fix(transfer_engine): Add notify callback registration in RPC metadata handling

[iBenzene]

If the registration for the Notify callback function is not added in the addRpcMetaEntry function, the P2PHANDSHAKE method will not work. An example is the testing program for the NIXL backend Mooncake integration plugin.

I1025 16:48:45.144167 87366 transfer_metadata_plugin.cpp:752] SocketHandShakePlugin: received handshake message of type 2, json string length: 38, json string content: {"name":"Agent1","notify_msg":"test"}
I1025 16:48:45.144309 87366 transfer_metadata_plugin.cpp:778] SocketHandShakePlugin: [RECEIVER] Before callback, local is empty
I1025 16:48:45.144345 87366 transfer_metadata_plugin.cpp:780] SocketHandShakePlugin: [RECEIVER] After callback, local content: null
I1025 16:48:45.144373 87366 transfer_metadata_plugin.cpp:790] SocketHandShakePlugin: [RECEIVER] Sending reply, type=2, json length=5, content: null
I1025 16:48:45.144551 87363 transfer_metadata_plugin.cpp:1035] SocketHandShakePlugin: [SENDER] Received reply, type=2, json length=5, content: null
I1025 16:48:45.144593 87363 transfer_metadata_plugin.cpp:1047] SocketHandShakePlugin: received notify message: null
E1025 16:48:45.144726 87363 transfer_metadata.cpp:745] Notify rejected by 192.168.0.27:16430:

[gemini-code-assist]

Summary of Changes

Hello @iBenzene, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical bug in the transfer engine where the P2P handshake process was failing because a necessary Notify callback was not being registered. By adding the registration for this callback within the RPC metadata handling logic, the change ensures that the P2P handshake can complete successfully, thereby enabling proper peer-to-peer communication and resolving a key functional impediment.

Highlights

• P2P Handshake Fix: Resolved an issue where the P2P handshake mechanism would fail due to the Notify callback not being registered, preventing proper communication.
• Callback Registration: Ensured that the registerOnNotifyCallBack function is called within the addRpcMetaEntry method when p2p_handshake_mode_ is active, allowing the receivePeerNotify function to be properly handled.
• Daemon Start Order: Reordered the startDaemon call to occur after both OnMetadataCallBack and OnNotifyCallBack registrations, ensuring all necessary callbacks are set up before the daemon begins.

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[gemini-code-assist]

Code Review

This pull request fixes an issue where the P2PHANDSHAKE method would fail by adding the missing registration for the Notify callback. The change also correctly moves the startDaemon call to after all callbacks are registered, which is a good practice to prevent race conditions. I've added one suggestion to improve robustness by checking for a null handshake_plugin_ before use, preventing a potential crash.