1 files changed, 63 insertions, 0 deletions
diff --git a/src/io/fs/readfile.go b/src/io/fs/readfile.go
new file mode 100644
index 0000000000..7ee9eadac4
--- /dev/null
+++ b/src/io/fs/readfile.go
@@ -0,0 +1,63 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fs
+
+import "io"
+
+// ReadFileFS is the interface implemented by a file system
+// that provides an optimized implementation of ReadFile.
+type ReadFileFS interface {
+	FS
+
+	// ReadFile reads the named file and returns its contents.
+	// A successful call returns a nil error, not io.EOF.
+	// (Because ReadFile reads the whole file, the expected EOF
+	// from the final Read is not treated as an error to be reported.)
+	ReadFile(name string) ([]byte, error)
+}
+
+// ReadFile reads the named file from the file system fs and returns its contents.
+// A successful call returns a nil error, not io.EOF.
+// (Because ReadFile reads the whole file, the expected EOF
+// from the final Read is not treated as an error to be reported.)
+//
+// If fs implements ReadFileFS, ReadFile calls fs.ReadFile.
+// Otherwise ReadFile calls fs.Open and uses Read and Close
+// on the returned file.
+func ReadFile(fsys FS, name string) ([]byte, error) {
+	if fsys, ok := fsys.(ReadFileFS); ok {
+		return fsys.ReadFile(name)
+	}
+
+	file, err := fsys.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	defer file.Close()
+
+	var size int
+	if info, err := file.Stat(); err == nil {
+		size64 := info.Size()
+		if int64(int(size64)) == size64 {
+			size = int(size64)
+		}
+	}
+
+	data := make([]byte, 0, size+1)
+	for {
+		if len(data) >= cap(data) {
+			d := append(data[:cap(data)], 0)
+			data = d[:len(data)]
+		}
+		n, err := file.Read(data[len(data):cap(data)])
+		data = data[:len(data)+n]
+		if err != nil {
+			if err == io.EOF {
+				err = nil
+			}
+			return data, err
+		}
+	}
+}