Revert "internal/poll: remove fallback path in accept"

This reverts CL 422375.

Reason for revert: We still need the fallback path on Solaris.

For #45964
For #59359

Change-Id: Ie598b9ef180708fb157080015aee44f67f6737c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/501275
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@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>

src/internal/poll/sock_cloexec.go

@@ -15,8 +15,36 @@ import "syscall"
 // descriptor as nonblocking and close-on-exec.
 func accept(s int) (int, syscall.Sockaddr, string, error) {
 	ns, sa, err := Accept4Func(s, syscall.SOCK_NONBLOCK|syscall.SOCK_CLOEXEC)
-	if err != nil {
+	// On Linux the accept4 system call was introduced in 2.6.28
+	// kernel and on FreeBSD it was introduced in 10 kernel. If we
+	// get an ENOSYS error on both Linux and FreeBSD, or EINVAL
+	// error on Linux, fall back to using accept.
+	switch err {
+	case nil:
+		return ns, sa, "", nil
+	default: // errors other than the ones listed
 		return -1, sa, "accept4", err
+	case syscall.ENOSYS: // syscall missing
+	case syscall.EINVAL: // some Linux use this instead of ENOSYS
+	case syscall.EACCES: // some Linux use this instead of ENOSYS
+	case syscall.EFAULT: // some Linux use this instead of ENOSYS
+	}
+
+	// See ../syscall/exec_unix.go for description of ForkLock.
+	// It is probably okay to hold the lock across syscall.Accept
+	// because we have put fd.sysfd into non-blocking mode.
+	// However, a call to the File method will put it back into
+	// blocking mode. We can't take that risk, so no use of ForkLock here.
+	ns, sa, err = AcceptFunc(s)
+	if err == nil {
+		syscall.CloseOnExec(ns)
+	}
+	if err != nil {
+		return -1, nil, "accept", err
+	}
+	if err = syscall.SetNonblock(ns, true); err != nil {
+		CloseFunc(ns)
+		return -1, nil, "setnonblock", err
 	}
 	return ns, sa, "", nil
 }