os: add TestReadAtOffset

In the Plan 9 kernel, there used to be a bug in the implementation of
the pread syscall, where the channel offset was erroneously updated after
calling pread on a file.

This test verifies that ReadAt is behaving as expected.

Fixes #14534.

Change-Id: Ifc9fd40a1f94879ee7eb09b2ffc369aa2bec2926
Reviewed-on: https://go-review.googlesource.com/22244
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

diff --git a/src/os/os_test.go b/src/os/os_test.go
index de25f26614d452059fef8dba316a8c76b9741c2f..baa2f07fd2b0f322bb4cc025af6fd8206b85eabb 100644 (file)
--- a/src/os/os_test.go
+++ b/src/os/os_test.go
@@ -1376,6 +1376,38 @@ func TestReadAt(t *testing.T) {
        }
 }
 
+// Verify that ReadAt doesn't affect seek offset.
+// In the Plan 9 kernel, there used to be a bug in the implementation of
+// the pread syscall, where the channel offset was erroneously updated after
+// calling pread on a file.
+func TestReadAtOffset(t *testing.T) {
+       f := newFile("TestReadAtOffset", t)
+       defer Remove(f.Name())
+       defer f.Close()
+
+       const data = "hello, world\n"
+       io.WriteString(f, data)
+
+       f.Seek(0, 0)
+       b := make([]byte, 5)
+
+       n, err := f.ReadAt(b, 7)
+       if err != nil || n != len(b) {
+               t.Fatalf("ReadAt 7: %d, %v", n, err)
+       }
+       if string(b) != "world" {
+               t.Fatalf("ReadAt 7: have %q want %q", string(b), "world")
+       }
+
+       n, err = f.Read(b)
+       if err != nil || n != len(b) {
+               t.Fatalf("Read: %d, %v", n, err)
+       }
+       if string(b) != "hello" {
+               t.Fatalf("Read: have %q want %q", string(b), "hello")
+       }
+}
+
 func TestWriteAt(t *testing.T) {
        f := newFile("TestWriteAt", t)
        defer Remove(f.Name())