Avoid assertion for legitimate case where reply received for handler that was closed (#1437)

Avoid assertion for legitimate case where reply received for handler that was closed

Author: rconnell9

Sources/SwiftBuild/SWBBuildServiceConnection.swift

@@ -57,6 +57,12 @@ typealias swb_build_service_connection_message_handler_t = @Sendable (UInt64, SW
 
         /// Track whether the channels have been cleared because the service has crashed.
         var channelsHaveBeenCleared = false
+
+        /// A nil handler for a known channel is expected, service crash or client closed the channel before the reply arrived.
+        /// A nil handler for an unknown channel is unexpected.
+        func isChannelKnown(_ channel: UInt64) -> Bool {
+            return channel <= nextChannelID
+        }
     }
 
     /// An "unfair lock" that protects the channels state.  Should be held only for very short periods of time.
@@ -303,14 +309,6 @@ typealias swb_build_service_connection_message_handler_t = @Sendable (UInt64, SW
                         let channel = headerFrame.channel
                         let msgSize = Int(headerFrame.messageSize)
 
-                        // Look up the channel by its number.
-                        let handler = self.channelState.withLock({ $0.channels[channel] })
-                        if handler == nil {
-                            // It’s an error if we didn’t find a channel here.
-                            // FIXME: We need to handle this error in a better way.
-                            assertionFailure("no handler for channel: \(channel)")
-                        }
-
                         // If we don’t have all the data yet, we can go no further.
                         if offset + msgSize > data.count {
                             // Move the offset back to just before the header, so we’ll see it again next time around.
@@ -320,7 +318,15 @@ typealias swb_build_service_connection_message_handler_t = @Sendable (UInt64, SW
                         }
 
                         // Otherwise, look up the channel by its number.
-                        handler?(channel, data.subdata(in: offset..<(offset + msgSize)))
+                        let (handler, isChannelKnown) = self.channelState.withLock {
+                            return ($0.channels[channel], $0.isChannelKnown(channel))
+                        }
+
+                        if let handler {
+                            handler(channel, data.subdata(in: offset..<(offset + msgSize)))
+                        } else {
+                            assert(isChannelKnown, "Received reply for unknown channel: \(channel)")
+                        }
 
                         // Move on to the next message.
                         offset += msgSize