From 4b48e464dc2082d2aa3f01fa6bdf8df7c025da0c Mon Sep 17 00:00:00 2001
From: Antonio Jesus Navarro Perez <antonio@sourced.tech>
Date: Tue, 18 Apr 2017 13:38:53 +0200
Subject: [PATCH] memfs: bug fixing

In this PR we fix two different bugs in memfs implementation:

- Using ReadAt, if the provided slice of bytes has a size bigger than the data to read, it must return the result and a io.EOF error.
- ReadAt must not affect to the offset position: https://golang.org/src/io/io.go?s=7545:7620#L203
---
 memfs/memory.go  |  7 ++++---
 memfs/storage.go | 11 ++++++++---
 test/fs_suite.go | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 60 insertions(+), 6 deletions(-)

diff --git a/memfs/memory.go b/memfs/memory.go
index 3f10d13..c9f40bb 100644
--- a/memfs/memory.go
+++ b/memfs/memory.go
@@ -170,8 +170,10 @@ type file struct {
 
 func (f *file) Read(b []byte) (int, error) {
 	n, err := f.ReadAt(b, f.position)
-	if err != nil {
-		return 0, err
+	f.position += int64(n)
+
+	if err == io.EOF && n != 0 {
+		err = nil
 	}
 
 	return n, err
@@ -187,7 +189,6 @@ func (f *file) ReadAt(b []byte, off int64) (int, error) {
 	}
 
 	n, err := f.content.ReadAt(b, off)
-	f.position += int64(n)
 
 	return n, err
 }
diff --git a/memfs/storage.go b/memfs/storage.go
index 03e39c3..0856f82 100644
--- a/memfs/storage.go
+++ b/memfs/storage.go
@@ -185,7 +185,7 @@ func (c *content) WriteAt(p []byte, off int64) (int, error) {
 	return len(p), nil
 }
 
-func (c *content) ReadAt(b []byte, off int64) (int, error) {
+func (c *content) ReadAt(b []byte, off int64) (n int, err error) {
 	size := int64(len(c.bytes))
 	if off >= size {
 		return 0, io.EOF
@@ -196,6 +196,11 @@ func (c *content) ReadAt(b []byte, off int64) (int, error) {
 		l = size - off
 	}
 
-	n := copy(b, c.bytes[off:off+l])
-	return n, nil
+	btr := c.bytes[off : off+l]
+	if len(btr) < len(b) {
+		err = io.EOF
+	}
+	n = copy(b, btr)
+
+	return
 }
diff --git a/test/fs_suite.go b/test/fs_suite.go
index ebe1f2b..e819d69 100644
--- a/test/fs_suite.go
+++ b/test/fs_suite.go
@@ -715,6 +715,54 @@ func (s *FilesystemSuite) TestReadAtOnReadOnly(c *C) {
 	c.Assert(f.Close(), IsNil)
 }
 
+func (s *FilesystemSuite) TestReadAtEOF(c *C) {
+	err := WriteFile(s.FS, "foo", []byte("TEST"), 0644)
+	c.Assert(err, IsNil)
+
+	f, err := s.FS.Open("foo")
+	c.Assert(err, IsNil)
+
+	rf, ok := f.(io.ReaderAt)
+	c.Assert(ok, Equals, true)
+
+	b := make([]byte, 5)
+	n, err := rf.ReadAt(b, 0)
+	c.Assert(err, Equals, io.EOF)
+	c.Assert(n, Equals, 4)
+	c.Assert(string(b), Equals, "TEST\x00")
+
+	err = f.Close()
+	c.Assert(err, IsNil)
+}
+
+func (s *FilesystemSuite) TestReadAtOffset(c *C) {
+	err := WriteFile(s.FS, "foo", []byte("TEST"), 0644)
+	c.Assert(err, IsNil)
+
+	f, err := s.FS.Open("foo")
+	c.Assert(err, IsNil)
+
+	rf, ok := f.(io.ReaderAt)
+	c.Assert(ok, Equals, true)
+
+	o, err := f.Seek(0, io.SeekCurrent)
+	c.Assert(err, IsNil)
+	c.Assert(o, Equals, int64(0))
+
+	b := make([]byte, 4)
+	n, err := rf.ReadAt(b, 0)
+	c.Assert(err, IsNil)
+	c.Assert(n, Equals, 4)
+	c.Assert(string(b), Equals, "TEST")
+
+	o, err = f.Seek(0, io.SeekCurrent)
+	c.Assert(err, IsNil)
+	c.Assert(o, Equals, int64(0))
+
+	err = f.Close()
+	c.Assert(err, IsNil)
+}
+
 func (s *FilesystemSuite) TestReadWriteLargeFile(c *C) {
 	f, err := s.FS.Create("foo")
 	c.Assert(err, IsNil)