1 files changed, 75 insertions, 0 deletions
diff --git a/misc/open_file.go b/misc/open_file.go
new file mode 100644
index 0000000..6767961
--- /dev/null
+++ b/misc/open_file.go
@@ -0,0 +1,75 @@
+package misc
+
+import (
+	"os"
+	"errors"
+	"strings"
+	"syscall"
+	"path"
+)
+
+type Dir_t struct {
+	fd int
+	name string
+}
+
+var (
+	Err_illegal_filename = errors.New("illegal filename")
+	Err_invalid_file_descriptor = errors.New("invalid file descriptor")
+)
+
+// Open a file at exactly the given directory.
+func Open_file_at(dir Dir_t, filename string, flags int, perms os.FileMode) (*os.File, error) {
+	if strings.IndexByte(filename, '/') != -1 {
+		return nil, Err_illegal_filename
+	}
+	return Open_file_beneath(dir, filename, flags, perms)
+}
+
+// Open a file at or beneath the given directory.
+func Open_file_beneath(dir Dir_t, filename string, flags int, perms os.FileMode) (*os.File, error) {
+	fd, err := Openat2(dir.fd, filename, &Open_how_t{
+		Flags: uint64(flags),
+		Mode: uint64(syscallMode(perms)),
+		Resolve: RESOLVE_BENEATH|RESOLVE_NO_SYMLINKS,
+	})
+	if err != nil {
+		return nil, err
+	}
+	file := os.NewFile(uintptr(fd), path.Join(dir.name, filename))
+	if file == nil {
+		return nil, Err_invalid_file_descriptor
+	} else {
+		return file, nil
+	}
+}
+
+// Open a directory as read-only and return a Dir_t to it. The caller is
+// responsible for closing the directory with [[Close_directory]].
+func Open_directory_readonly(path string) (Dir_t, error) {
+	_fd, err := syscall.Open(path, os.O_RDONLY|syscall.O_DIRECTORY, 0)
+	return Dir_t{fd: _fd, name: path}, err
+}
+
+// Close a directory returned by [[Open_directory_readonly]].
+func Close_directory(dir Dir_t) error {
+	return syscall.Close(dir.fd)
+}
+
+// syscallMode returns the syscall-specific mode flags from Go's portable mode flags.
+func syscallMode(i os.FileMode) (o uint32) {
+	// This part actually came from Go's os/file_posix.go but IMO it's too
+	// small and should fall under fair use.
+	o |= uint32(i.Perm())
+	if i&os.ModeSetuid != 0 {
+		o |= syscall.S_ISUID
+	}
+	if i&os.ModeSetgid != 0 {
+		o |= syscall.S_ISGID
+	}
+	if i&os.ModeSticky != 0 {
+		o |= syscall.S_ISVTX
+	}
+	// No mapping for Go's ModeTemporary (plan9 only).
+	return
+}