[net processing] Sample the last 199 outbound time offsets

src/net_processing.cpp

@@ -187,6 +187,51 @@ struct QueuedBlock {
     std::unique_ptr<PartiallyDownloadedBlock> partialBlock;
 };
 
+class TimeOffsets
+{
+private:
+    // Historic maximum number of time offsets we include in our median.
+    static constexpr size_t N{199};
+
+    mutable Mutex m_mutex;
+    std::array<int64_t, N> m_offsets GUARDED_BY(m_mutex){};
+    size_t m_index GUARDED_BY(m_mutex){0};
+
+public:
+    /** Add a new time offset sample. */
+    void Add(int64_t offset) EXCLUSIVE_LOCKS_REQUIRED(!m_mutex)
+    {
+        LOCK(m_mutex);
+
+        // Stop sampling after N samples.
+        // TODO This maintains a historic bug of adjusted time and could be
+        // changed in the future.
+        if (m_index >= N) return;
+
+        m_offsets[m_index] = offset;
+        ++m_index;
+    }
+
+    /** Compute and return the median of the collected time offset samples. */
+    int64_t Median() const EXCLUSIVE_LOCKS_REQUIRED(!m_mutex)
+    {
+        LOCK(m_mutex);
+
+        // Only compute the median from 5 or more samples.
+        if (m_index < 4) return 0;
+
+        std::array<int64_t, N> sorted_copy = m_offsets;
+        std::sort(sorted_copy.begin(), sorted_copy.begin() + m_index);
+
+        if (m_index % 2 == 0) {
+            return (sorted_copy[m_index / 2] / 2) +
+                   (sorted_copy[m_index / 2 - 1] / 2);
+        } else {
+            return sorted_copy[m_index / 2];
+        }
+    }
+};
+
 /**
  * Data structure for an individual peer. This struct is not protected by
  * cs_main since it does not contain validation-critical data.
@@ -729,6 +774,8 @@ class PeerManagerImpl final : public PeerManager
     /** Next time to check for stale tip */
     std::chrono::seconds m_stale_tip_check_time GUARDED_BY(cs_main){0s};
 
+    TimeOffsets m_outbound_time_offsets;
+
     const Options m_opts;
 
     bool RejectIncomingTxs(const CNode& peer) const;
@@ -3558,6 +3605,8 @@ void PeerManagerImpl::ProcessMessage(CNode& pfrom, const std::string& msg_type,
             // Don't use timedata samples from inbound peers to make it
             // harder for others to tamper with our adjusted time.
             AddTimeData(pfrom.addr, peer->m_time_offset);
+
+            m_outbound_time_offsets.Add(peer->m_time_offset);
         }
 
         // If the peer is old enough to have the old alert system, send it the final alert.