Tolerate new request after connection error happened#688
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dnadoba
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May 15, 2023
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| switch self.state { | ||
| case .initialized, .closing, .inRequest: | ||
| case .initialized, .inRequest: | ||
| // These states are unreachable as the connection pool state machine has put the | ||
| // connection into these states. In other words the connection pool state machine must | ||
| // be aware about these states before the connection itself. For this reason the | ||
| // connection pool state machine must not send a new request to the connection, if the | ||
| // connection is `.initialized`, `.closing` or `.inRequest` | ||
| preconditionFailure("Invalid state: \(self.state)") | ||
| fatalError("Invalid state: \(self.state)") | ||
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| case .closed: | ||
| case .closing, .closed: | ||
| // The remote may have closed the connection and the connection pool state machine | ||
| // was not updated yet because of a race condition. New request vs. marking connection | ||
| // as closed. |
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This is the actual bug fix.
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Motivation
A connection and the connection pool may be on different threads and therefore run concurrently. If a connection error happens the connections closes itself and notifies the pool about it. While this is happening the connection pool could already decide to run a new connection on the closing connection. We need to tolerate this happening. We already partially do but only if the connection is fully closed but not currently still closing.
Result
preconditionFailures inHTTP1ConnectionStateMachine.swifttofatalErrors to see the message in release builds.closingin addition to.closedinrunNewRequestResult
AHC no longer crashes in the race condition described above.