Fix #2514, change CFE_MSG_Message from union to struct

Having this abstract type as a "struct" makes it match the Command
and Telemetry abstract types.  Futhermore, it better conveys the
intent that this is an abstract object and should not be directly
used or accessed in other ways.

It may still be implemented as a union underneath (depending on
how MSG module chooses to implement) but that is hidden from
public API.  In the case of the default MSG module implementation,
there are just a handful of cases where it is accessed internally
as bytes, and those are simple enough to do with a cast.

modules/core_api/fsw/inc/cfe_msg_api_typedefs.h

@@ -99,7 +99,7 @@ typedef enum CFE_MSG_PlaybackFlag
 /**
  * \brief cFS generic base message
  */
-typedef union CFE_MSG_Message CFE_MSG_Message_t;
+typedef struct CFE_MSG_Message CFE_MSG_Message_t;
 
 /**
  * \brief cFS command header

modules/core_api/ut-stubs/src/cfe_sb_handlers.c

@@ -348,7 +348,7 @@ void UT_DefaultHandler_CFE_SB_GetUserData(void *UserObj, UT_EntryKey_t FuncKey,
  if (UT_Stub_CopyToLocal(UT_KEY(CFE_SB_GetUserData), &Result, sizeof(Result)) != sizeof(Result))
  {
  BytePtr = (uint8 *)MsgPtr;
- if ((MsgPtr->Byte[0] & 0x10) != 0)
+ if ((*BytePtr & 0x10) != 0)
  {
  HdrSize = sizeof(CFE_MSG_CommandHeader_t);
  }

modules/core_private/ut-stubs/src/ut_support.c

@@ -342,13 +342,15 @@ int32 UT_SoftwareBusSnapshotHook(void *UserObj, int32 StubRetcode, uint32 CallCo
 {
  UT_SoftwareBusSnapshot_Entry_t *Snapshot = UserObj;
  const CFE_MSG_Message_t * MsgPtr = UT_Hook_GetArgValueByName(Context, "MsgPtr", CFE_MSG_Message_t *);
+ const uint8_t * BytePtr;
 
  if (MsgPtr != NULL && Snapshot != NULL)
  {
  ++Snapshot->Count;
  if (Snapshot->SnapshotSize > 0 && Snapshot->SnapshotBuffer != NULL)
  {
- memcpy(Snapshot->SnapshotBuffer, &MsgPtr->Byte[Snapshot->SnapshotOffset], Snapshot->SnapshotSize);
+ BytePtr = (const uint8 *)MsgPtr;
+ memcpy(Snapshot->SnapshotBuffer, &BytePtr[Snapshot->SnapshotOffset], Snapshot->SnapshotSize);
  }
  }
 
@@ -535,7 +537,7 @@ uint16 UT_GetNumEventsSent(void)
 */
 void UT_DisplayPkt(CFE_MSG_Message_t *MsgPtr, size_t size)
 {
- uint8 *BytePtr = MsgPtr->Byte;
+ uint8 *BytePtr = (uint8 *)MsgPtr;
  size_t i;
  size_t BufSize = UT_MAX_MESSAGE_LENGTH;
  char DisplayMsg[UT_MAX_MESSAGE_LENGTH];

modules/msg/fsw/src/cfe_msg_sechdr_checksum.c

@@ -34,7 +34,7 @@
 CFE_MSG_Checksum_t CFE_MSG_ComputeCheckSum(const CFE_MSG_Message_t *MsgPtr)
 {
  CFE_MSG_Size_t PktLen = 0;
- const uint8 * BytePtr = MsgPtr->Byte;
+ const uint8 * BytePtr = (const uint8 *)MsgPtr;
  CFE_MSG_Checksum_t chksum = 0xFF;
 
  /* Message already checked, no error case reachable */

modules/msg/option_inc/default_cfe_msg_hdr_pri.h

@@ -89,10 +89,9 @@ typedef struct
  *
  * This provides the definition of CFE_MSG_Message_t
  */
-union CFE_MSG_Message
+struct CFE_MSG_Message
 {
- CCSDS_SpacePacket_t CCSDS; /**< \brief CCSDS Header (Pri or Pri + Ext) */
- uint8 Byte[sizeof(CCSDS_SpacePacket_t)]; /**< \brief Byte level access */
+ CCSDS_SpacePacket_t CCSDS; /**< \brief CCSDS Header (Pri or Pri + Ext) */
 };
 
 /**

modules/msg/option_inc/default_cfe_msg_hdr_priext.h

@@ -96,10 +96,9 @@ typedef struct
 /**
  * \brief cFS generic base message
  */
-union CFE_MSG_Message
+struct CFE_MSG_Message
 {
- CCSDS_SpacePacket_t CCSDS; /**< \brief CCSDS Header (Pri or Pri + Ext) */
- uint8 Byte[sizeof(CCSDS_SpacePacket_t)]; /**< \brief Byte level access */
+ CCSDS_SpacePacket_t CCSDS; /**< \brief CCSDS Header (Pri or Pri + Ext) */
 };
 
 /**