Add PeerMaximumMessageLength option for SSH child channels.

Sources/NIOSSH/Child Channels/ChildChannelOptions.swift

@@ -26,6 +26,9 @@ public struct SSHChildChannelOptions {
 
     /// - seealso: `SSHChannelTypeOption`.
     public static let sshChannelType: SSHChildChannelOptions.Types.SSHChannelTypeOption = .init()
+
+    /// - seealso: `PeerMaximumMessageLengthOption`.
+    public static let peerMaximumMessageLength: SSHChildChannelOptions.Types.PeerMaximumMessageLengthOption = .init()
 }
 
 extension SSHChildChannelOptions {
@@ -53,4 +56,11 @@ extension SSHChildChannelOptions.Types {
 
         public init() {}
     }
+
+    /// `PeerMaximumMessageLengthOption` allows users to query the maximum packet size value reported by the remote peer for a given channel.
+    public struct PeerMaximumMessageLengthOption: ChannelOption {
+        public typealias Value = UInt32
+
+        public init() {}
+    }
 }

Sources/NIOSSH/Child Channels/SSHChildChannel.swift

@@ -222,6 +222,8 @@ extension SSHChildChannel: Channel, ChannelCore {
             // This force-unwrap is safe: we set type before we call the initializer, so
             // users can only get this after this value is set.
             return self.type! as! Option.Value
+        case _ as SSHChildChannelOptions.Types.PeerMaximumMessageLengthOption:
+            return self.peerMaxMessageSize as! Option.Value
         case _ as ChannelOptions.Types.AutoReadOption:
             return self.autoRead as! Option.Value
         case _ as ChannelOptions.Types.AllowRemoteHalfClosureOption:

Tests/NIOSSHTests/ChildChannelMultiplexerTests.swift

@@ -1729,6 +1729,31 @@ final class ChildChannelMultiplexerTests: XCTestCase {
 
         self.assertChannelOpenConfirmation(harness.flushedMessages.first, recipientChannel: 1)
     }
+
+    func testChildChannelMaxMessageLengthOption() throws {
+        let harness = self.harnessForbiddingInboundChannels()
+        defer {
+            harness.finish()
+        }
+
+        var childChannel: Channel?
+        harness.multiplexer.createChildChannel(channelType: .session) { channel, _ in
+            childChannel = channel
+            return channel.eventLoop.makeSucceededFuture(())
+        }
+
+        guard let channel = childChannel else {
+            XCTFail("Did not create child channel")
+            return
+        }
+
+        // Activate channel.
+        let channelID = self.assertChannelOpen(harness.flushedMessages.first)
+        XCTAssertNoThrow(try harness.multiplexer.receiveMessage(self.openConfirmation(originalChannelID: channelID!, peerChannelID: 1, initialWindowSize: 5, maxPacketSize: 4247)))
+        XCTAssertTrue(channel.isWritable)
+
+        XCTAssertEqual(try channel.getOption(SSHChildChannelOptions.peerMaximumMessageLength).wait(), 4247)
+    }
 }
 
 extension ByteBuffer {