script/verify_flags: make script_verify_flags type safe

`using script_verify_flags = uint32_t` allows implicit conversion to
and from int, so replace it with a class to have the compiler ensure we
use the correct type. Provide from_int and as_int to allow for explicit
conversions when desired.

Introduces the type `script_verify_flag_name` for the individual flag
name enumeration.

src/script/interpreter.cpp

@@ -2164,7 +2164,7 @@ size_t CountWitnessSigOps(const CScript& scriptSig, const CScript& scriptPubKey,
 }
 
 #define FLAG_NAME(flag) {std::string(#flag), SCRIPT_VERIFY_##flag}
-const std::map<std::string, uint32_t> g_verify_flag_names{
+const std::map<std::string, script_verify_flag_name> g_verify_flag_names{
     FLAG_NAME(P2SH),
     FLAG_NAME(STRICTENC),
     FLAG_NAME(DERSIG),
@@ -2203,7 +2203,7 @@ std::vector<std::string> GetScriptFlagNames(script_verify_flags flags)
         }
     }
     if (leftover != 0) {
-        res.push_back(strprintf("0x%08x", leftover));
+        res.push_back(strprintf("0x%08x", leftover.as_int()));
     }
     return res;
 }

src/script/interpreter.h

@@ -14,6 +14,7 @@
 #include <span.h>
 #include <uint256.h>
 
+#include <bit>
 #include <cstddef>
 #include <cstdint>
 #include <optional>
@@ -43,9 +44,10 @@ enum
  *  All flags are intended to be soft forks: the set of acceptable scripts under
  *  flags (A | B) is a subset of the acceptable scripts under flag (A).
  */
-enum : uint32_t {
-    SCRIPT_VERIFY_NONE      = 0,
 
+static constexpr script_verify_flags SCRIPT_VERIFY_NONE{0};
+
+enum class script_verify_flag_name : uint32_t {
     // Evaluate P2SH subscripts (BIP16).
     SCRIPT_VERIFY_P2SH      = (1U << 0),
 
@@ -148,6 +150,13 @@ enum : uint32_t {
     //
     SCRIPT_VERIFY_END_MARKER
 };
+using enum script_verify_flag_name;
+
+// assert there is still a spare bit
+static_assert(static_cast<script_verify_flags::value_type>(SCRIPT_VERIFY_END_MARKER) < (1u << 31));
+
+static constexpr script_verify_flags::value_type MAX_SCRIPT_VERIFY_FLAGS = ((static_cast<script_verify_flags::value_type>(SCRIPT_VERIFY_END_MARKER) - 1) << 1) - 1;
+static constexpr int MAX_SCRIPT_VERIFY_FLAGS_BITS = std::bit_width(MAX_SCRIPT_VERIFY_FLAGS);
 
 bool CheckSignatureEncoding(const std::vector<unsigned char> &vchSig, script_verify_flags flags, ScriptError* serror);
 
@@ -372,7 +381,7 @@ size_t CountWitnessSigOps(const CScript& scriptSig, const CScript& scriptPubKey,
 
 int FindAndDelete(CScript& script, const CScript& b);
 
-extern const std::map<std::string, uint32_t> g_verify_flag_names;
+extern const std::map<std::string, script_verify_flag_name> g_verify_flag_names;
 
 std::vector<std::string> GetScriptFlagNames(script_verify_flags flags);
 

src/script/verify_flags.h

@@ -6,8 +6,66 @@
 #ifndef BITCOIN_SCRIPT_VERIFY_FLAGS_H
 #define BITCOIN_SCRIPT_VERIFY_FLAGS_H
 
+#include <compare>
 #include <cstdint>
 
-using script_verify_flags = uint32_t;
+enum class script_verify_flag_name : uint32_t;
+
+class script_verify_flags
+{
+public:
+    using value_type = uint32_t;
+
+    consteval script_verify_flags() = default;
+
+    // also allow construction with hard-coded 0 (but not other integers)
+    consteval explicit(false) script_verify_flags(value_type f) : m_value{f} { if (f != 0) throw 0; }
+
+    // implicit construction from a hard-coded SCRIPT_VERIFY_* constant is also okay
+    constexpr explicit(false) script_verify_flags(script_verify_flag_name f) : m_value{static_cast<value_type>(f)} { }
+
+    // rule of 5
+    constexpr script_verify_flags(const script_verify_flags&) = default;
+    constexpr script_verify_flags(script_verify_flags&&) = default;
+    constexpr script_verify_flags& operator=(const script_verify_flags&) = default;
+    constexpr script_verify_flags& operator=(script_verify_flags&&) = default;
+    constexpr ~script_verify_flags() = default;
+
+    // integer conversion needs to be very explicit
+    static constexpr script_verify_flags from_int(value_type f) { script_verify_flags r; r.m_value = f; return r; }
+    constexpr value_type as_int() const { return m_value; }
+
+    // bitwise operations
+    constexpr script_verify_flags operator~() const { return from_int(~m_value); }
+    friend constexpr script_verify_flags operator|(script_verify_flags a, script_verify_flags b) { return from_int(a.m_value | b.m_value); }
+    friend constexpr script_verify_flags operator&(script_verify_flags a, script_verify_flags b) { return from_int(a.m_value & b.m_value); }
+
+    // in-place bitwise operations
+    constexpr script_verify_flags& operator|=(script_verify_flags vf) { m_value |= vf.m_value; return *this; }
+    constexpr script_verify_flags& operator&=(script_verify_flags vf) { m_value &= vf.m_value; return *this; }
+
+    // tests
+    constexpr explicit operator bool() const { return m_value != 0; }
+    constexpr bool operator==(script_verify_flags other) const { return m_value == other.m_value; }
+
+    /** Compare two script_verify_flags. <, >, <=, and >= are auto-generated from this. */
+    friend constexpr std::strong_ordering operator<=>(const script_verify_flags& a, const script_verify_flags& b) noexcept
+    {
+        return a.m_value <=> b.m_value;
+    }
+
+private:
+    value_type m_value{0}; // default value is SCRIPT_VERIFY_NONE
+};
+
+inline constexpr script_verify_flags operator~(script_verify_flag_name f)
+{
+    return ~script_verify_flags{f};
+}
+
+inline constexpr script_verify_flags operator|(script_verify_flag_name f1, script_verify_flag_name f2)
+{
+    return script_verify_flags{f1} | f2;
+}
 
 #endif // BITCOIN_SCRIPT_VERIFY_FLAGS_H

src/test/fuzz/coins_view.cpp

@@ -288,7 +288,7 @@ void TestCoinsView(FuzzedDataProvider& fuzzed_data_provider, CCoinsView& backend
                     // consensus/tx_verify.cpp:130: unsigned int GetP2SHSigOpCount(const CTransaction &, const CCoinsViewCache &): Assertion `!coin.IsSpent()' failed.
                     return;
                 }
-                const auto flags{fuzzed_data_provider.ConsumeIntegral<uint32_t>()};
+                const auto flags = script_verify_flags::from_int(fuzzed_data_provider.ConsumeIntegral<script_verify_flags::value_type>());
                 if (!transaction.vin.empty() && (flags & SCRIPT_VERIFY_WITNESS) != 0 && (flags & SCRIPT_VERIFY_P2SH) == 0) {
                     // Avoid:
                     // script/interpreter.cpp:1705: size_t CountWitnessSigOps(const CScript &, const CScript &, const CScriptWitness *, unsigned int): Assertion `(flags & SCRIPT_VERIFY_P2SH) != 0' failed.

src/test/fuzz/eval_script.cpp

@@ -12,7 +12,7 @@
 FUZZ_TARGET(eval_script)
 {
     FuzzedDataProvider fuzzed_data_provider(buffer.data(), buffer.size());
-    const script_verify_flags flags = fuzzed_data_provider.ConsumeIntegral<unsigned int>();
+    const auto flags = script_verify_flags::from_int(fuzzed_data_provider.ConsumeIntegral<script_verify_flags::value_type>());
     const std::vector<uint8_t> script_bytes = [&] {
         if (fuzzed_data_provider.remaining_bytes() != 0) {
             return fuzzed_data_provider.ConsumeRemainingBytes<uint8_t>();

src/test/fuzz/script.cpp

@@ -118,8 +118,8 @@ FUZZ_TARGET(script, .init = initialize_script)
             (void)FindAndDelete(script_mut, *other_script);
         }
         const std::vector<std::string> random_string_vector = ConsumeRandomLengthStringVector(fuzzed_data_provider);
-        const uint32_t u32{fuzzed_data_provider.ConsumeIntegral<uint32_t>()};
+        const auto flags_rand{fuzzed_data_provider.ConsumeIntegral<script_verify_flags::value_type>()};
-        const script_verify_flags flags{u32 | SCRIPT_VERIFY_P2SH};
+        const auto flags = script_verify_flags::from_int(flags_rand) | SCRIPT_VERIFY_P2SH;
         {
             CScriptWitness wit;
             for (const auto& s : random_string_vector) {

src/test/fuzz/script_assets_test_minimizer.cpp

@@ -90,7 +90,7 @@ CScriptWitness ScriptWitnessFromJSON(const UniValue& univalue)
     return scriptwitness;
 }
 
-const std::map<std::string, uint32_t> FLAG_NAMES = {
+const std::map<std::string, script_verify_flag_name> FLAG_NAMES = {
     {std::string("P2SH"), SCRIPT_VERIFY_P2SH},
     {std::string("DERSIG"), SCRIPT_VERIFY_DERSIG},
     {std::string("NULLDUMMY"), SCRIPT_VERIFY_NULLDUMMY},

src/test/fuzz/script_flags.cpp

@@ -15,6 +15,14 @@
 #include <utility>
 #include <vector>
 
+static DataStream& operator>>(DataStream& ds, script_verify_flags& f)
+{
+    script_verify_flags::value_type n{0};
+    ds >> n;
+    f = script_verify_flags::from_int(n);
+    return ds;
+}
+
 FUZZ_TARGET(script_flags)
 {
     if (buffer.size() > 100'000) return;

src/test/fuzz/signature_checker.cpp

@@ -51,7 +51,7 @@ public:
 FUZZ_TARGET(signature_checker)
 {
     FuzzedDataProvider fuzzed_data_provider(buffer.data(), buffer.size());
-    const script_verify_flags flags = fuzzed_data_provider.ConsumeIntegral<unsigned int>();
+    const auto flags = script_verify_flags::from_int(fuzzed_data_provider.ConsumeIntegral<script_verify_flags::value_type>());
     const SigVersion sig_version = fuzzed_data_provider.PickValueInArray({SigVersion::BASE, SigVersion::WITNESS_V0});
     const auto script_1{ConsumeScript(fuzzed_data_provider)};
     const auto script_2{ConsumeScript(fuzzed_data_provider)};

src/test/script_tests.cpp

@@ -96,7 +96,7 @@ static ScriptErrorDesc script_errors[]={
     {SCRIPT_ERR_SIG_FINDANDDELETE, "SIG_FINDANDDELETE"},
 };
 
-static std::string FormatScriptFlags(uint32_t flags)
+static std::string FormatScriptFlags(script_verify_flags flags)
 {
     return util::Join(GetScriptFlagNames(flags), ",");
 }
@@ -134,13 +134,13 @@ void DoTest(const CScript& scriptPubKey, const CScript& scriptSig, const CScript
     BOOST_CHECK_MESSAGE(err == scriptError, FormatScriptError(err) + " where " + FormatScriptError((ScriptError_t)scriptError) + " expected: " + message);
 
     // Verify that removing flags from a passing test or adding flags to a failing test does not change the result.
-    for (int i = 0; i < 16; ++i) {
+    for (int i = 0; i < 256; ++i) {
-        uint32_t extra_flags(m_rng.randbits(16));
+        script_verify_flags extra_flags = script_verify_flags::from_int(m_rng.randbits(MAX_SCRIPT_VERIFY_FLAGS_BITS));
-        uint32_t combined_flags{expect ? (flags & ~extra_flags) : (flags | extra_flags)};
+        script_verify_flags combined_flags{expect ? (flags & ~extra_flags) : (flags | extra_flags)};
         // Weed out some invalid flag combinations.
         if (combined_flags & SCRIPT_VERIFY_CLEANSTACK && ~combined_flags & (SCRIPT_VERIFY_P2SH | SCRIPT_VERIFY_WITNESS)) continue;
         if (combined_flags & SCRIPT_VERIFY_WITNESS && ~combined_flags & SCRIPT_VERIFY_P2SH) continue;
-        BOOST_CHECK_MESSAGE(VerifyScript(scriptSig, scriptPubKey, &scriptWitness, combined_flags, MutableTransactionSignatureChecker(&tx, 0, txCredit.vout[0].nValue, MissingDataBehavior::ASSERT_FAIL), &err) == expect, message + strprintf(" (with flags %x)", combined_flags));
+        BOOST_CHECK_MESSAGE(VerifyScript(scriptSig, scriptPubKey, &scriptWitness, combined_flags, MutableTransactionSignatureChecker(&tx, 0, txCredit.vout[0].nValue, MissingDataBehavior::ASSERT_FAIL), &err) == expect, message + strprintf(" (with flags %x)", combined_flags.as_int()));
     }
 }
 }; // struct ScriptTest
@@ -1716,9 +1716,9 @@ BOOST_AUTO_TEST_CASE(formatscriptflags)
 {
     // quick check that FormatScriptFlags reports any unknown/unexpected bits
     BOOST_CHECK_EQUAL(FormatScriptFlags(SCRIPT_VERIFY_P2SH), "P2SH");
-    BOOST_CHECK_EQUAL(FormatScriptFlags(SCRIPT_VERIFY_P2SH | (1u<<31)), "P2SH,0x80000000");
+    BOOST_CHECK_EQUAL(FormatScriptFlags(SCRIPT_VERIFY_P2SH | script_verify_flags::from_int(1u<<31)), "P2SH,0x80000000");
-    BOOST_CHECK_EQUAL(FormatScriptFlags(SCRIPT_VERIFY_TAPROOT | (1u<<27)), "TAPROOT,0x08000000");
+    BOOST_CHECK_EQUAL(FormatScriptFlags(SCRIPT_VERIFY_TAPROOT | script_verify_flags::from_int(1u<<27)), "TAPROOT,0x08000000");
-    BOOST_CHECK_EQUAL(FormatScriptFlags(1u<<26), "0x04000000");
+    BOOST_CHECK_EQUAL(FormatScriptFlags(script_verify_flags::from_int(1u<<26)), "0x04000000");
 }
 
 BOOST_AUTO_TEST_SUITE_END()

src/test/transaction_tests.cpp

@@ -50,7 +50,7 @@ typedef std::vector<unsigned char> valtype;
 static CFeeRate g_dust{DUST_RELAY_TX_FEE};
 static bool g_bare_multi{DEFAULT_PERMIT_BAREMULTISIG};
 
-static const std::map<std::string, unsigned int>& mapFlagNames = g_verify_flag_names;
+static const std::map<std::string, script_verify_flag_name>& mapFlagNames = g_verify_flag_names;
 
 script_verify_flags ParseScriptFlags(std::string strFlags)
 {
@@ -230,9 +230,9 @@ BOOST_AUTO_TEST_CASE(tx_valid)
                     BOOST_ERROR("Tx unexpectedly failed with flag " << name << " unset: " << strTest);
                 }
                 // Removing random combinations of flags
-                flags = TrimFlags(~(verify_flags | (unsigned int)m_rng.randbits(mapFlagNames.size())));
+                flags = TrimFlags(~(verify_flags | script_verify_flags::from_int(m_rng.randbits(MAX_SCRIPT_VERIFY_FLAGS_BITS))));
                 if (!CheckTxScripts(tx, mapprevOutScriptPubKeys, mapprevOutValues, flags, txdata, strTest, /*expect_valid=*/true)) {
-                    BOOST_ERROR("Tx unexpectedly failed with random flags " << ToString(flags) << ": " << strTest);
+                    BOOST_ERROR("Tx unexpectedly failed with random flags " << ToString(flags.as_int()) << ": " << strTest);
                 }
             }
 
@@ -321,7 +321,7 @@ BOOST_AUTO_TEST_CASE(tx_invalid)
                     BOOST_ERROR("Tx unexpectedly passed with flag " << name << " set: " << strTest);
                 }
                 // Adding random combinations of flags
-                flags = FillFlags(verify_flags | (unsigned int)m_rng.randbits(mapFlagNames.size()));
+                flags = FillFlags(verify_flags | script_verify_flags::from_int(m_rng.randbits(MAX_SCRIPT_VERIFY_FLAGS_BITS)));
                 if (!CheckTxScripts(tx, mapprevOutScriptPubKeys, mapprevOutValues, flags, txdata, strTest, /*expect_valid=*/false)) {
                     BOOST_ERROR("Tx unexpectedly passed with random flags " << name << ": " << strTest);
                 }

src/test/txvalidationcache_tests.cpp

@@ -130,7 +130,7 @@ static void ValidateCheckInputsForAllFlags(const CTransaction &tx, script_verify
         TxValidationState state;
 
         // Randomly selects flag combinations
-        script_verify_flags test_flags = (uint32_t) insecure_rand.randrange((SCRIPT_VERIFY_END_MARKER - 1) << 1);
+        script_verify_flags test_flags = script_verify_flags::from_int(insecure_rand.randrange(MAX_SCRIPT_VERIFY_FLAGS));
 
         // Filter out incompatible flag choices
         if ((test_flags & SCRIPT_VERIFY_CLEANSTACK)) {