Move uint64 fields that are accessed using sync/atomic to the beginni…

…ng of struct to ensure proper alignment in memory

On 32-bit platforms (e.g. Raspberry Pi) it is caller's responsibility to arrange 64-bit alignment for sync/atomic operation parameters, see: https://golang.org/pkg/sync/atomic/#pkg-note-BUG. This commit fixes "SIGSEGV: segmentation violation" errors on Raspberry Pi 3.

tcp/session.go

@@ -18,6 +18,9 @@ import (
 type EventFunc func()
 
 type Session struct {
+	// This field needs to be the first in the struct to ensure proper word alignment on 32-bit platforms.
+	// See: https://golang.org/pkg/sync/atomic/#pkg-note-BUG
+	sequence              uint64
 	connection *coapNet.Conn
 
 	maxMessageSize                  int
@@ -29,7 +32,6 @@ type Session struct {
 	errors                          ErrorFunc
 	closeSocket                     bool
 
-	sequence              uint64
 	tokenHandlerContainer *HandlerContainer
 	midHandlerContainer   *HandlerContainer
 	handler               HandlerFunc

udp/client/clientconn.go

@@ -36,6 +36,9 @@ type Session interface {
 
 // ClientConn represents a virtual connection to a conceptual endpoint, to perform COAPs commands.
 type ClientConn struct {
+	// This field needs to be the first in the struct to ensure proper word alignment on 32-bit platforms.
+	// See: https://golang.org/pkg/sync/atomic/#pkg-note-BUG
+	sequence              uint64
 	session                        Session
 	handler                        HandlerFunc
 	observationTokenHandler        *HandlerContainer
@@ -51,7 +54,6 @@ type ClientConn struct {
 
 	tokenHandlerContainer *HandlerContainer
 	midHandlerContainer   *HandlerContainer
-	sequence              uint64
 }
 
 // NewClientConn creates connection over session and observation.