[move] Move PoissonNextSend to src/random and update comment

PoissonNextSend is used by net and net_processing and is stateless, so
place it in the utility random.cpp translation unit.

src/net.cpp

@@ -3069,12 +3069,6 @@ std::chrono::microseconds CConnman::PoissonNextSendInbound(std::chrono::microsec
     return m_next_send_inv_to_incoming;
 }
 
-std::chrono::microseconds PoissonNextSend(std::chrono::microseconds now, std::chrono::seconds average_interval)
-{
-    double unscaled = -log1p(GetRand(1ULL << 48) * -0.0000000000000035527136788 /* -1/2^48 */);
-    return now + std::chrono::duration_cast<std::chrono::microseconds>(unscaled * average_interval + 0.5us);
-}
-
 CSipHasher CConnman::GetDeterministicRandomizer(uint64_t id) const
 {
     return CSipHasher(nSeed0, nSeed1).Write(id);

src/net.h

@@ -1270,9 +1270,6 @@ private:
     friend struct ConnmanTestMsg;
 };
 
-/** Return a timestamp in the future (in microseconds) for exponentially distributed events. */
-std::chrono::microseconds PoissonNextSend(std::chrono::microseconds now, std::chrono::seconds average_interval);
-
 /** Dump binary message to file, with timestamp */
 void CaptureMessage(const CAddress& addr, const std::string& msg_type, const Span<const unsigned char>& data, bool is_incoming);
 

src/random.cpp

@@ -19,6 +19,7 @@
 #include <sync.h>     // for Mutex
 #include <util/time.h> // for GetTimeMicros()
 
+#include <cmath>
 #include <stdlib.h>
 #include <thread>
 
@@ -714,3 +715,9 @@ void RandomInit()
 
     ReportHardwareRand();
 }
+
+std::chrono::microseconds PoissonNextSend(std::chrono::microseconds now, std::chrono::seconds average_interval)
+{
+    double unscaled = -log1p(GetRand(1ULL << 48) * -0.0000000000000035527136788 /* -1/2^48 */);
+    return now + std::chrono::duration_cast<std::chrono::microseconds>(unscaled * average_interval + 0.5us);
+}

src/random.h

@@ -10,7 +10,7 @@
 #include <crypto/common.h>
 #include <uint256.h>
 
-#include <chrono> // For std::chrono::microseconds
+#include <chrono>
 #include <cstdint>
 #include <limits>
 
@@ -82,6 +82,18 @@ D GetRandomDuration(typename std::common_type<D>::type max) noexcept
 };
 constexpr auto GetRandMicros = GetRandomDuration<std::chrono::microseconds>;
 constexpr auto GetRandMillis = GetRandomDuration<std::chrono::milliseconds>;
+
+/**
+ * Return a timestamp in the future sampled from an exponential distribution
+ * (https://en.wikipedia.org/wiki/Exponential_distribution). This distribution
+ * is memoryless and should be used for repeated network events (e.g. sending a
+ * certain type of message) to minimize leaking information to observers.
+ *
+ * The probability of an event occuring before time x is 1 - e^-(x/a) where a
+ * is the average interval between events.
+ * */
+std::chrono::microseconds PoissonNextSend(std::chrono::microseconds now, std::chrono::seconds average_interval);
+
 int GetRandInt(int nMax) noexcept;
 uint256 GetRandHash() noexcept;