Python: Erroneous transmission of non-8-byte-aligned unions within structs

[fabiandiewald]

Hello,

the issue is related to Python API.

Assuming a union
enum Kind { ONE, TWO };
union ExampleUnion switch(Kind) { case ONE: double foo; case TWO: float bar; };

inside a structure
struct NoIssueStruct { double some8ByteVal; ExampleUnion unionData; };

everything is fine. E.g. publishing
Published NoIssueStruct using foo: some8ByteVal: 42.0, unionData: ExampleUnion(discrimitator=Kind.ONE, foo=3.1415)
Published NoIssueStruct using bar: some8ByteVal: 42.0, unionData: ExampleUnion(discrimitator=Kind.TWO, bar=3.1415)
leads at receiver side to
Content of NoIssueExample some8ByteVal: 42.0, unionData: ExampleUnion(discrimitator=Kind.ONE, foo=3.1415)
Content of NoIssueExample some8ByteVal: 42.0, unionData: ExampleUnion(discrimitator=Kind.TWO, bar=3.1414999961853027)

If the union is not 8 byte aligned, like in e.g.
struct AnotherExampleStruct { float some4ByteVal; ExampleUnion unionData; };

data of the union is not received correctly by a subscriber. Sending side:
Published AnotherExampleStruct using foo: some4ByteVal: 42.0, unionData: ExampleUnion(discrimitator=Kind.ONE, foo=3.1415)
Published AnotherExampleStruct using bar: some4ByteVal: 42.0, unionData: ExampleUnion(discrimitator=Kind.TWO, bar=3.1415)

is received as
Content of AnotherExample some4ByteVal: 42.0, unionData: ExampleUnion(discrimitator=Kind.ONE, foo=3.141498565673828)
Content of AnotherExample some4ByteVal: 42.0, unionData: ExampleUnion(discrimitator=Kind.ONE, foo=0.0)

Full example code:
alignmentIssue.zip

It seems proper 8 byte alignment is done for the union itself (in _calc_union_formats inside ddsutil.py), but aligning properly when the unions are within structs fails.