Add TimeOffsets helper class

This helper class is an alternative to CMedianFilter, but without a
lot of the special logic and exceptions that we needed while it was
still used for consensus.

src/node/timeoffsets.cpp

@@ -0,0 +1,69 @@
+// Copyright (c) 2024-present The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <logging.h>
+#include <node/interface_ui.h>
+#include <node/timeoffsets.h>
+#include <sync.h>
+#include <tinyformat.h>
+#include <util/time.h>
+#include <util/translation.h>
+#include <warnings.h>
+
+#include <algorithm>
+#include <chrono>
+#include <cstdint>
+#include <deque>
+#include <limits>
+#include <optional>
+
+using namespace std::chrono_literals;
+
+void TimeOffsets::Add(std::chrono::seconds offset)
+{
+    LOCK(m_mutex);
+
+    if (m_offsets.size() >= MAX_SIZE) {
+        m_offsets.pop_front();
+    }
+    m_offsets.push_back(offset);
+    LogDebug(BCLog::NET, "Added time offset %+ds, total samples %d\n",
+             Ticks<std::chrono::seconds>(offset), m_offsets.size());
+}
+
+std::chrono::seconds TimeOffsets::Median() const
+{
+    LOCK(m_mutex);
+
+    // Only calculate the median if we have 5 or more offsets
+    if (m_offsets.size() < 5) return 0s;
+
+    auto sorted_copy = m_offsets;
+    std::sort(sorted_copy.begin(), sorted_copy.end());
+    return sorted_copy[sorted_copy.size() / 2];  // approximate median is good enough, keep it simple
+}
+
+bool TimeOffsets::WarnIfOutOfSync() const
+{
+    // when median == std::numeric_limits<int64_t>::min(), calling std::chrono::abs is UB
+    auto median{std::max(Median(), std::chrono::seconds(std::numeric_limits<int64_t>::min() + 1))};
+    if (std::chrono::abs(median) <= WARN_THRESHOLD) {
+        SetMedianTimeOffsetWarning(std::nullopt);
+        uiInterface.NotifyAlertChanged();
+        return false;
+    }
+
+    bilingual_str msg{strprintf(_(
+        "Your computer's date and time appear to be more than %d minutes out of sync with the network, "
+        "this may lead to consensus failure. After you've confirmed your computer's clock, this message "
+        "should no longer appear when you restart your node. Without a restart, it should stop showing "
+        "automatically after you've connected to a sufficient number of new outbound peers, which may "
+        "take some time. You can inspect the `timeoffset` field of the `getpeerinfo` and `getnetworkinfo` "
+        "RPC methods to get more info."
+    ), Ticks<std::chrono::minutes>(WARN_THRESHOLD))};
+    LogWarning("%s\n", msg.original);
+    SetMedianTimeOffsetWarning(msg);
+    uiInterface.NotifyAlertChanged();
+    return true;
+}

src/node/timeoffsets.h

@@ -0,0 +1,39 @@
+// Copyright (c) 2024-present The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_NODE_TIMEOFFSETS_H
+#define BITCOIN_NODE_TIMEOFFSETS_H
+
+#include <sync.h>
+
+#include <chrono>
+#include <cstddef>
+#include <deque>
+
+class TimeOffsets
+{
+    //! Maximum number of timeoffsets stored.
+    static constexpr size_t MAX_SIZE{50};
+    //! Minimum difference between system and network time for a warning to be raised.
+    static constexpr std::chrono::minutes WARN_THRESHOLD{10};
+
+    mutable Mutex m_mutex;
+    /** The observed time differences between our local clock and those of our outbound peers. A
+     * positive offset means our peer's clock is ahead of our local clock. */
+    std::deque<std::chrono::seconds> m_offsets GUARDED_BY(m_mutex){};
+
+public:
+    /** Add a new time offset sample. */
+    void Add(std::chrono::seconds offset) EXCLUSIVE_LOCKS_REQUIRED(!m_mutex);
+
+    /** Compute and return the median of the collected time offset samples. The median is returned
+     * as 0 when there are less than 5 samples. */
+    std::chrono::seconds Median() const EXCLUSIVE_LOCKS_REQUIRED(!m_mutex);
+
+    /** Raise warnings if the median time offset exceeds the warnings threshold. Returns true if
+     * warnings were raised. */
+    bool WarnIfOutOfSync() const EXCLUSIVE_LOCKS_REQUIRED(!m_mutex);
+};
+
+#endif // BITCOIN_NODE_TIMEOFFSETS_H