handle readahead 403 errors

this usually shouldn't happen because the original read request should
have failed first before we trigger readahead, but eventual consistency
or IAM changes can cause readahead range GET to fail

fixes #243

internal/buffer_pool.go

@@ -375,6 +375,10 @@ func (b *Buffer) Read(p []byte) (n int, err error) {
 	for b.reader == nil && b.err == nil {
 		bufferLog.Debugf("waiting for stream")
 		b.cond.Wait()
+		if b.err != nil {
+			err = b.err
+			return
+		}
 	}
 
 	if b.buf != nil {

internal/goofys_test.go

@@ -1927,3 +1927,27 @@ func (s *GoofysTest) TestRenameOverwrite(t *C) {
 	err = os.Rename(file, rename)
 	t.Assert(err, IsNil)
 }
+
+func (s *GoofysTest) TestRead403(t *C) {
+	s.fs.flags.StatCacheTTL = 1 * time.Minute
+	s.fs.flags.TypeCacheTTL = 1 * time.Minute
+
+	// cache the inode first so we don't get 403 when we lookup
+	in, err := s.LookUpInode(t, "file1")
+	t.Assert(err, IsNil)
+
+	fh, err := in.OpenFile()
+	t.Assert(err, IsNil)
+
+	s.fs.awsConfig.Credentials = credentials.AnonymousCredentials
+	s.fs.sess = session.New(s.fs.awsConfig)
+	s.fs.s3 = s.fs.newS3()
+
+	// fake enable read-ahead
+	fh.seqReadAmount = uint64(READAHEAD_CHUNK)
+
+	buf := make([]byte, 5)
+
+	_, err = fh.ReadFile(0, buf)
+	t.Assert(err, Equals, syscall.EACCES)
+}