Add rate-limiting for notifications

Add token-bucket based rate limiting for falco notifications.

The token bucket is implemented in token_bucket.cpp (actually in the
engine directory, just to make it easier to include in other
programs). It maintains a current count of tokens (i.e. right to send a
notification). Its main method is claim(), which attemps to claim a
token and returns true if one was claimed successfully. It has a
configurable configurable max burst size and rate. The token bucket
gains "rate" tokens per second, up to a maximum of max_burst tokens.

These parameters are configurable in falco.yaml via the config
options (defaults shown):

outputs:
  rate: 1
  max_burst: 1000

In falco_outputs::handle_event(), try to claim a token, and if
unsuccessful log a debug message and return immediately.

userspace/engine/CMakeLists.txt

@@ -4,7 +4,7 @@ include_directories("${PROJECT_SOURCE_DIR}/../sysdig/userspace/libsinsp")
 include_directories("${PROJECT_BINARY_DIR}/userspace/engine")
 include_directories("${LUAJIT_INCLUDE}")
 
-add_library(falco_engine STATIC rules.cpp falco_common.cpp falco_engine.cpp formats.cpp)
+add_library(falco_engine STATIC rules.cpp falco_common.cpp falco_engine.cpp token_bucket.cpp formats.cpp)
 
 target_include_directories(falco_engine PUBLIC
   "${LUAJIT_INCLUDE}")

userspace/engine/token_bucket.cpp

@@ -0,0 +1,78 @@
+/*
+Copyright (C) 2016 Draios inc.
+
+This file is part of falco.
+
+falco is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License version 2 as
+published by the Free Software Foundation.
+
+falco is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with falco.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#include <cstddef>
+#include <sys/time.h>
+
+#include "token_bucket.h"
+
+token_bucket::token_bucket()
+{
+	init(1, 1);
+}
+
+token_bucket::~token_bucket()
+{
+}
+
+void token_bucket::init(uint32_t rate, uint32_t max_tokens)
+{
+	m_rate = rate;
+	m_max_tokens = max_tokens;
+	m_tokens = max_tokens;
+	m_last_seen = get_epoch_ns();
+}
+
+bool token_bucket::claim()
+{
+	// Determine the number of tokens gained. Delta between
+	// last_seen and now, divided by the rate.
+	uint64_t now = get_epoch_ns();
+	uint64_t tokens_gained = (now - m_last_seen) / (m_rate * 1000000000);
+	m_last_seen = now;
+
+	m_tokens += tokens_gained;
+
+	//
+	// Cap at max_tokens
+	//
+	if(m_tokens > m_max_tokens)
+	{
+		m_tokens = m_max_tokens;
+	}
+
+	//
+	// If tokens is < 1, can't claim.
+	//
+	if(m_tokens < 1)
+	{
+		return false;
+	}
+
+	m_tokens--;
+
+	return true;
+}
+
+uint64_t token_bucket::get_epoch_ns()
+{
+    struct timeval tv;
+    gettimeofday(&tv, NULL);
+
+    return tv.tv_sec * (uint64_t) 1000000000 + (tv.tv_usec * 1000);
+}

userspace/engine/token_bucket.h

@@ -0,0 +1,68 @@
+/*
+Copyright (C) 2016 Draios inc.
+
+This file is part of falco.
+
+falco is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License version 2 as
+published by the Free Software Foundation.
+
+falco is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with falco.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#pragma once
+
+#include <cstdint>
+
+// A simple token bucket that accumulates tokens at a fixed rate and allows
+// for limited bursting in the form of "banked" tokens.
+class token_bucket
+{
+public:
+	token_bucket();
+	virtual ~token_bucket();
+
+	//
+	// Initialize the token bucket and start accumulating tokens
+	//
+	void init(uint32_t rate, uint32_t max_tokens);
+
+	//
+	// Returns true if a token can be claimed. Also updates
+	// internal metrics.
+	//
+	bool claim();
+private:
+
+	// Utility function to get the time in nanoseconds since the epoch.
+	uint64_t get_epoch_ns();
+
+	//
+	// The number of tokens generated per second.
+	//
+	uint64_t m_rate;
+
+	//
+	// The maximum number of tokens that can be banked for future
+	// claim()s.
+	//
+	uint64_t m_max_tokens;
+
+	//
+	// The current number of tokens
+	//
+	uint64_t m_tokens;
+
+	//
+	// The last time claim() was called (or the object was created).
+	// Nanoseconds since the epoch.
+	//
+	uint64_t m_last_seen;
+};
+