PosixTest.WriteAtDoesntChangeOffset_AppendMode

samples/memfs/posix_test.go

@@ -187,7 +187,35 @@ func (t *PosixTest) WriteAtDoesntChangeOffset_NotAppendMode() {
 }
 
 func (t *PosixTest) WriteAtDoesntChangeOffset_AppendMode() {
-	AssertTrue(false, "TODO")
+	var err error
+	var n int
+
+	// Create a file in append mode.
+	f, err := os.OpenFile(
+		path.Join(t.dir, "foo"),
+		os.O_RDWR|os.O_APPEND|os.O_CREATE,
+		0600)
+
+	t.toClose = append(t.toClose, f)
+	AssertEq(nil, err)
+
+	// Make it 16 bytes long.
+	err = f.Truncate(16)
+	AssertEq(nil, err)
+
+	// Seek to offset 4.
+	_, err = f.Seek(4, 0)
+	AssertEq(nil, err)
+
+	// Write the range [10, 14).
+	n, err = f.WriteAt([]byte("taco"), 2)
+	AssertEq(nil, err)
+	AssertEq(4, n)
+
+	// We should still be at offset 4.
+	offset, err := getFileOffset(f)
+	AssertEq(nil, err)
+	ExpectEq(4, offset)
 }
 
 func (t *PosixTest) ReadOverlapsEndOfFile() {