Force explicit double -> int conversion for CFeeRate constructor

This resolves an issue where estimatesmartfee would return 999
sat/byte instead of 1000, due to floating point loss of precision

Thanks to sipa for suggesting is_integral.

2 files changed, 8 insertions, 3 deletions

diff --git a/src/policy/feerate.h b/src/policy/feerate.h
index 7e519e3ef..3449cdd69 100644
--- a/src/policy/feerate.h
+++ b/src/policy/feerate.h
@@ -20,10 +20,15 @@ class CFeeRate
 {
 private:
     CAmount nSatoshisPerK; // unit is satoshis-per-1,000-bytes
+
 public:
     /** Fee rate of 0 satoshis per kB */
     CFeeRate() : nSatoshisPerK(0) { }
-    explicit CFeeRate(const CAmount& _nSatoshisPerK): nSatoshisPerK(_nSatoshisPerK) { }
+    template<typename I>
+    CFeeRate(const I _nSatoshisPerK): nSatoshisPerK(_nSatoshisPerK) {
+        // We've previously had bugs creep in from silent double->int conversion...
+        static_assert(std::is_integral<I>::value, "CFeeRate should be used without floats");
+    }
     /** Constructor for a fee rate in satoshis per kB. The size in bytes must not exceed (2^63 - 1)*/
     CFeeRate(const CAmount& nFeePaid, size_t nBytes);
     /**
diff --git a/src/policy/fees.cpp b/src/policy/fees.cpp
index ca774cd74..8056f385a 100644
--- a/src/policy/fees.cpp
+++ b/src/policy/fees.cpp
@@ -714,7 +714,7 @@ CFeeRate CBlockPolicyEstimator::estimateRawFee(int confTarget, double successThr
     if (median < 0)
         return CFeeRate(0);
 
-    return CFeeRate(median);
+    return CFeeRate(llround(median));
 }
 
 unsigned int CBlockPolicyEstimator::HighestTargetTracked(FeeEstimateHorizon horizon) const
@@ -901,7 +901,7 @@ CFeeRate CBlockPolicyEstimator::estimateSmartFee(int confTarget, FeeCalculation
 
     if (median < 0) return CFeeRate(0); // error condition
 
-    return CFeeRate(median);
+    return CFeeRate(llround(median));
 }