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Wait for closeFuture instead of close promise in NIOAsyncChannel's executeThenClose #3032

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merged 5 commits into from
Dec 17, 2024
Merged

Wait for closeFuture instead of close promise in NIOAsyncChannel's executeThenClose #3032

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d050010
Wait for closeFuture instead of close promise in executeThenClose
gjcairo Dec 16, 2024
e57de80
Try to fix docs
gjcairo Dec 17, 2024
7ee9d27
Fix up docs
gjcairo Dec 17, 2024
908ff24
Merge branch 'main' into async-channel-executethenclose
gjcairo Dec 17, 2024
01c568c
Add an assertion failure if the channel's closeFuture is failed
gjcairo Dec 17, 2024
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16 changes: 7 additions & 9 deletions Sources/NIOCore/AsyncChannel/AsyncChannel.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -301,15 +301,13 @@ public struct NIOAsyncChannel<Inbound: Sendable, Outbound: Sendable>: Sendable {
}
}

do {
self._outbound.finish()
try await self.channel.close().get()
} catch {
if let error = error as? ChannelError, error == .alreadyClosed {
return result
}
throw error
}
self._outbound.finish()
// We ignore errors from close, since all we care about is that the channel has been closed
// at this point.
self.channel.close(promise: nil)
// `closeFuture` is never failed, so we can ignore the error
try? await self.channel.closeFuture.get()
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Not sure whether we decided to ignore closeFuture errors entirely or not – it seems completely reasonable to ignore them though.

Suspect @FranzBusch has opinions.

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This future should never be failed; if we're considering a failing closeFuture the result of a bad implementation of a Channel but not something that should happen in any other scenario, it seemed reasonable to me to ignore all errors.

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The original goals were:

  1. Make sure we only excited from executeThenClose when the channel is actually closed
  2. Surface any errors if closing fails

Now 1. is still being upheld here since we wait for the closeFuture. It also turns out that closing can't fail and a call to close() must always result in the closeFuture completing. Strictly speaking succeeding actually.

Now we have two options here. We can just assume that the "strict" part is true so there will never be any errors. However, there might be some code out there that actually fails the close future. What if we catch the error and add an assert that triggers if this close future actually fails to help us spot incorrect implementations?

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I agree, it just depends whether we want to help flush out the different implementations or not.

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Fair enough, adding an assertion sounds useful.


return result
}
}
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6 changes: 6 additions & 0 deletions Sources/NIOCore/Channel.swift
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Expand Up @@ -107,6 +107,12 @@ public protocol Channel: AnyObject, ChannelOutboundInvoker, _NIOPreconcurrencySe
var allocator: ByteBufferAllocator { get }

/// The `closeFuture` will fire when the `Channel` has been closed.
///
/// - Important: This future should never be failed: it signals when the channel has been closed, and this action should not fail,
/// regardless of whether the close happenned cleanly or not.
/// If you are interested in any errors thrown during `close` to diagnose any unclean channel closures, you
/// should instead use the future returned from ``ChannelOutboundInvoker/close(mode:file:line:)-7hlgf``
/// or pass a promise via ``ChannelOutboundInvoker/close(mode:promise:)``.
var closeFuture: EventLoopFuture<Void> { get }

/// The `ChannelPipeline` which handles all I/O events and requests associated with this `Channel`.
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