syscallcompat: add Fstatat + emulation + test

Fstatat has recently been added to x/sys/unix. Make
it available for use in gocryptfs.

internal/syscallcompat/emulate.go

@@ -210,3 +210,23 @@ func emulateMkdirat(dirfd int, path string, mode uint32) (err error) {
 	}
 	return syscall.Mkdir(path, mode)
 }
+
+// emulateFstatat emulates the syscall for platforms that don't have it
+// in the kernel (darwin).
+func emulateFstatat(dirfd int, path string, stat *unix.Stat_t, flags int) (err error) {
+	if !filepath.IsAbs(path) {
+		chdirMutex.Lock()
+		defer chdirMutex.Unlock()
+		cwd, err := syscall.Open(".", syscall.O_RDONLY, 0)
+		if err != nil {
+			return err
+		}
+		defer syscall.Close(cwd)
+		err = syscall.Fchdir(dirfd)
+		if err != nil {
+			return err
+		}
+		defer syscall.Fchdir(cwd)
+	}
+	return unix.Lstat(path, stat)
+}

internal/syscallcompat/emulate_test.go

@@ -256,3 +256,38 @@ func TestEmulateMkdirat(t *testing.T) {
 		t.Fatalf("mkdirat did not create a directory")
 	}
 }
+
+func TestEmulateFstatat(t *testing.T) {
+	var st unix.Stat_t
+	// stat a normal file (size 3)
+	f, err := os.Create(tmpDir + "/fstatat")
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = f.Write([]byte("foo"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	f.Close()
+	err = emulateFstatat(tmpDirFd, "fstatat", &st, unix.AT_SYMLINK_NOFOLLOW)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if st.Size != 3 {
+		t.Errorf("wrong file size: %d", st.Size)
+	}
+	// stat a symlink and check that the size matches the length of the
+	// symlink target. This proves that we have stat'ed the symlink itself.
+	err = os.Symlink(tmpDir+"/fstatat", tmpDir+"/fstatatSymlink")
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = emulateFstatat(tmpDirFd, "fstatatSymlink", &st, unix.AT_SYMLINK_NOFOLLOW)
+	if err != nil {
+		t.Fatal(err)
+	}
+	expectedSize := int64(len(tmpDir + "/fstatat"))
+	if st.Size != expectedSize {
+		t.Errorf("symlink size: expected=%d, got=%d", expectedSize, st.Size)
+	}
+}

internal/syscallcompat/sys_darwin.go

@@ -3,6 +3,10 @@ package syscallcompat
 import (
 	"log"
 	"syscall"
+
+	"golang.org/x/sys/unix"
+
+	"github.com/hanwen/go-fuse/fuse"
 )
 
 // Sorry, fallocate is not available on OSX at all and
@@ -60,3 +64,11 @@ func Symlinkat(oldpath string, newdirfd int, newpath string) (err error) {
 func Mkdirat(dirfd int, path string, mode uint32) (err error) {
 	return emulateMkdirat(dirfd, path, mode)
 }
+
+func Fstatat(dirfd int, path string, stat *unix.Stat_t, flags int) (err error) {
+	return emulateFstatat(dirfd, path, stat, flags)
+}
+
+func Getdents(fd int) ([]fuse.DirEntry, error) {
+	return emulateGetdents(fd)
+}

internal/syscallcompat/sys_linux.go

@@ -105,3 +105,13 @@ func Symlinkat(oldpath string, newdirfd int, newpath string) (err error) {
 func Mkdirat(dirfd int, path string, mode uint32) (err error) {
 	return syscall.Mkdirat(dirfd, path, mode)
 }
+
+// Fstatat syscall.
+func Fstatat(dirfd int, path string, stat *unix.Stat_t, flags int) (err error) {
+	// Why would we ever want to call this without AT_SYMLINK_NOFOLLOW?
+	if flags&unix.AT_SYMLINK_NOFOLLOW == 0 {
+		tlog.Warn.Printf("Fstatat: adding missing AT_SYMLINK_NOFOLLOW flag")
+		flags |= unix.AT_SYMLINK_NOFOLLOW
+	}
+	return unix.Fstatat(dirfd, path, stat, flags)
+}