Implement Health service

[clintonpi]

Motivation:

Implement the Health service defined in Protos/upstream/grpc/health/v1/health.proto.

Modifications:

• Implement the "check" and "watch" methods for the service.
• Implement providers (public APIs) to interact with the service.
• Add tests.

Result:

The Health service will be available for use in grpc-swift.

[glbrntt]

This should be .v6

[glbrntt]

This should be Sendable

[glbrntt]

Can we nest this within Health?

extension Health {
  public struct Provider {
    // ...
  }
}

[glbrntt]

Can we nest this within Health?

extension Health {
  public struct Service {
    // ...
  }
}

[glbrntt]

Also this can be a struct and should be Sendable

[glbrntt]

We don't need a continuation here, we can just get the first N elements from the response stream and collect them into an array and then return that from the RPC:

try await response.messages.prefix(N).reduce(into: []) { $0.append($1) }

Breaking that down:

// Get an async sequence which only returns the first N messages from `response.messages`
let prefix = response.messages.prefix(N) 

// Wait for the prefix sequence to end (i.e. collect all N elements) and store them in an array:
let reduced = prefix.reduce(into: []) { elements, nextElement in 
  elements.append(nextElement)
}

[glbrntt]

We can then reduce this test to:

try await withThrowingTaskGroup(of: [Status].self) { group in
  let concurrentStreams = 2
  for _ in 0 ..< concurrentStreams {
    group.addTask {
      try await watch()
    }
  }

  for status in statuses {
    // update status
  }

  for try await next in group {
    XCTAssertEqual(next, expected)
  }
}

[glbrntt]

Rather than spelling out the descriptor every time we can add an extension within this file to ServiceDescriptor, e.g.:

extension ServiceDescriptor {
  fileprivate static let echo = ServiceDescriptor(package: "echo", service: "Echo")
}

[glbrntt]

This is a 'value preserving type conversion' so should be:

  package init(_ status: ServingStatus) {

see: https://www.swift.org/documentation/api-design-guidelines/#type-conversion

[glbrntt]

prefer each line doing one thing only:

Suggested change

	switch status.value {
	case .serving: self = .serving
	case .notServing: self = .notServing
	}
	switch status.value {
	case .serving:
	self = .serving
	case .notServing:
	self = .notServing
	}

[glbrntt]

This'll need to be (see also #1995):

Suggested change

	swiftSettings: [.swiftLanguageVersion(.v6), .enableUpcomingFeature("ExistentialAny")]
	swiftSettings: [._swiftLanguageMode(.v6), .enableUpcomingFeature("ExistentialAny")]

[glbrntt]

We don't need to store this here.

[glbrntt]

nit: forService would follow the shape of the API that Dictionary has (which is updateValue(_:forKey:))

Suggested change

	ofService service: ServiceDescriptor
	forService service: ServiceDescriptor

[glbrntt]

No need to call this InternalHealthService, HealthService is fine.

[glbrntt]

Let's avoid the repeated hashing by yielding the value in addContinuation

[glbrntt]

We can early exit here:

guard status != self.currentStatus else { return }

...

[glbrntt]

Can you add a comment as to why we send this status before starting the RPC because that's quite important.

[glbrntt]

You can just do:

let message = Grpc_Health_V1_HealthCheckRequest.with { 
  $0.service = ...
}

[glbrntt]

I don't think this test needs a task group, we should be able to do everything we need within watch:

healthClient.watch(...) { response in 
  var iterator = response.messages.makeAsyncIterator()
  // wait for 'serviceUnknown'
  // send status updates
  // read updates
}

[glbrntt]

/// An implementation of the 'grpc.health.v1.Health' service.

[glbrntt]

/// Provides status updates to the health service.

[glbrntt]

No need to document this

[glbrntt]

This comment doesn't tell us anything new other than the key is the service name. You might want to say something like "The state of each service keyed by the fully qualified service name."

[glbrntt]

It probably makes sense to default this to "unknown"

[glbrntt]

Suggested change

	/// - `ServingStatus.serving` indicates that a service is healthy.
	/// - `ServingStatus.notServing` indicates that a service is unhealthy.
	/// - ``ServingStatus/serving`` indicates that a service is healthy.
	/// - ``ServingStatus/notServing`` indicates that a service is unhealthy.

[glbrntt]

No point creating a ServiceDescriptor to immediately throw it away. All we want is a string for a service which doesn't exist.

Suggested change

	$0.service =
	ServiceDescriptor(package: "does.not", service: "Exist").fullyQualifiedService
	$0.service = "does.not/Exist"

[glbrntt]

This should be removed

[glbrntt]

Should be possible for this to be a struct

[glbrntt]

Can you update the health module and health tests to use explicit imports like in Gus' PR #2003?

I'm happy for this to be done in a separate PR as well (actually it probably needs to be done separately because Gus needs to do the imports on the code-gen too).