os: implement fs.ReadFileFS for DirFS

Use the os.ReadFile implementation to handle
sysfs files not reporting size properly via stat.

Fixes #53761

Change-Id: I6f34515e8a211e3659f4f6c3598fae7ec0c86975
Reviewed-on: https://go-review.googlesource.com/c/go/+/416775
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Rob Pike <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Rob Pike <[email protected]>
Reviewed-by: hopehook <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>

Author: seankhliao

src/os/file.go

@@ -652,6 +652,18 @@ func (dir dirFS) Open(name string) (fs.File, error) {
 	return f, nil
 }
 
+// The ReadFile method calls the [ReadFile] function for the file
+// with the given name in the directory. The function provides
+// robust handling for small files and special file systems.
+// Through this method, dirFS implements [io/fs.ReadFileFS].
+func (dir dirFS) ReadFile(name string) ([]byte, error) {
+	fullname, err := dir.join(name)
+	if err != nil {
+		return nil, &PathError{Op: "readfile", Path: name, Err: err}
+	}
+	return ReadFile(fullname)
+}
+
 func (dir dirFS) Stat(name string) (fs.FileInfo, error) {
 	fullname, err := dir.join(name)
 	if err != nil {

src/os/os_test.go

@@ -3114,6 +3114,23 @@ func TestReadFileProc(t *testing.T) {
 	}
 }
 
+func TestDirFSReadFileProc(t *testing.T) {
+	t.Parallel()
+
+	fsys := DirFS("/")
+	name := "proc/sys/fs/pipe-max-size"
+	if _, err := fs.Stat(fsys, name); err != nil {
+		t.Skip()
+	}
+	data, err := fs.ReadFile(fsys, name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(data) == 0 || data[len(data)-1] != '\n' {
+		t.Fatalf("read %s: not newline-terminated: %q", name, data)
+	}
+}
+
 func TestWriteStringAlloc(t *testing.T) {
 	if runtime.GOOS == "js" {
 		t.Skip("js allocates a lot during File.WriteString")