Move filesystem code to its own file

fs.go

@@ -0,0 +1,81 @@
+package gmi
+
+import (
+	"errors"
+	"io"
+	"os"
+	"path"
+)
+
+// FileServer errors.
+var (
+	ErrNotAFile = errors.New("gemini: not a file")
+)
+
+// FileServer takes a filesystem and returns a Handler which uses that filesystem.
+// The returned Handler sanitizes paths before handling them.
+func FileServer(fsys FS) Handler {
+	return fsHandler{fsys}
+}
+
+type fsHandler struct {
+	FS
+}
+
+func (fsh fsHandler) Serve(rw *ResponseWriter, req *Request) {
+	path := path.Clean(req.URL.Path)
+	f, err := fsh.Open(path)
+	if err != nil {
+		NotFound(rw, req)
+		return
+	}
+	// TODO: detect mimetype
+	rw.SetMimetype("text/gemini")
+	// Copy file to response writer
+	io.Copy(rw, f)
+}
+
+// TODO: replace with io/fs.FS when available
+type FS interface {
+	Open(name string) (File, error)
+}
+
+// TODO: replace with io/fs.File when available
+type File interface {
+	Stat() (os.FileInfo, error)
+	Read([]byte) (int, error)
+	Close() error
+}
+
+// Dir implements FS using the native filesystem restricted to a specific directory.
+type Dir string
+
+// Open tries to open the file with the given name.
+// If the file is a directory, it tries to open the index file in that directory.
+func (d Dir) Open(name string) (File, error) {
+	p := path.Join(string(d), name)
+	f, err := os.OpenFile(p, os.O_RDONLY, 0644)
+	if err != nil {
+		return nil, err
+	}
+
+	if stat, err := f.Stat(); err == nil {
+		if stat.IsDir() {
+			f, err := os.Open(path.Join(p, "index.gmi"))
+			if err != nil {
+				return nil, err
+			}
+			stat, err := f.Stat()
+			if err != nil {
+				return nil, err
+			}
+			if stat.Mode().IsRegular() {
+				return f, nil
+			}
+			return nil, ErrNotAFile
+		} else if !stat.Mode().IsRegular() {
+			return nil, ErrNotAFile
+		}
+	}
+	return f, nil
+}