLock()
+	s, err = socketFunc(family, sotype, proto)
+	if err == nil {
+		syscall.CloseOnExec(s)
+	}
+	syscall.ForkLock.RUnlock()
 	if err != nil {
 		return -1, os.NewSyscallError("socket", err)
 	}
+	if err = syscall.SetNonblock(s, true); err != nil {
+		poll.CloseFunc(s)
+		return -1, os.NewSyscallError("setnonblock", err)
+	}
 	return s, nil
 }