stats: miscellaneous changes and housekeeping

- `timing` cannot contain a negative value, change to `uint64_t`
- remove `sendDouble`, use template specialization of `Send` instead,
  that can be reused as `send`
- rename `value` in `count()` to `delta`
- add more comments

`gaugeDouble` has been kept around because utilizing templates for it
would require us to either write template specializations for every kind
of `gauge` value (implicit conversions will not save us from this) or
move all logic to the header, neither option seems desirable.

src/net.cpp

@@ -4145,7 +4145,7 @@ void CConnman::RecordBytesRecv(uint64_t bytes)
 {
     nTotalBytesRecv += bytes;
     ::g_stats_client->count("bandwidth.bytesReceived", bytes, 0.1f);
-    ::g_stats_client->gauge("bandwidth.totalBytesReceived", nTotalBytesRecv, 0.01f);
+    ::g_stats_client->gauge("bandwidth.totalBytesReceived", nTotalBytesRecv.load(), 0.01f);
 }
 
 void CConnman::RecordBytesSent(uint64_t bytes)

src/stats/client.cpp

@@ -49,6 +49,12 @@ static constexpr float EPSILON{0.0001f};
 
 /** Delimiter segmenting two fully formed Statsd messages */
 static constexpr char STATSD_MSG_DELIMITER{'\n'};
+/** Character used to denote Statsd message type as count */
+static constexpr char STATSD_METRIC_COUNT[]{"c"};
+/** Character used to denote Statsd message type as gauge */
+static constexpr char STATSD_METRIC_GAUGE[]{"g"};
+/** Characters used to denote Statsd message type as timing */
+static constexpr char STATSD_METRIC_TIMING[]{"ms"};
 } // anonymous namespace
 
 std::unique_ptr<StatsdClient> g_stats_client;
@@ -82,28 +88,28 @@ bool StatsdClient::dec(const std::string& key, float sample_rate) { return count
 
 bool StatsdClient::inc(const std::string& key, float sample_rate) { return count(key, 1, sample_rate); }
 
-bool StatsdClient::count(const std::string& key, int64_t value, float sample_rate)
+bool StatsdClient::count(const std::string& key, int64_t delta, float sample_rate)
 {
-    return send(key, value, "c", sample_rate);
+    return send(key, delta, STATSD_METRIC_COUNT, sample_rate);
 }
 
 bool StatsdClient::gauge(const std::string& key, int64_t value, float sample_rate)
 {
-    return send(key, value, "g", sample_rate);
+    return send(key, value, STATSD_METRIC_GAUGE, sample_rate);
 }
 
 bool StatsdClient::gaugeDouble(const std::string& key, double value, float sample_rate)
 {
-    return sendDouble(key, value, "g", sample_rate);
+    return send(key, value, STATSD_METRIC_GAUGE, sample_rate);
 }
 
-bool StatsdClient::timing(const std::string& key, int64_t ms, float sample_rate)
+bool StatsdClient::timing(const std::string& key, uint64_t ms, float sample_rate)
 {
-    return send(key, ms, "ms", sample_rate);
+    return send(key, ms, STATSD_METRIC_TIMING, sample_rate);
 }
 
 template <typename T1>
-bool StatsdClient::Send(const std::string& key, T1 value, const std::string& type, float sample_rate)
+bool StatsdClient::send(const std::string& key, T1 value, const std::string& type, float sample_rate)
 {
     static_assert(std::is_arithmetic<T1>::value, "Must specialize to an arithmetic type");
 
@@ -137,3 +143,9 @@ bool StatsdClient::Send(const std::string& key, T1 value, const std::string& typ
 
     return true;
 }
+
+template bool StatsdClient::send(const std::string& key, double value, const std::string& type, float sample_rate);
+template bool StatsdClient::send(const std::string& key, int32_t value, const std::string& type, float sample_rate);
+template bool StatsdClient::send(const std::string& key, int64_t value, const std::string& type, float sample_rate);
+template bool StatsdClient::send(const std::string& key, uint32_t value, const std::string& type, float sample_rate);
+template bool StatsdClient::send(const std::string& key, uint64_t value, const std::string& type, float sample_rate);

src/stats/client.h

@@ -15,10 +15,15 @@
 
 class RawSender;
 
+/** Default state of Statsd enablement */
 static constexpr bool DEFAULT_STATSD_ENABLE{false};
+/** Default port used to connect to a Statsd server */
 static constexpr uint16_t DEFAULT_STATSD_PORT{8125};
+/** Default host assumed to be running a Statsd server */
 static const std::string DEFAULT_STATSD_HOST{"127.0.0.1"};
+/** Default suffix appended to Statsd message keys */
 static const std::string DEFAULT_STATSD_HOSTNAME{""};
+/** Default prefix prepended to Statsd message keys */
 static const std::string DEFAULT_STATSD_NAMESPACE{""};
 
 /** Default number of milliseconds between flushing a queue of messages */
@@ -40,39 +45,34 @@ public:
     ~StatsdClient();
 
 public:
-    bool inc(const std::string& key, float sample_rate = 1.f);
+    /* Statsd-defined APIs */
     bool dec(const std::string& key, float sample_rate = 1.f);
-    bool count(const std::string& key, int64_t value, float sample_rate = 1.f);
+    bool inc(const std::string& key, float sample_rate = 1.f);
+    bool count(const std::string& key, int64_t delta, float sample_rate = 1.f);
     bool gauge(const std::string& key, int64_t value, float sample_rate = 1.f);
     bool gaugeDouble(const std::string& key, double value, float sample_rate = 1.f);
-    bool timing(const std::string& key, int64_t ms, float sample_rate = 1.f);
+    bool timing(const std::string& key, uint64_t ms, float sample_rate = 1.f);
 
-    /* (Low Level Api) manually send a message
+    /* Statsd-compatible APIs */
-        * type = "c", "g" or "ms"
-        */
-    bool send(const std::string& key, int64_t value, const std::string& type, float sample_rate = 1.f)
-    {
-        return Send(key, value, type, sample_rate);
-    }
-    bool sendDouble(const std::string& key, double value, const std::string& type, float sample_rate = 1.f)
-    {
-        return Send(key, value, type, sample_rate);
-    }
-
-private:
     template <typename T1>
-    bool Send(const std::string& key, T1 value, const std::string& type, float sample_rate);
+    bool send(const std::string& key, T1 value, const std::string& type, float sample_rate = 1.f);
 
 private:
+    /* Mutex to protect PRNG */
     mutable Mutex cs;
+    /* PRNG used to dice-roll messages that are 0 < f < 1 */
     mutable FastRandomContext insecure_rand GUARDED_BY(cs);
 
+    /* Broadcasts messages crafted by StatsdClient */
     std::unique_ptr<RawSender> m_sender{nullptr};
 
+    /* Phrase prepended to keys */
     const std::string m_nodename;
+    /* Phrase appended to keys */
     const std::string m_ns;
 };
 
+/** Global smart pointer containing StatsdClient instance */
 extern std::unique_ptr<StatsdClient> g_stats_client;
 
 #endif // BITCOIN_STATS_CLIENT_H