Fix #1949, update msgid testcase to match implementation

Updates the test case for "CFE_MSG_SetMsgId()" to write a value that
is outside the set of storable values for MsgId.  This test has a
caveat that it is somewhat implementation-dependent, but by passing
a MsgId value with all bits set, at least one of those bits is
likely not correlated with a real header bit.

modules/cfe_testcase/src/message_id_test.c

@@ -45,7 +45,20 @@ void TestMsgId(void)
     UtAssert_UINT32_EQ(msgid, expectedmsgid);
 
     UtAssert_INT32_EQ(CFE_MSG_SetMsgId(NULL, msgid), CFE_MSG_BAD_ARGUMENT);
-    UtAssert_INT32_EQ(CFE_MSG_SetMsgId(&msg, CFE_SB_INVALID_MSG_ID), CFE_MSG_BAD_ARGUMENT);
+
+    /*
+     * The purpose of this test is to attempt to set a MsgId beyond the set of values that can
+     * be stored in the MSG header. However the criteria for being "storable" depends on the
+     * actual MSG implementation, and may be different in other implementations.  Currently both
+     * "v1" and "v2" implementations do define a specific highest MsgId value, but another
+     * implementation might not have a highest number concept at all.
+     *
+     * By passing the value of -1, when converted to a an unsigned value (either 16 or 32 bit)
+     * it should translate to a MsgId value with all bits being set.  In theory, at least some of
+     * those bits will be not mappable to the packet header bits, and it should therefore elicit
+     * the CFE_MSG_BAD_ARGUMENT response.
+     */
+    UtAssert_INT32_EQ(CFE_MSG_SetMsgId(&msg, CFE_SB_ValueToMsgId(-1)), CFE_MSG_BAD_ARGUMENT);
 
     UtAssert_INT32_EQ(CFE_MSG_GetMsgId(NULL, &msgid), CFE_MSG_BAD_ARGUMENT);
     UtAssert_INT32_EQ(CFE_MSG_GetMsgId(&msg, NULL), CFE_MSG_BAD_ARGUMENT);

modules/core_api/fsw/inc/cfe_msg.h

@@ -670,6 +670,11 @@ CFE_Status_t CFE_MSG_GetMsgId(const CFE_MSG_Message_t *MsgPtr, CFE_SB_MsgId_t *M
  *        This API only sets the bits in the header that make up the message ID.
  *        No other values in the header are modified.
  *
+ *        The user should ensure that this function is only called with a valid
+ *        MsgId parameter value.  If called with an invalid value, the results
+ *        are implementation-defined.  The implementation may or may not return
+ *        the error code #CFE_MSG_BAD_ARGUMENT in this case.
+ *
  * \param[in, out]  MsgPtr      A pointer to the buffer that contains the message @nonnull.
  * \param[in]       MsgId       Message id
  *