Add reload.FS for hot reloading during development

fs.go

@@ -0,0 +1,87 @@
+package buffalo
+
+import (
+	"fmt"
+	"io"
+	"io/fs"
+	"os"
+)
+
+// FS wraps a directory and an embed FS that are expected to have the same contents.
+// it prioritizes the directory FS and falls back to the embedded FS if the file cannot
+// be found on disk. This is useful during development or when deploying with
+// assets not embedded in the binary.
+//
+// Additionally FS hiddes any file named embed.go from the FS.
+type FS struct {
+	embed fs.FS
+	dir   fs.FS
+}
+
+// NewFS returns a new FS that wraps the given directory and embedded FS.
+// the embed.FS is expected to embed the same files as the directory FS.
+func NewFS(embed fs.ReadDirFS, dir string) FS {
+	return FS{
+		embed: embed,
+		dir:   os.DirFS(dir),
+	}
+}
+
+// Open implements the FS interface.
+func (f FS) Open(name string) (fs.File, error) {
+	if name == "embed.go" {
+		return nil, fs.ErrNotExist
+	}
+	file, err := f.getFile(name)
+	if name == "." {
+		return rootFile{file}, err
+	}
+	return file, err
+}
+
+func (f FS) getFile(name string) (fs.File, error) {
+	file, err := f.dir.Open(name)
+	if err == nil {
+		return file, nil
+	}
+
+	return f.embed.Open(name)
+}
+
+// rootFile wraps the "." directory for hidding the embed.go file.
+type rootFile struct {
+	fs.File
+}
+
+// ReadDir implements the fs.ReadDirFile interface.
+func (f rootFile) ReadDir(n int) (entries []fs.DirEntry, err error) {
+	dir, ok := f.File.(fs.ReadDirFile)
+	if !ok {
+		return nil, fmt.Errorf("%T is not a directory", f.File)
+	}
+
+	if n <= 0 {
+		entries, err = dir.ReadDir(n)
+		entries = hideEmbedFile(entries)
+	} else {
+		entries, err = dir.ReadDir(n + 1)
+		entries = hideEmbedFile(entries)
+		if len(entries) > n {
+			entries = entries[:n-1]
+		}
+	}
+
+	if len(entries) == 0 {
+		return entries, io.EOF
+	}
+	return entries, err
+}
+
+func hideEmbedFile(entries []fs.DirEntry) []fs.DirEntry {
+	for i, entry := range entries {
+		if entry.Name() == "embed.go" {
+			entries = append(entries[:i], entries[i+1:]...)
+		}
+	}
+	return entries
+}