io/fs: add ReadFile and ReadFileFS

Add ReadFile helper function, ReadFileFS interface, and test.
Add ReadFile method to fstest.MapFS.
Add testing of ReadFile method to fstest.TestFS.

For #41190.

Change-Id: I5b6a41e2e582824e570463b698b635abaa436c32
Reviewed-on: https://go-review.googlesource.com/c/go/+/243912
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

2 files changed, 106 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
+		}
+	}
+}
diff --git a/src/io/fs/readfile_test.go b/src/io/fs/readfile_test.go
new file mode 100644
index 0000000000..0afa334ace
--- /dev/null
+++ b/src/io/fs/readfile_test.go
@@ -0,0 +1,43 @@
+// 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_test
+
+import (
+	. "io/fs"
+	"testing"
+	"testing/fstest"
+	"time"
+)
+
+var testFsys = fstest.MapFS{
+	"hello.txt": {
+		Data:    []byte("hello, world"),
+		Mode:    0456,
+		ModTime: time.Now(),
+		Sys:     &sysValue,
+	},
+}
+
+var sysValue int
+
+type readFileOnly struct{ ReadFileFS }
+
+func (readFileOnly) Open(name string) (File, error) { return nil, ErrNotExist }
+
+type openOnly struct{ FS }
+
+func TestReadFile(t *testing.T) {
+	// Test that ReadFile uses the method when present.
+	data, err := ReadFile(readFileOnly{testFsys}, "hello.txt")
+	if string(data) != "hello, world" || err != nil {
+		t.Fatalf(`ReadFile(readFileOnly, "hello.txt") = %q, %v, want %q, nil`, data, err, "hello, world")
+	}
+
+	// Test that ReadFile uses Open when the method is not present.
+	data, err = ReadFile(openOnly{testFsys}, "hello.txt")
+	if string(data) != "hello, world" || err != nil {
+		t.Fatalf(`ReadFile(openOnly, "hello.txt") = %q, %v, want %q, nil`, data, err, "hello, world")
+	}
+}