io/fs: document that caller can modify slice returned by ReadFile

Also add a test to testing/fstest.

Fixes #45186

Change-Id: I00e5f46ccd5269dbc266a8f2ebc9a62ebb1297b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/311649
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/io/fs/readfile.go

@@ -15,6 +15,9 @@ type ReadFileFS interface {
 	// 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)
 }
 

src/testing/fstest/testfs.go

@@ -537,6 +537,18 @@ func (t *fsTester) checkFile(file string) {
 		}
 		t.checkFileRead(file, "ReadAll vs fsys.ReadFile", data, data2)
 
+		// Modify the data and check it again. Modifying the
+		// returned byte slice should not affect the next call.
+		for i := range data2 {
+			data2[i]++
+		}
+		data2, err = fsys.ReadFile(file)
+		if err != nil {
+			t.errorf("%s: second call to fsys.ReadFile: %v", file, err)
+			return
+		}
+		t.checkFileRead(file, "Readall vs second fsys.ReadFile", data, data2)
+
 		t.checkBadPath(file, "ReadFile",
 			func(name string) error { _, err := fsys.ReadFile(name); return err })
 	}