Add error property to "degraded" events
#6188
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mcmire
changed the title
error property to "degraded" events
| [Data] extends [void] | ||
| ? (data: AdditionalData) => void | ||
| : (data: Data & AdditionalData) => void | ||
| ? (data: Data extends void ? AdditionalData : Data & AdditionalData) => void |
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This was an interesting one to figure out.
The purpose of this utility type is to extend the type for a Cockatiel event with additional payload data. We need to do this because NetworkController:rpcEndpointDegraded and NetworkController:rpcEndpointAvailable are based on existing Cockatiel events but with additional properties we add.
However, this type did not properly account for when EventListener was CockatielEvent<FailureReason<unknown> | void> (which is the same thing as saying (data: void | { error: Error } | { value: unknown }) => void).
If we say the following
AddToCockatielEventData<
(data: void | { error: Error } | { value: unknown }) => void,
{ foo: 'bar' }
>we want this to resolve to:
(data: { foo: 'bar' } | ({ error: Error } & { foo: 'bar' }) | ({ value: unknown } & { foo: 'bar' })) => void(Essentially, we want the void to be treated as {}.)
But instead this was resolving as:
(data: ((void | { error: Error } | { value: unknown }) & { foo: 'bar' }) => voidwhich distributes to:
(data: (void & { foo: 'bar' }) | ({ error: Error } & { foo: 'bar' }) | ({ value: unknown } & { foo: 'bar' })) => voidThis is incorrect, because void & { foo: 'bar' } doesn't make sense.
What we want is for TypeScript to distribute void | { error: Error } | { value: unknown } over the condition. So first, despite the comment here, we don't want [Data] extends [void], because that prevents TypeScript from applying the distribution.
But the second thing is that even if we say Data extends void then TypeScript still won't apply the distribution, because the condition is actually in the wrong place. What we want is not a type union of three function types, but a single type with a type union for the arguments.
Even if the condition were in the place we wanted it, TypeScript seems to distribute the type union only in a particular circumstance. Looking a bit deeper at the TypeScript 2.8 release notes where this concept was introduced, it says: "Distributive conditional types are automatically distributed over union types during instantiation." In this case, we also need to place the condition so it becomes a part of the type that we want to "return" from this utility type and is not used to simply determine the return type. This way TypeScript evaluates the condition when that return type is used and not beforehand.
Explanation
We would like to start tracking the HTTP status code in metrics when the
NetworkController:rpcEndpointDegradedevent is published because the maximum number of retries for a request has been exceeded. The HTTP status is stored in the error that Cockatiel captures, so we just need to include it in the event payload.References
Closes #6190.
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