tx fees, policy: do not read estimates of old fee_estimates.dat

Old fee estimates could cause transactions to become stuck in the
mempool. This commit prevents the node from using stale estimates
from an old file.

src/policy/fees.cpp

@@ -24,6 +24,7 @@
 
 #include <algorithm>
 #include <cassert>
+#include <chrono>
 #include <cmath>
 #include <cstddef>
 #include <cstdint>
@@ -545,9 +546,22 @@ CBlockPolicyEstimator::CBlockPolicyEstimator(const fs::path& estimation_filepath
     shortStats = std::unique_ptr<TxConfirmStats>(new TxConfirmStats(buckets, bucketMap, SHORT_BLOCK_PERIODS, SHORT_DECAY, SHORT_SCALE));
     longStats = std::unique_ptr<TxConfirmStats>(new TxConfirmStats(buckets, bucketMap, LONG_BLOCK_PERIODS, LONG_DECAY, LONG_SCALE));
 
-    // If the fee estimation file is present, read recorded estimations
     AutoFile est_file{fsbridge::fopen(m_estimation_filepath, "rb")};
-    if (est_file.IsNull() || !Read(est_file)) {
+
+    // Whenever the fee estimation file is not present return early
+    if (est_file.IsNull()) {
+        LogPrintf("%s is not found. Continue anyway.\n", fs::PathToString(m_estimation_filepath));
+        return;
+    }
+
+    std::chrono::hours file_age = GetFeeEstimatorFileAge();
+    // fee estimate file must not be too old to avoid wrong fee estimates.
+    if (file_age > MAX_FILE_AGE) {
+        LogPrintf("Fee estimation file %s too old (age=%lld > %lld hours) and will not be used to avoid serving stale estimates.\n", fs::PathToString(m_estimation_filepath), Ticks<std::chrono::hours>(file_age), Ticks<std::chrono::hours>(MAX_FILE_AGE));
+        return;
+    }
+
+    if (!Read(est_file)) {
         LogPrintf("Failed to read fee estimates from %s. Continue anyway.\n", fs::PathToString(m_estimation_filepath));
     }
 }
@@ -1017,6 +1031,13 @@ void CBlockPolicyEstimator::FlushUnconfirmed()
     LogPrint(BCLog::ESTIMATEFEE, "Recorded %u unconfirmed txs from mempool in %gs\n", num_entries, Ticks<SecondsDouble>(endclear - startclear));
 }
 
+std::chrono::hours CBlockPolicyEstimator::GetFeeEstimatorFileAge()
+{
+    auto file_time = std::filesystem::last_write_time(m_estimation_filepath);
+    auto now = std::filesystem::file_time_type::clock::now();
+    return std::chrono::duration_cast<std::chrono::hours>(now - file_time);
+}
+
 static std::set<double> MakeFeeSet(const CFeeRate& min_incremental_fee,
                                    double max_filter_fee_rate,
                                    double fee_filter_spacing)

src/policy/fees.h

@@ -25,6 +25,11 @@
 // How often to flush fee estimates to fee_estimates.dat.
 static constexpr std::chrono::hours FEE_FLUSH_INTERVAL{1};
 
+/** fee_estimates.dat that are more than 60 hours (2.5 days) will not be read,
+ * as the estimates in the file are stale.
+ */
+static constexpr std::chrono::hours MAX_FILE_AGE{60};
+
 class AutoFile;
 class CTxMemPoolEntry;
 class TxConfirmStats;
@@ -248,6 +253,9 @@ class CBlockPolicyEstimator
     void FlushFeeEstimates()
         EXCLUSIVE_LOCKS_REQUIRED(!m_cs_fee_estimator);
 
+    /** Calculates the age of the file, since last modified */
+    std::chrono::hours GetFeeEstimatorFileAge();
+
 private:
     mutable Mutex m_cs_fee_estimator;