memfs: Fix memory FS implementation bug

Now, using memory FS, is possible to seek outside the size of the file and write something, adding '0' between the old file content and the new one.

Author: ajnavarro

memfs/memory.go

@@ -282,7 +282,7 @@ func (f *file) Seek(offset int64, whence int) (int64, error) {
 	case io.SeekStart:
 		f.position = offset
 	case io.SeekEnd:
-		f.position = int64(f.content.Len()) - offset
+		f.position = int64(f.content.Len()) + offset
 	}
 
 	return f.position, nil
@@ -368,6 +368,12 @@ type content struct {
 
 func (c *content) WriteAt(p []byte, off int64) (int, error) {
 	prev := len(c.bytes)
+
+	diff := int(off) - prev
+	if diff > 0 {
+		c.bytes = append(c.bytes, make([]byte, diff)...)
+	}
+
 	c.bytes = append(c.bytes[:off], p...)
 	if len(c.bytes) < prev {
 		c.bytes = c.bytes[:prev]

test/fs_suite.go

@@ -142,6 +142,26 @@ func (s *FilesystemSuite) TestOpenFileReadWrite(c *C) {
 	s.testReadClose(c, f, "quxbar")
 }
 
+func (s *FilesystemSuite) TestSeekToEndAndWrite(c *C) {
+	defaultMode := os.FileMode(0666)
+
+	f, err := s.FS.OpenFile("foo1", os.O_CREATE|os.O_TRUNC|os.O_RDWR, defaultMode)
+	c.Assert(err, IsNil)
+	c.Assert(f.Filename(), Equals, "foo1")
+
+	_, err = f.Seek(10, io.SeekEnd)
+	c.Assert(err, IsNil)
+
+	n, err := f.Write([]byte(`TEST`))
+	c.Assert(err, IsNil)
+	c.Assert(n, Equals, 4)
+
+	_, err = f.Seek(0, io.SeekStart)
+	c.Assert(err, IsNil)
+
+	s.testReadClose(c, f, "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00TEST")
+}
+
 func (s *FilesystemSuite) TestOpenFile(c *C) {
 	defaultMode := os.FileMode(0666)