Source File
	readfile.go

Belonging Package
	io/fs

// 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.)
	//
	// The caller is permitted to modify the returned byte slice.
	// This method should return a copy of the underlying data.
	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
		}
	}
}

The pages are generated with Golds v0.6.7. (GOOS=linux GOARCH=amd64)
Golds is a Go 101 project developed by Tapir Liu.
PR and bug reports are welcome and can be submitted to the issue list.
Please follow @Go100and1 (reachable from the left QR code) to get the latest news of Golds.