Icinga DB Check: fix race-condition with IcingaDB::Start()

IcingaDB::GetConnection() uses IcingaDB::m_Rcon which is only initialized in
IcingaDB::Start(), therefore add a nullptr check to the check command.
Additionally, as m_Rcon is potentially accessed concurrently, add a copy of the
value that is safe for concurrent use.

lib/icingadb/icingadb.cpp

@@ -32,7 +32,7 @@ REGISTER_TYPE(IcingaDB);
 IcingaDB::IcingaDB()
 	: m_Rcon(nullptr)
 {
-	m_Rcon = nullptr;
+	m_RconLocked.store(nullptr);
 
 	m_WorkQueue.SetName("IcingaDB");
 
@@ -80,6 +80,7 @@ void IcingaDB::Start(bool runtimeCreated)
 	m_Rcon = new RedisConnection(GetHost(), GetPort(), GetPath(), GetPassword(), GetDbIndex(),
 		GetEnableTls(), GetInsecureNoverify(), GetCertPath(), GetKeyPath(), GetCaPath(), GetCrlPath(),
 		GetTlsProtocolmin(), GetCipherList(), GetConnectTimeout(), GetDebugInfo());
+	m_RconLocked.store(m_Rcon);
 
 	for (const Type::Ptr& type : GetTypes()) {
 		auto ctype (dynamic_cast<ConfigType*>(type.get()));

lib/icingadb/icingadb.hpp

@@ -5,6 +5,7 @@
 
 #include "icingadb/icingadb-ti.hpp"
 #include "icingadb/redisconnection.hpp"
+#include "base/atomic.hpp"
 #include "base/bulker.hpp"
 #include "base/timer.hpp"
 #include "base/workqueue.hpp"
@@ -46,7 +47,7 @@ public:
 
 	inline RedisConnection::Ptr GetConnection()
 	{
-		return m_Rcon;
+		return m_RconLocked.load();
 	}
 
 	template<class T>
@@ -215,6 +216,10 @@ private:
 	bool m_ConfigDumpDone;
 
 	RedisConnection::Ptr m_Rcon;
+	// m_RconLocked containes a copy of the value in m_Rcon where all accesses are guarded by a mutex to allow safe
+	// concurrent access like from the icingadb check command. It's a copy to still allow fast access without additional
+	// syncronization to m_Rcon within the IcingaDB feature itself.
+	Locked<RedisConnection::Ptr> m_RconLocked;
 	std::unordered_map<ConfigType*, RedisConnection::Ptr> m_Rcons;
 	std::atomic_size_t m_PendingRcons;
 

lib/icingadb/icingadbchecktask.cpp

@@ -101,7 +101,7 @@ void IcingadbCheckTask::ScriptFunc(const Checkable::Ptr& checkable, const CheckR
 
 	auto redis (conn->GetConnection());
 
-	if (!redis->GetConnected()) {
+	if (!redis || !redis->GetConnected()) {
 		ReportIcingadbCheck(checkable, commandObj, cr, "Icinga DB CRITICAL: Could not connect to Redis.", ServiceCritical);
 		return;
 	}