LibSQL+SQLServer: Build SQLServer system service

This patch introduces the SQLServer system server. This service is
supposed to be the only process/application talking to database storage.
This makes things like locking and caching more reliable, easier to
implement, and more efficient.

In LibSQL we added a client component that does the ugly IPC nitty-
gritty for you. All that's needed is setting a number of event handler
lambdas and you can connect to databases and execute statements on them.

Applications that wish to use this SQLClient class obviously need to
link LibSQL and LibIPC.

Userland/Services/SQLServer/ClientConnection.cpp

@@ -0,0 +1,84 @@
+/*
+ * Copyright (c) 2021, Jan de Visser <jan@de-visser.net>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/String.h>
+#include <AK/Vector.h>
+#include <LibSQL/SQLResult.h>
+#include <SQLServer/ClientConnection.h>
+#include <SQLServer/DatabaseConnection.h>
+#include <SQLServer/SQLStatement.h>
+
+namespace SQLServer {
+
+static HashMap<int, RefPtr<ClientConnection>> s_connections;
+
+RefPtr<ClientConnection> ClientConnection::client_connection_for(int client_id)
+{
+    if (s_connections.contains(client_id))
+        return *s_connections.get(client_id).value();
+    dbgln_if(SQLSERVER_DEBUG, "Invalid client_id {}", client_id);
+    return nullptr;
+}
+
+ClientConnection::ClientConnection(AK::NonnullRefPtr<Core::LocalSocket> socket, int client_id)
+    : IPC::ClientConnection<SQLClientEndpoint, SQLServerEndpoint>(*this, move(socket), client_id)
+{
+    s_connections.set(client_id, *this);
+}
+
+ClientConnection::~ClientConnection()
+{
+}
+
+void ClientConnection::die()
+{
+    s_connections.remove(client_id());
+}
+
+Messages::SQLServer::ConnectResponse ClientConnection::connect(String const& database_name)
+{
+    dbgln_if(SQLSERVER_DEBUG, "ClientConnection::connect(database_name: {})", database_name);
+    auto database_connection = DatabaseConnection::construct(database_name, client_id());
+    return { database_connection->connection_id() };
+}
+
+void ClientConnection::disconnect(int connection_id)
+{
+    dbgln_if(SQLSERVER_DEBUG, "ClientConnection::disconnect(connection_id: {})", connection_id);
+    auto database_connection = DatabaseConnection::connection_for(connection_id);
+    if (database_connection)
+        database_connection->disconnect();
+    else
+        dbgln("Database connection has disappeared");
+}
+
+Messages::SQLServer::SqlStatementResponse ClientConnection::sql_statement(int connection_id, String const& sql)
+{
+    dbgln_if(SQLSERVER_DEBUG, "ClientConnection::sql_statement(connection_id: {}, sql: '{}')", connection_id, sql);
+    auto database_connection = DatabaseConnection::connection_for(connection_id);
+    if (database_connection) {
+        auto statement_id = database_connection->sql_statement(sql);
+        dbgln_if(SQLSERVER_DEBUG, "ClientConnection::sql_statement -> statement_id = {}", statement_id);
+        return { statement_id };
+    } else {
+        dbgln("Database connection has disappeared");
+        return { -1 };
+    }
+}
+
+void ClientConnection::statement_execute(int statement_id)
+{
+    dbgln_if(SQLSERVER_DEBUG, "ClientConnection::statement_execute_query(statement_id: {})", statement_id);
+    auto statement = SQLStatement::statement_for(statement_id);
+    if (statement && statement->connection()->client_id() == client_id()) {
+        statement->execute();
+    } else {
+        dbgln_if(SQLSERVER_DEBUG, "Statement has disappeared");
+        async_execution_error(statement_id, (int)SQL::SQLErrorCode::StatementUnavailable, String::formatted("{}", statement_id));
+    }
+}
+
+}