Docs: add a design document for the Client

Add a document that states the requirements for the Client, and then
proposes a design that fulfills them.

docs/client-design.md

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+# Tntcxx Client Design
+
+## Scope
+
+This document describes the design of the Tntcxx Client. First we state the 
+requirements and use cases we have in mind, and then we propose a design that 
+fulfills them.
+
+## Requirements
+
+### Functional Requirements
+
+We envision Tntcxx to be used primarily as a part of network applications (e.g.,
+an HTTP server), the Tntcxx Client backing requests to Tarantool. Such 
+applications are usually built around a central event processing loop, which is 
+responsible for multiplexing network I/O.
+
+#### Application Event Processing Loop Integration
+
+The first and foremost requirement is that there must be a convenient 
+way to integrate the Tntcxx Client into the event processing loop used by the 
+application. Moreover, the Tntcxx Client must never run the event processing 
+loop itself.
+
+At the same time, since event processing loops are inherently single threaded, 
+we do expect the Tntcxx Client to be used in a multithreaded environment, i.e., 
+when connections and reqeust futures are in different threads. So we do not aim
+to design the Tntcxx Client to be thread-safe.
+
+#### Asynchronous Request Processing
+
+Since the Tntcxx Client is constrained from running the event processing loop, 
+the Tntcxx Client must support asynchronous request processing through 
+application-provided callbacks or futures.  
+
+#### Connection State
+
+The application must be able to check the state of a Tntcxx Client.
+
+#### Connection Error Handling
+
+There must be a convenient way for the application to handle errors arising
+throughout the Tntcxx Client lifecycle. A connection error must be returned 
+through the request callback and through the request object.
+
+#### Request Handling
+
+In order for the application to be able to manage a request, a request object is 
+always returned. The application can check the request status, cancel the 
+request, handle request errors and retrieve the response through this handle 
+(only once). However, if the response was retrieved by other means (either 
+returned through a callback or collected through scatter-gather), the handle
+cannot return the response.
+
+#### Request Status
+
+The application must be able to check the status of a Tntcxx Client request.
+
+#### Request Timeout
+
+Since the Tntcxx Client does not have control over the application's event 
+processing loop, the application must implement its own request timeouts. The 
+application can cancel stale requests.
+
+#### Request Cancelling
+
+The application must be able to cancel a Tntcxx Client request. Cancelling a 
+request explicitly ends the lifetime of the corresponding response.
+
+#### Request Retrying
+
+Since the Tntcxx Client does not have control over the application's event 
+processing loop, the application must implement its own request retrying.
+
+#### Request Fan-Out
+
+A common Tntcxx Client use case is fan-out to multiple Tarantool instances, and 
+collection of responses received after some deadline, and discarding of requests
+that are not ready by the deadline.
+
+#### Response Lifetime
+
+The response lifetime is managed implicitly through the lifetime of the request.
+The response is not copyable. It can be retrieved only once, and the response 
+ownership is moved to the application.
+
+#### Reconnection
+
+The Tntcxx Client must support implicit reconnection with the same session 
+settings it was created with.
+
+#### Connection Pool
+
+TBD.
+
+#### Transactions
+
+TBD.
+
+#### Failover
+
+TBD.
+
+## Design
+
+### I/O Event Providers
+
+```c++
+/** Callback called on a read event. */ 
+using read_ready_cb_f = void (*)(int fd, Data *data);
+/** Callback called on a write event. */
+using write_ready_cb_f = void (*)(int fd, Data *data); 
+
+/**
+ * An I/O event provider encapsulates the notification about events for a 
+ * collection of file descriptors.
+ * 
+ * `Data` is an opaque context type passed to the notification callbacks. 
+ */
+template<class Data *>
+class IOEventProvider {
+public:
+    /**
+     * Register a file descriptor. Returns 0 on success, -1 on failure.
+     */
+	int register(int fd, Data *data, read_ready_cb_f read_ready_cb, 
+		         write_ready_cb_f write_ready_cb);
+
+	/**
+	 * Unregister a file descriptor. Returns 0 on success, -1 on failure.
+	 */
+	int unregister(int fd);
+};
+```
+
+#### Epoll
+
+```c++
+/**
+ * Since `epoll` does not have a facility for storing callbacks, we delegate 
+ * calling the notification callbacks to the application. 
+ * 
+ * In order to distinguish between application file descriptors and tntcxx 
+ * sockets, we provide a wrapper class around the `Data`, which should be passed 
+ * as the `ptr` argument of `epoll_data` to `epoll` by the application for its 
+ * own file descriptors.
+ */
+template<class Data *>
+class EpollIOEventProviderData {
+public:
+    EpollIOEventProviderData(int type, Data *data);
+    
+    /** 
+     * Type of file descriptor, needed for distinguishing application file 
+     * descriptors from tntcxx sockets. 
+     */
+    int type;
+    Data *data;
+};
+
+/**
+ * Encapsulates calling of notification callbacks from epoll for tntcxx. 
+ */
+template<class Data *>
+class EpollIoEventProviderDataTntcxx : public EpollIOEventProviderData<Data *> {
+public:
+    EpollIoEventProviderData(int fd, Data *data, read_ready_cb_f read_ready_cb, write_ready_cb_f write_ready_cb);
+    
+    /** Needs to be called by the application on a read event. */
+    void read_ready();
+    /** Needs to be called by the application on a write event. */
+    void write_ready();
+};
+```
+
+### Connections
+
+```c++
+/**
+ * A connection encapsulates sending requests and receiving responses from one 
+ * Tarantool instance.
+ */
+template<class IOEventProvider>
+class Connection {
+public:
+    Connection(IOEventProvider &net_provider, 
+	           const ConnectionOptions &connection_options);
+
+    /** 
+	 * Return the connection's state.
+	 */
+	enum ConnectionState get_state() const;
+
+	/** 
+	 * If the connection is an erroneous state (see `status`), return the 
+	 * connection error that caused it. The same error is also passed to 
+	 * response callbacks, and the same error is returned by 
+	 * `Request::get_error`. 
+	 */
+	std::optional<ConnectionError> &get_error() & const;
+
+	/* An abstract request's interface. A request object is always returned. */
+	Request some_request(/* options */);
+};
+```
+
+#### Connection State
+
+```c++
+enum ConnectionState {
+	CONNECTION_INITIAL = 0,
+	CONNECTION_AUTH = 1,
+	CONNECTION_ACTIVE = 2,
+	CONNECTION_ERROR = 3,
+	CONNECTION_ERROR_RECONNECT = 4,
+};
+```
+
+#### Connect Options
+
+```c++
+/* Extend `ConnectionOptions` with an option for the reconnection feature. */
+struct ConnectOptions {
+	/* All existing options. */
+
+	/** 
+	 * In the event of a broken connection, the interval in which the stream 
+	 * tries to re-establish the connection. 
+	 */
+	static constexpr size_t DEFAULT_RECONNECTION_INTERVAL = 2;
+	size_t reconnection_interval = DEFAULT_RECONNECTION_INTERVAL;
+};
+```
+
+### Requests
+
+```c++
+/** Encapsulates management of a request issued through a connection. */
+class Request {
+public:
+	/** 
+	 * Callback called when response is ready. Since the callback has a fixed 
+	 * signature, we need to allow for capturing additional context using 
+	 * lambdas. Hence, we use `std::function` for type erasure.
+	 * 
+	 * See `Connection::get_error` for details about the `error` parameter.
+	 */ 
+	using request_cb_f = 
+		std::function<void(Request &&request, Connection &connection)>;
+	
+	/** Set a callback called when the response is ready. */
+	void set_callback(request_cb_f request_cb) &&;
+	
+	/** Return the request's status. */
+	enum RequestStatus get_status();
+	
+	/**
+	 * Return a connection error, if any. See `Connection::get_error` for 
+	 * details.
+	 */
+	std::optional<ConnectionError> &get_error() & const;
+	
+	/**
+	 * Cancel the request, ending the lifetime of the corresponding response.
+	 */
+	void cancel();
+	
+	/** 
+	 * Return the response, if any. A response is available, iff:
+	 * 1. The response has been received by the connection.
+	 * 2. The response has not been dispatched to a callback.
+	 * 3. The response has not already been retrieved previously.
+	 */
+	std::optional<Response> get_response();
+};
+```
+
+#### Request Status
+
+```c++
+enum RequestStatus {
+	REQUEST_SUCCESS = 0,
+	REQUEST_ERROR = -1,
+	REQUEST_IN_PROGRESS = 1,
+};
+```
+
+#### Request Fan-Out
+
+```c++
+/**
+ * A fan-out encapsulates the collection of responses for a collection of 
+ * requests.
+ */
+class FanOut {
+	template<class InputIt>
+	FanOut(MoveIt first, MoveIt last)
+
+	/**
+	 * Return a list of ready request, and cancel the requests for which a 
+	 * response is not available. 
+	 */
+	tnt::List<Request> collect();
+};
+```