More robust partial data handling

[micimize]

• Adds a path to PartialDataException to surface invalid paths
• Adds acceptPartialData=true to normalizeFragment and normalizeOperation
  to allow for the rejection of structurally incomplete results (i.e. {} != '{ foo: null }')
• denormalizeFragment and denormalizeOperation now have a handleException=true flag which can be disabled to throw rather than return null
• Adds a validateOperationDataStructure closing [normalize feature req] validation and safety utils #102 (handleException=false by default)
• moved returnPartialData to config.allowPartialData in denormalizeNode as partial data is now a cross cutting concern
• add tests and a few docstrings elsewhere

[micimize]

Sorry this does more than #102 specified – I realized there's an issue with write/read round-trips always resulting in valid data, because normalizeOperation will fill in nulls and then denormalizeOperation will accept them.

[micimize]

hmm... validateOperationDataStructure won't work for purely virtualized fields because it ignores typePolicies, but we don't have the ability to do that yet anyways. And once we do we'll be stripping out purely client-side virtualized fields from the query so the validation will stand.

[smkhalsa]

hmm... validateOperationDataStructure won't work for purely virtualized fields because it ignores typePolicies, but we don't have the ability to do that yet anyways. And once we do we'll be stripping out purely client-side virtualized fields from the query so the validation will stand.

Is there a reason why validateOperationDataStructure shouldn't just call denormalizeOperation internally rather than re-implementing it? It seems like it'd resolve this issue and keep things more DRY.

[micimize]

@smkhalsa right you are – did that before implementing the handleException flag.

Went ahead and refactored, added validateFragmentDataStructure and null rejection, along with tests and some docs on my rationale there.

[micimize]

@smkhalsa could you take a look at this when you get the chance? I kind of ended up making zino-hofmann/graphql-flutter#754 fairly large, and it depends on this

[smkhalsa]

LGTM