Experimental Physics and
Industrial Control System

Hi:

The PV Access protocol documentation for the Destroy channel (0x08) message, https://github.com/epics-base/pvAccessCPP/wiki/protocol#destroy-channel-0x08,  claims that the format is CID first, SID second:

struct destroyChannelRequest {
    int clientChannelID;
    int serverChannelID;
};

struct destroyChannelResponse {
    int clientChannelID;
    int serverChannelID;
};

The server implementations are in fact the other way round.
Both 
https://github.com/epics-base/pvAccessCPP/blob/master/src/server/responseHandlers.cpp#L984 
and 
https://github.com/epics-base/epicsCoreJava/blob/master/pvAccessJava/src/org/epics/pvaccess/server/impl/remote/handlers/DestroyChannelHandler.java#L50

basically have this:
    transport->ensureData(8);
    const pvAccessID sid = payloadBuffer->getInt();
    const pvAccessID cid = payloadBuffer->getInt();

The servers also send a reply with SID first, followed by CID.


The client, however, follows the protocol documentation, and takes the first ID to be the CID,
https://github.com/epics-base/pvAccessCPP/blob/master/src/remoteClient/clientContextImpl.cpp#L2934 :

        pvAccessID cid = payloadBuffer->getInt();
     /*pvAccessID sid =*/ payloadBuffer->getInt();


==> Client and server destroy different channels, which results in "Trying to destroy a channel that no longer exists (SID: 4, CID 3, client: 127.0.0.1:51886)" messages and dropped data.

My suggestion:

Update protocol to what the servers are actually do, i.e. 

struct destroyChannelRequest {
    int serverChannelID;
    int clientChannelID;
};

struct destroyChannelResponse {
    int serverChannelID;
    int clientChannelID;
};

No change to already deployed servers.
Clients need to be fixed.

Thanks,
Kay

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