[Bug] Diagram editor errors when using named keys to Join

[mxgrey]

The join operation supports joining messages from buffers based on name (to create structs/maps) or based on sequence (to create vectors or tuples). The UI of the diagram editor is made to support either, but an overly eager exception makes it impossible to use named keys.

To reproduce the problem, follow these steps:

• Open the diagram editor
• Create a Join operation
• Create two buffer operations
• Connect each buffer to the join
• Click the edge from either buffer to the join
• Under Slot, change Index to Key

Depending on which buffer you changed, you'll get one of these two exceptions:

• 'a sequential buffer edge must be assigned to an array of buffers'
• 'a keyed buffer edge must be assigned to a keyed buffer selection'

After the exception is thrown, the editor crashes, making it impossible to use keys in the join operation.

Instead of an exception, the change from index to key or vice-versa should either:

1. Update all the other connections into the Join operation to match the Slot type.
2. Highlight that an error exists in the diagram until all the connections match. The ability to export diagrams needs to be blocked or diminished (e.g. can't export the connections of the broken Join) until the issue is resolved.

An alternative solution would be to toggle between named keys and sequential keys from inside the Join operation itself and then update the state of all connections into it accordingly.

As an alternative to any of the above, we could update the BufferSelection schema to support a mix of indices and keys. This is technically something the executor is able to support, but so far there isn't any data structure that makes sense to join with a combination of indices and named fields, so having a mix of the two is almost certainly a user error.

[xiyuoh]

I'd like to add an observation - the diagram editor also crashes when I attempt to connect multiple buffers of different slot types (Index vs. Key) to a single buffer access, without creating any Join operation. This leads me to think that we should tackle this extend the solution beyond Join to also Listen and Buffer Access.

About the BufferSelection approach - it may not be a bad alternative if we add an error message until all the connections match. I understand that there shouldn't be a situation where there is a mix of slot types, but given the step-by-step nature of creating a diagram on the editor, as a user I wouldn't be able to set the input slot for both buffers simultaneously. The intermediate step towards setting both buffers to Key is the cause of the exception, even when the intended outcome is to set a coherent Key slot type.

To reproduce, create this diagram from buffer_schema. The app crashes for me when there is an attempt to connect 2 buffers of different slot types to the same buffer access, which can happen in different sequences of steps:

• Create two Buffer operations, create one Buffer Access operation, connect both Buffers to Buffer Access, then change one of them to Key, OR
• Create two Buffer operations, create one Buffer Access operation, connect one of them to Buffer Access, change its input slot to Key, then connect the other Buffer to the same Buffer Access

[aaronchongth]

#175 mitigates the issue for now. Due to the way the we are baking some of the rules of connections into the diagram editor (slots from buffers for example), this PR is only highlighting the problematic edges for this particular scenario.

The more generic way would be to set up an application-wide provider to allow highlighting edges with issues, so different parts of the program can trigger these visible cues. If yall know of any other connections that would have similar results (connect-able, but errors will arise), let me know, and I'll bake those in together with the more generic highlighting.