Difference between revisions of "Projects/Windows CCAPI"

Revision as of 13:12, 24 January 2008

RPC design for Windows CCAPI clients and server

The CCAPI is a local service provided to local clients. The implementation is operating system independent, but the transport between the clients and server is not. The transport is what's described here.

The clients are threads of processes belonging to a login session.

The proposal is for a single user; the solution is replicated for each user logged onto the PC.

-Kevin 13:44, 8 January 2008 (EST) This is a cut and paste of the original html document. Responses to Danny Mayer's email exchange have not been incorporated yet.

Conventions & clarifications

The CCAPI client acts as both an RPC client and RPC server and the CCAPI server acts as both an RPC client and RPC server.

• The RPC call from the CCAPI client to the CCAPI server is called the "request." In this mode, the CCAPI client is the RPC client and the CCAPI server is the RPC server. The CCAPI client calls the "request routine" and the RPC mechanism invokes the identically named "request handler" on the CCAPI server.
• The RPC call from the CCAPI server to the CCAPI client is called the "reply." In this mode, the CCAPI client is the RPC server and the CCAPI server is the RPC client. The CCAPI server calls the "reply routine" and the RPC mechanism invokes the identically named "reply handler" on the CCAPI client.

<UUID> is a UUID for a client thread.

<SST> means server start time, a time_t.

<LSID> is a logon-session-specific identifier.

Design Requirements

• The server's OS-independent code is single threaded, because it must operate on platforms that do not allow multiple threads.
• The client and server must be able to maintain connections, where state is maintained between individual messages.
• Individual messages must be handled in a single threaded server.
• The server must be able to detect when a client dies, so that any connection state can be cleaned up.

Design

The design of server endpoint names, server authentication and RPC thread safety is carried over from the previous CCAPI implementation. References to terminology from that implementation are annotated with '[CC-]' and assume some familiarity with the old code.

References to terminology from the new os-independent CCAPI implementation are annotated '[CC+]' and assume some familiarity with the new code.

The server and each client create an RPC endpoint. The server's endpoint is krbcc.<LSID>.ep [CC-] and the client's endpoint is CCAPI_<UUID>.

The server's ccs_pipe_t type [CC+] is a struct containing the UUID and a handle to the client's TSP. <TODO: Move this to implementation details.>

How is the request handled in the server and the reply sent to the client?

One straightforward way is for the reply to be the returned data in the request RPC call (an [out] parameter). That is, data passed from the RPC server to the RPC client. The request handler calls ccs_server_handle_request. Eventually, the server code calls ccs_os_server_send_reply, which saves the reply somewhere. When the server eventually returns to the request handler, the handler returns the saved reply to the client.

But this doesn't work. Consider client A taking a lock and then client B asking for the same lock. A single threaded server waiting for the lock on behalf of B will never be able to process A's unlock request.

Instead, the server asynchronously sends a reply for each request. Each client must wait for a reply to its request.

This will resolve the deadlock described above. The server will complete B's request. B will wait for a separate reply from the server that indicates that the lock has been acquired. The server will be free to process A's unlock request.

More details:

The Windows RPC mechanism runs each RPC procedure in its own thread.

Client threads use Microsoft Thread Local Storage to store a pointer to the client's thread-specific data [TSP].

Request and reply RPC calls include the client's UUID and TSP*.

The request handler in the CCAPI server builds a struct describing the request and adds it to an interlocked queue. This enables the server to handle simultaneous requests and requests that overlap with operations.

The server's main thread removes requests from the queue and processes them sequentially. The call into ccs_server_handle_request is the interface to the OS-independent CCAPI code [OSIC]. This call includes the request data and a ccs_pipe_t [CC+]. The Windows ccs_pipe_t is a ccs_win_pipe_t*. ccs_win_pipe_t is a struct containing the UUID and TSP*.

Eventually, the processing initiated in ccs_server_handle_request produces a call to ccs_os_server_send_reply. The OSIC passed the ccs_pipe_t along and ccs_os_server_send_reply makes the client endpoint from the included UUID and calls ccs_rpc_reply.

The client's cci_os_ipc function calls ccs_rpc_request and then waits for ccs_rpc_reply to set an event flag. How does the reply data go from the reply procedure thread to the client thread? One field in the TSP is a cci_stream_t*. The reply procedure builds the stream from the reply data, stores the cci_stream_t* in the TSP and sets a UUID-based event flag. Then the client can retrieve the stream* from the TSP.

Connections

Client state exists on the server. The server needs to know when the client is finished so it can clean up the client's state.

The Windows CCAPI client code manages a 'connection' to the server. This happens automatically, without any participation from the client application. The connection is established in cci_os_ipc() [CC+]when no connection was previously established. This 'lazy' technique avoids creating connections for client threads that do not use the CCAPI.

The connection is discarded when the client thread exits.

Detecting client exit

The server must be able to detect when clients disappear, so the server can free any resources that had been held for the client.

The Windows RPC API does not appear to provide a notification for an endpoint disappearing. It does provide a way to ask if an endpoint is listening. This is useful for polling, but we want a better performing solution than that.

The client has an isAlive function on its endpoint.

To detect the client disappearing without using polling, the server makes an asynchronous call to the isAlive function on the client's endpoint. The isAlive function never returns. When the client exits for any reason, it's endpoint will be closed and the server's function call will return an error. The asynchronous call on the server means no additional threads are used.

Windows provides a number of notification methods to signal I/O completion. Among them are I/O completion ports and callback functions. I chose callback functions because they appear to consume fewer resources.

RPC Endpoint / Function summary

• The server creates one CCS_<LSID> endpoint to listen for connection requests and client requests. It has the functions
  • ccs_rpc_connect(msgtype, UUIDlen, <UUID>, status)
  • ccs_rpc_request(msgtype, UUIDlen, <UUID>, msglen, msg, SST, status) called by client. NB: The windows server sets the in_client_pipe to the in_reply_pipe.

• Each client thread creates a CCAPI_<UUID> endpoint. It has the functions
  • isAlive [function never returns.]
  • ccs_rpc_request_reply(msgtype, SST, replylen, reply, status)
  • ccs_rpc_connect_reply(msgtype, SST, status

Windows-specific implementation details

Client CCAPI library initialization:

This code runs when the CCAPI DLL is loaded.

• <placeholder>

Client initialization:

This code runs when cci_os_ipc_thread_init is called:

• Generate <UUID> and save in thread-specific storage. This serves as the client ID / ccs_pipe_t.
• Create client endpoint.
• Listen on client endpoint.
• Create canonical server connection endpoint from the <LSID>, which the client and server should have in common.
• Test if server is listening to the CCS_<LSID> endpoint.
  • If not, quit. (! Start it?)
• Call ccs_connect(<UUID>) on the CCS_<LSID> endpoint.
• Save SST in thread-specific storage.

Server initialization:

[old]

Server is initialized by client starting a new process. There should be only one server process per Windows username. [new]

• Server is started by kfwlogon (as is done currently).
• Capture server start time (SST).
• Start listener thread, create listener endpoint, listen on CCS_<LSID> endpoint.

Establishing a connection:

• Client calls ccs_connect(<UUID>) on server's CCS_<LSID> endpoint.
• Client gets back and stores SST in thread-specific storage.
• If new connection, server ...
  • adds connection to connection table
  • calls isAlive on CCAPI_<UUID>.
    • NB: isAlive never returns.

Client request:

The server's reply to the client's request is not synchronous.

• Client calls ccs_rpc_request(msglen, msg, msgtype, UUIDlen, <UUID>, SST, status) on server's endpoint.
• Server listen thread receives message, queues request.
• Server worker thread dequeues request, processes, calls ccs_rpc_reply(replylen, reply, msgtype, status) on CCAPI_<UUID>.
• Server checks SST. If server's SST is different, it means server has restarted since client created connection.
• Client receives reply.

Detecting client exit

• When connection created, client created an endpoint.
• Server calls isAlive on client's endpoint.
• When isAlive returns, the server's notification callback will be called. Call back routine queues a DISCONNECT pseudo-message. When the server's worker thread handles the DISCONNECT, it will release connection resources.

Detecting server exit

• Client's call to ccs_rpc_request will return an error if the server has gone away.

To do

Code / implementation

Notes to myself:

• Recover from weird error in RpcMgmtIsServerListening
• Convert makefiles to work with existing build system

Administration

Explorations

• Verify: If a user has tickets and then starts a process with elevated privileges, the elevated process will not be able to communicate with the pre-existing credential cache.

Review

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Approvals

Discussion