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>

1 files changed, 12 insertions, 0 deletions

diff --git a/src/testing/fstest/testfs.go b/src/testing/fstest/testfs.go
index 80ca0e9a1d..5c4f30af16 100644
--- a/src/testing/fstest/testfs.go
+++ b/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 })
 	}