util: Avoid invalid integer negation in FormatMoney: make FormatMoney(const CAmount& n) well-defined also when n is std::numeric_limits<CAmount>::min()

Author: practicalswift

src/test/fuzz/integer.cpp

@@ -84,8 +84,7 @@ FUZZ_TARGET_INIT(integer, initialize_integer)
     (void)DecompressAmount(u64);
     (void)FormatISO8601Date(i64);
     (void)FormatISO8601DateTime(i64);
-    // FormatMoney(i) not defined when i == std::numeric_limits<int64_t>::min()
-    if (i64 != std::numeric_limits<int64_t>::min()) {
+    {
         int64_t parsed_money;
         if (ParseMoney(FormatMoney(i64), parsed_money)) {
             assert(parsed_money == i64);

src/test/util_tests.cpp

@@ -1180,6 +1180,16 @@ BOOST_AUTO_TEST_CASE(util_FormatMoney)
     BOOST_CHECK_EQUAL(FormatMoney(COIN/1000000), "0.000001");
     BOOST_CHECK_EQUAL(FormatMoney(COIN/10000000), "0.0000001");
     BOOST_CHECK_EQUAL(FormatMoney(COIN/100000000), "0.00000001");
+
+    BOOST_CHECK_EQUAL(FormatMoney(std::numeric_limits<CAmount>::max()), "92233720368.54775807");
+    BOOST_CHECK_EQUAL(FormatMoney(std::numeric_limits<CAmount>::max() - 1), "92233720368.54775806");
+    BOOST_CHECK_EQUAL(FormatMoney(std::numeric_limits<CAmount>::max() - 2), "92233720368.54775805");
+    BOOST_CHECK_EQUAL(FormatMoney(std::numeric_limits<CAmount>::max() - 3), "92233720368.54775804");
+    // ...
+    BOOST_CHECK_EQUAL(FormatMoney(std::numeric_limits<CAmount>::min() + 3), "-92233720368.54775805");
+    BOOST_CHECK_EQUAL(FormatMoney(std::numeric_limits<CAmount>::min() + 2), "-92233720368.54775806");
+    BOOST_CHECK_EQUAL(FormatMoney(std::numeric_limits<CAmount>::min() + 1), "-92233720368.54775807");
+    BOOST_CHECK_EQUAL(FormatMoney(std::numeric_limits<CAmount>::min()), "-92233720368.54775808");
 }
 
 BOOST_AUTO_TEST_CASE(util_ParseMoney)

src/util/moneystr.cpp

@@ -9,13 +9,17 @@
 #include <util/strencodings.h>
 #include <util/string.h>
 
-std::string FormatMoney(const CAmount& n)
+std::string FormatMoney(const CAmount n)
 {
     // Note: not using straight sprintf here because we do NOT want
     // localized number formatting.
-    int64_t n_abs = (n > 0 ? n : -n);
-    int64_t quotient = n_abs/COIN;
-    int64_t remainder = n_abs%COIN;
+    static_assert(COIN > 1);
+    int64_t quotient = n / COIN;
+    int64_t remainder = n % COIN;
+    if (n < 0) {
+        quotient = -quotient;
+        remainder = -remainder;
+    }
     std::string str = strprintf("%d.%08d", quotient, remainder);
 
     // Right-trim excess zeros before the decimal point:

src/util/moneystr.h

@@ -17,7 +17,7 @@
 /* Do not use these functions to represent or parse monetary amounts to or from
  * JSON but use AmountFromValue and ValueFromAmount for that.
  */
-std::string FormatMoney(const CAmount& n);
+std::string FormatMoney(const CAmount n);
 /** Parse an amount denoted in full coins. E.g. "0.0034" supplied on the command line. **/
 [[nodiscard]] bool ParseMoney(const std::string& str, CAmount& nRet);