From 76190489e6c89ad091ebba0781c585afc9a39f1b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?L=C5=91rinc?= <pap.lorinc@gmail.com>
Date: Mon, 9 Dec 2024 13:30:04 +0100
Subject: [PATCH] coins: pack `Coin` height/coinbase consistently

Serialize `Coin` metadata using the canonical (height << 1) | coinbase packing across `Coin` serialization, undo records, and coinstats hashing.

Cast the 31-bit `nHeight` bitfield to `uint32_t` before shifting to avoid signed promotion undefined behaviour.
---
 src/coins.h              | 4 ++--
 src/kernel/coinstats.cpp | 2 +-
 src/undo.h               | 3 ++-
 3 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/src/coins.h b/src/coins.h
index e58586dd6cd..bfd6c5b0d7e 100644
--- a/src/coins.h
+++ b/src/coins.h
@@ -27,7 +27,7 @@
  * A UTXO entry.
  *
  * Serialized format:
- * - VARINT((coinbase ? 1 : 0) | (height << 1))
+ * - VARINT((height << 1) | (coinbase ? 1 : 0))
  * - the non-spent CTxOut (via TxOutCompression)
  */
 class Coin
@@ -62,7 +62,7 @@ public:
     template<typename Stream>
     void Serialize(Stream &s) const {
         assert(!IsSpent());
-        uint32_t code = nHeight * uint32_t{2} + fCoinBase;
+        uint32_t code{(uint32_t{nHeight} << 1) | uint32_t{fCoinBase}};
         ::Serialize(s, VARINT(code));
         ::Serialize(s, Using<TxOutCompression>(out));
     }
diff --git a/src/kernel/coinstats.cpp b/src/kernel/coinstats.cpp
index d287ec4be6e..53039e57101 100644
--- a/src/kernel/coinstats.cpp
+++ b/src/kernel/coinstats.cpp
@@ -47,7 +47,7 @@ template <typename T>
 static void TxOutSer(T& ss, const COutPoint& outpoint, const Coin& coin)
 {
     ss << outpoint;
-    ss << static_cast<uint32_t>((coin.nHeight << 1) + coin.fCoinBase);
+    ss << ((uint32_t{coin.nHeight} << 1) | uint32_t{coin.fCoinBase});
     ss << coin.out;
 }
 
diff --git a/src/undo.h b/src/undo.h
index 5591fe6cc82..13b92349510 100644
--- a/src/undo.h
+++ b/src/undo.h
@@ -23,7 +23,8 @@ struct TxInUndoFormatter
 {
     template<typename Stream>
     void Ser(Stream &s, const Coin& txout) {
-        ::Serialize(s, VARINT(txout.nHeight * uint32_t{2} + txout.fCoinBase ));
+        uint32_t nCode{(uint32_t{txout.nHeight} << 1) | uint32_t{txout.fCoinBase}};
+        ::Serialize(s, VARINT(nCode));
         if (txout.nHeight > 0) {
             // Required to maintain compatibility with older undo format.
             ::Serialize(s, (unsigned char)0);