Add ReadFile, interface guard, and drone CI

Signed-off-by: jolheiser <john.olheiser@gmail.com>

.drone.yml

@@ -0,0 +1,18 @@
+---
+kind: pipeline
+name: compliance
+trigger:
+  event:
+    - pull_request
+steps:
+  - name: build
+    pull: always
+    image: golang:1.16
+    commands:
+      - make test
+      - make build
+  - name: check
+    pull: always
+    image: golang:1.16
+    commands:
+      - make vet

overlay.go

@@ -6,6 +6,10 @@ import (
 	"path"
 )
 
+// Interface guard
+// fs.ReadFileFS also fulfills fs.FS
+var _ fs.ReadFileFS = (*FS)(nil)
+
 // FS is an overlay File System
 type FS struct {
 	fs      fs.FS
@@ -39,6 +43,14 @@ func (f *FS) Open(name string) (fs.File, error) {
 	return f.fs.Open(name)
 }
 
+// ReadFile reads a file, preferring disk
+func (f *FS) ReadFile(name string) ([]byte, error) {
+	if f.exists(name) {
+		return os.ReadFile(f.apn(name))
+	}
+	return fs.ReadFile(f.fs, name)
+}
+
 // ReadDir reads []fs.DirEntry
 // This method will prefer EMBEDDED, because that is the "real" FS for overlay
 func (f *FS) ReadDir(name string) ([]fs.DirEntry, error) {
@@ -66,6 +78,15 @@ func New(root string, fs fs.FS, opts ...Option) (*FS, error) {
 	return x, nil
 }
 
+// MustNew returns New and panics on error
+func MustNew(root string, fs fs.FS, opts ...Option) *FS {
+	f, err := New(root, fs, opts...)
+	if err != nil {
+		panic(err)
+	}
+	return f
+}
+
 // WithSub sets a fs.Sub for an FS
 func WithSub(sub string) Option {
 	return func(x *FS) (err error) {

overlay_test.go

@@ -2,7 +2,6 @@ package overlay
 
 import (
 	"embed"
-	"io"
 	"io/fs"
 	"os"
 	"strings"
@@ -49,7 +48,7 @@ func TestOverlay(t *testing.T) {
 			}
 			defer fi.Close()
 
-			contents, err := io.ReadAll(fi)
+			contents, err := x.ReadFile(tc.File)
 			if err != nil {
 				t.Log(err)
 				t.FailNow()