From 709edc9d8f5dc20bfb6327262b13534f20dfc643 Mon Sep 17 00:00:00 2001
From: Brian Tiger Chow <btc@users.noreply.github.com>
Date: Tue, 20 Nov 2018 13:29:30 -0500
Subject: [PATCH 1/2] add ReadAt/Seek test

---
 mem/file_test.go | 53 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 53 insertions(+)

diff --git a/mem/file_test.go b/mem/file_test.go
index 5769067..bb54db6 100644
--- a/mem/file_test.go
+++ b/mem/file_test.go
@@ -1,6 +1,7 @@
 package mem
 
 import (
+	"io"
 	"testing"
 	"time"
 )
@@ -152,3 +153,55 @@ func TestFileDataSizeRace(t *testing.T) {
 		t.Errorf("Failed to read correct value for dir, was %v", s.Size())
 	}
 }
+
+func TestFileReadAtSeekOffset(t *testing.T) {
+	t.Parallel()
+
+	fd := CreateFile("foo")
+	f := NewFileHandle(fd)
+
+	_, err := f.WriteString("TEST")
+	if err != nil {
+		t.Fatal(err)
+	}
+	offset, err := f.Seek(0, io.SeekStart)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if offset != 0 {
+		t.Fail()
+	}
+
+	offsetBeforeReadAt, err := f.Seek(0, io.SeekCurrent)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if offsetBeforeReadAt != 0 {
+		t.Fatal("expected 0")
+	}
+
+	b := make([]byte, 4)
+	n, err := f.ReadAt(b, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != 4 {
+		t.Fail()
+	}
+	if string(b) != "TEST" {
+		t.Fail()
+	}
+
+	offsetAfterReadAt, err := f.Seek(0, io.SeekCurrent)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if offsetAfterReadAt != offsetBeforeReadAt {
+		t.Fatal("ReadAt should not affect offset")
+	}
+
+	err = f.Close()
+	if err != nil {
+		t.Fatal(err)
+	}
+}

From e151fe9493955bc38d3f4ea834bfe9bdbd6562e9 Mon Sep 17 00:00:00 2001
From: Brian Tiger Chow <btc@users.noreply.github.com>
Date: Tue, 20 Nov 2018 13:34:41 -0500
Subject: [PATCH 2/2] fix ReadAt

---
 mem/file.go | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mem/file.go b/mem/file.go
index 7af2fb5..f99590e 100644
--- a/mem/file.go
+++ b/mem/file.go
@@ -193,8 +193,11 @@ func (f *File) Read(b []byte) (n int, err error) {
 }
 
 func (f *File) ReadAt(b []byte, off int64) (n int, err error) {
+	prev := atomic.LoadInt64(&f.at)
 	atomic.StoreInt64(&f.at, off)
-	return f.Read(b)
+	n, err = f.Read(b)
+	atomic.StoreInt64(&f.at, prev)
+	return
 }
 
 func (f *File) Truncate(size int64) error {