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/mkall.sh

@@ -214,11 +214,6 @@ esac
 			if [ "$GOOSARCH" == "aix_ppc64" ]; then
 				# aix/ppc64 script generates files instead of writing to stdin.
 				echo "$mksyscall -tags $GOOS,$GOARCH $syscall_goos $GOOSARCH_in && gofmt -w zsyscall_$GOOSARCH.go && gofmt -w zsyscall_"$GOOSARCH"_gccgo.go && gofmt -w zsyscall_"$GOOSARCH"_gc.go " ;
-			elif [ "$GOOS" == "darwin" ]; then
-			        # 1.12 and later, syscalls via libSystem
-				echo "$mksyscall -tags $GOOS,$GOARCH,go1.12 $syscall_goos $GOOSARCH_in |gofmt >zsyscall_$GOOSARCH.go";
-				# 1.13 and later, syscalls via libSystem (including syscallPtr)
-				echo "$mksyscall -tags $GOOS,$GOARCH,go1.13 syscall_darwin.1_13.go |gofmt >zsyscall_$GOOSARCH.1_13.go";
 			elif [ "$GOOS" == "illumos" ]; then
 			        # illumos code generation requires a --illumos switch
 			        echo "$mksyscall -illumos -tags illumos,$GOARCH syscall_illumos.go |gofmt > zsyscall_illumos_$GOARCH.go";

unix/mkasm.go

@@ -31,7 +31,7 @@ func archPtrSize(arch string) int {
 	}
 }
 
-func generateASMFile(arch string, inFileNames []string, outFileName, buildTags string) map[string]bool {
+func generateASMFile(arch string, inFileNames []string, outFileName string) map[string]bool {
 	trampolines := map[string]bool{}
 	var orderedTrampolines []string
 	for _, inFileName := range inFileNames {
@@ -59,11 +59,6 @@ func generateASMFile(arch string, inFileNames []string, outFileName, buildTags s
 	fmt.Fprintf(&out, "// go run mkasm.go %s\n", strings.Join(os.Args[1:], " "))
 	fmt.Fprintf(&out, "// Code generated by the command above; DO NOT EDIT.\n")
 	fmt.Fprintf(&out, "\n")
-	if buildTags != "" {
-		fmt.Fprintf(&out, "//go:build %s\n", buildTags)
-		fmt.Fprintf(&out, "// +build %s\n", buildTags)
-		fmt.Fprintf(&out, "\n")
-	}
 	fmt.Fprintf(&out, "#include \"textflag.h\"\n")
 	for _, fn := range orderedTrampolines {
 		fmt.Fprintf(&out, "\nTEXT %s_trampoline<>(SB),NOSPLIT,$0-0\n", fn)
@@ -121,7 +116,6 @@ func main() {
 	}
 	goos, arch := os.Args[1], os.Args[2]
 
-	buildTags := ""
 	syscallFilename := fmt.Sprintf("syscall_%s.go", goos)
 	syscallArchFilename := fmt.Sprintf("syscall_%s_%s.go", goos, arch)
 	zsyscallArchFilename := fmt.Sprintf("zsyscall_%s_%s.go", goos, arch)
@@ -133,25 +127,9 @@ func main() {
 		zsyscallArchFilename,
 	}
 
-	if goos == "darwin" {
-		buildTags = "go1.12"
-	}
-	trampolines := generateASMFile(arch, inFileNames, zsyscallASMFileName, buildTags)
-
-	if goos != "darwin" {
-		return
-	}
+	trampolines := generateASMFile(arch, inFileNames, zsyscallASMFileName)
 
-	inFileNames = []string{
-		"syscall_darwin.1_13.go",
-		fmt.Sprintf("zsyscall_darwin_%s.1_13.go", arch),
-	}
-	trampolines2 := generateASMFile(arch, inFileNames, fmt.Sprintf("zsyscall_darwin_%s.1_13.s", arch), "go1.13")
-
-	// merge trampolines
-	for trampoline := range trampolines2 {
-		trampolines[trampoline] = true
+	if goos == "darwin" {
+		writeDarwinTest(trampolines, fmt.Sprintf("darwin_%s_test.go", arch), arch)
 	}
-
-	writeDarwinTest(trampolines, fmt.Sprintf("darwin_%s_test.go", arch), arch)
 }

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