unix: remove Go 1.12 compatibility on darwin

Go 1.12 was released more than 3 years ago. Dropping support for it will
simplify generating the wrappers needed for Getdirentries
implementation.

Change-Id: If24bf6ff9076fb4e281948c67bac87744df98d7a
Reviewed-on: https://go-review.googlesource.com/c/sys/+/430176
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>

unix/syscall_darwin.go

@@ -19,6 +19,96 @@ import (
 	"unsafe"
 )
 
+//sys	closedir(dir uintptr) (err error)
+//sys	readdir_r(dir uintptr, entry *Dirent, result **Dirent) (res Errno)
+
+func fdopendir(fd int) (dir uintptr, err error) {
+	r0, _, e1 := syscall_syscallPtr(libc_fdopendir_trampoline_addr, uintptr(fd), 0, 0)
+	dir = uintptr(r0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+var libc_fdopendir_trampoline_addr uintptr
+
+//go:cgo_import_dynamic libc_fdopendir fdopendir "/usr/lib/libSystem.B.dylib"
+
+func Getdirentries(fd int, buf []byte, basep *uintptr) (n int, err error) {
+	// Simulate Getdirentries using fdopendir/readdir_r/closedir.
+	// We store the number of entries to skip in the seek
+	// offset of fd. See issue #31368.
+	// It's not the full required semantics, but should handle the case
+	// of calling Getdirentries or ReadDirent repeatedly.
+	// It won't handle assigning the results of lseek to *basep, or handle
+	// the directory being edited underfoot.
+	skip, err := Seek(fd, 0, 1 /* SEEK_CUR */)
+	if err != nil {
+		return 0, err
+	}
+
+	// We need to duplicate the incoming file descriptor
+	// because the caller expects to retain control of it, but
+	// fdopendir expects to take control of its argument.
+	// Just Dup'ing the file descriptor is not enough, as the
+	// result shares underlying state. Use Openat to make a really
+	// new file descriptor referring to the same directory.
+	fd2, err := Openat(fd, ".", O_RDONLY, 0)
+	if err != nil {
+		return 0, err
+	}
+	d, err := fdopendir(fd2)
+	if err != nil {
+		Close(fd2)
+		return 0, err
+	}
+	defer closedir(d)
+
+	var cnt int64
+	for {
+		var entry Dirent
+		var entryp *Dirent
+		e := readdir_r(d, &entry, &entryp)
+		if e != 0 {
+			return n, errnoErr(e)
+		}
+		if entryp == nil {
+			break
+		}
+		if skip > 0 {
+			skip--
+			cnt++
+			continue
+		}
+
+		reclen := int(entry.Reclen)
+		if reclen > len(buf) {
+			// Not enough room. Return for now.
+			// The counter will let us know where we should start up again.
+			// Note: this strategy for suspending in the middle and
+			// restarting is O(n^2) in the length of the directory. Oh well.
+			break
+		}
+
+		// Copy entry into return buffer.
+		s := unsafe.Slice((*byte)(unsafe.Pointer(&entry)), reclen)
+		copy(buf, s)
+
+		buf = buf[reclen:]
+		n += reclen
+		cnt++
+	}
+	// Set the seek offset of the input fd to record
+	// how many files we've already returned.
+	_, err = Seek(fd, cnt, 0 /* SEEK_SET */)
+	if err != nil {
+		return n, err
+	}
+
+	return n, nil
+}
+
 // SockaddrDatalink implements the Sockaddr interface for AF_LINK type sockets.
 type SockaddrDatalink struct {
 	Len    uint8