Extended handling of "optional" fields

Proposal:

Make the expression of an "optional" field optional.

When parsing an optional field the parser always saves the current position before starting to parse the field. 

If when parsing the field no "AssertionException" is being caught, this information is just discarded. If however such an exception is caught, then the parser resets the position to the initial position and the next field can continue parsing from that location.

In addition to this, we add a new field type: "assertion" ... these can generally be seen similar to "const" fields. However in contrast to "const" fields these need to be saved as properties in the model. An assert field contains an expression field. If the value parsed matches this, the value is saved in the property. If however this value doesn't match the expected value, an "AssertionException" is thrown.

These changes are intended especially for protocols like BacNET and PROFINET where there are optional fields, based on their content. 

Example:

['0x07' BACnetUnconfirmedServiceRequestWhoHas
    [optional BACnetComplexTagUnsignedInteger 'deviceInstanceRangeLowLimit'                                        ['0', 'BACnetDataType.UNSIGNED_INTEGER' ]]
    [optional BACnetComplexTagUnsignedInteger 'deviceInstanceRangeHighLimit' 'deviceInstanceRangeLowLimit != null' ['1', 'BACnetDataType.UNSIGNED_INTEGER' ]]
    [optional BACnetComplexTagOctetString     'objectIdentifier'                                                   ['2', 'BACnetDataType.OCTET_STRING'     ]]
    [optional BACnetComplexTagOctetString     'objectName'                   'objectIdentifier == null'            ['3', 'BACnetDataType.OCTET_STRING'     ]]
]

[discriminatedType 'BACnetComplexTag' [uint 4 'tagNumberArgument', BACnetDataType 'dataType']
    [assert        uint 4           'tagNumber'                 'tagNumberArgument'                                           ]
    [const         TagClass         'tagClass'                  'TagClass.CONTEXT_SPECIFIC_TAGS'                              ]
    [simple        uint 3           'lengthValueType'                                                                         ]
    [optional      uint 8           'extTagNumber'              'tagNumber == 15'                                             ]
    [virtual       uint 8           'actualTagNumber'           'tagNumber < 15 ? tagNumber : extTagNumber'                   ]
    [virtual       bit              'isPrimitiveAndNotBoolean'  '!(lengthValueType == 6) && tagNumber != 1'                   ]
    [optional      uint 8           'extLength'        'isPrimitiveAndNotBoolean && lengthValueType == 5'                     ]
    [optional      uint 16          'extExtLength'     'isPrimitiveAndNotBoolean && lengthValueType == 5 && extLength == 254' ]
    [optional      uint 32          'extExtExtLength'  'isPrimitiveAndNotBoolean && lengthValueType == 5 && extLength == 255' ]
    [virtual       uint 32          'actualLength'     'lengthValueType == 5 && extLength == 255 ? extExtExtLength : (lengthValueType == 5 && extLength == 254 ? extExtLength : (lengthValueType == 5 ? extLength : (isPrimitiveAndNotBoolean ? lengthValueType : 0)))']
    [typeSwitch 'dataType'
        ['NULL' BACnetComplexTagNull
        ]
        ['BOOLEAN' BACnetComplexTagBoolean
        ]
        ['UNSIGNED_INTEGER' BACnetComplexTagUnsignedInteger [uint 3 'lengthValueType', uint 8 'extLength']
            [array int 8 'data' length '(lengthValueType == 5) ? extLength : lengthValueType']
        ]
        ['SIGNED_INTEGER' BACnetComplexTagSignedInteger [uint 3 'lengthValueType', uint 8 'extLength']
            [array int 8 'data' length '(lengthValueType == 5) ? extLength : lengthValueType']
        ]
        ['REAL' BACnetComplexTagReal [uint 3 'lengthValueType', uint 8 'extLength']
            [simple float 8.23 'value']
        ]
        ['DOUBLE' BACnetComplexTagDouble [uint 3 'lengthValueType', uint 8 'extLength']
            [simple float 11.52 'value']
        ]
        ['OCTET_STRING' BACnetComplexTagOctetString [uint 32 'actualLength']
            // TODO: The reader expects int but uint32 get's mapped to long so even uint32 would easily overflow...
            [virtual    uint    16                           'actualLengthInBit' 'actualLength * 8']
            [simple     string 'actualLengthInBit' 'ASCII'   'theString']
        ]
        ['CHARACTER_STRING' BACnetComplexTagCharacterString
        ]
        ['BIT_STRING' BACnetComplexTagBitString [uint 3 'lengthValueType', uint 8 'extLength']
            [simple uint 8 'unusedBits']
            [array int 8 'data' length '(lengthValueType == 5) ? (extLength - 1) : (lengthValueType - 1)']
        ]
        ['ENUMERATED' BACnetComplexTagEnumerated [uint 3 'lengthValueType', uint 8 'extLength']
            [array int 8 'data' length '(lengthValueType == 5) ? extLength : lengthValueType']
        ]
        ['DATE' BACnetComplexTagDate
        ]
        ['TIME' BACnetComplexTagTime
        ]
        ['BACNET_OBJECT_IDENTIFIER' BACnetComplexTagObjectIdentifier
        ]
    ]
]

Controlling/Changing the endianess

Especially in parts of the PROFINET protocol, the endianess needs to change througout the protocol stack. So our current approach with one fixed endianess doesn't work well in this case. I've tried using manual fields, but the result was everything but ideal. 

In general I've encountered multiple situations:

• One layer of the protocol stack is fixed to a given endianess (UDP is Big Endian, for example, PROFINET IO CM Blocks are Big Endian too)
• Endianess of fields are dependent on some parsed values (In DCE/RPC there is a 4 bit field, that controlls if the following fields are BigEndian or LittleEndian)
• Endianess of all fields of a complex type are dependent on the endianess of the parent (In case of PROFINET IO CM the ProfinetIO CM packet has the same endianess of the DCE/RPC packet that contains it)

In order to address this we discussed 3 options:

• Adding a new block type "endianessSwitch" which sets the endianess of the parser to a given endianess, and hereby reads all the fields it contains using this endianess and then returns back to it's original endianess when leaving the block
• We add a flag to fields that allow setting the endianess for parsing this field (and if it's a complex one ... all of it's children
• We add a flat to the type definition

All options have their advantage/disadvantage.

Adding a flag to the field would simply require us to come up with a sensible notation, which I haven't currently found anything I would feel comfortable with.

With adding a flag to a type: This seems simple, but how would we control this from the usage?

Chris' thoughs on this

I would propose implementing two of these aboe scenarios:

1. We define a new block type of "endianessSwitch" ... this can either take a constant "LE" or "BE" (Or the long form, which might be more explicit) ... or an expression which evaluates to one of the two.
2. We add the BE/LE flag to type definitions (explicitly no dynamic here)

Now to my reasoning:

Usually, it will not only be one field that needs to be switched, so adding a flag to the fields would require duplicating a lot of the flagging-code to the mspec documents. Using a block of equally endianed fields simplifies things. And in addition to that it explicitly focusses on the detail of setting the endianess, which should simplify implementing this in the code-generation ... as a endianessSwitch simply gets converted into an allmost constant try-finally block.

Not allowing dynamic switching in types and simply forcing this to be constant solves the problem of how the system tells the parser which endianess to use. 

So taking into account the 3 scenarios above. This would then be solved the following way:

"One layer of the protocol stack is fixed to a given endianess"

Set the UDP or the PROFINET IO CM Block type definitions to "BE".

"Endianess of fields are dependent on some parsed values"

Use a endinanessSwitch.

"Endianess of all fields of a complex type are dependent on the endianess of the parent"

If the type definition doesn't have any BE/LE flags, it simply uses the endianess of the parent.