Add global access to the current ledger rules:

It can be difficult to make transaction breaking changes to low level
code because the low level code does not have access to a ledger and the
current activated amendments in that ledger (the "rules"). This patch
adds global access to the current ledger rules as a `std::optional`. If
the optional is not seated, then there is no active transaction.

src/ripple/app/tx/impl/Transactor.cpp

@@ -831,8 +831,11 @@ Transactor::operator()()
 {
     JLOG(j_.trace()) << "apply: " << ctx_.tx.getTransactionID();
 
+    // raii classes for the current ledger rules. fixSTAmountCanonicalize and
+    // fixSTAmountCanonicalize predate the rulesGuard and should be replaced.
     STAmountSO stAmountSO{view().rules().enabled(fixSTAmountCanonicalize)};
     NumberSO stNumberSO{view().rules().enabled(fixUniversalNumber)};
+    CurrentTransactionRulesGuard currentTransctionRulesGuard(view().rules());
 
 #ifdef DEBUG
     {

src/ripple/protocol/Rules.h

@@ -23,6 +23,7 @@
 #include <ripple/basics/base_uint.h>
 #include <ripple/beast/hash/uhash.h>
 #include <ripple/protocol/STVector256.h>
+
 #include <unordered_set>
 
 namespace ripple {
@@ -42,9 +43,14 @@ class Rules
 public:
     Rules(Rules const&) = default;
 
+    Rules(Rules&&) = default;
+
     Rules&
     operator=(Rules const&) = default;
 
+    Rules&
+    operator=(Rules&&) = default;
+
     Rules() = delete;
 
     /** Construct an empty rule set.
@@ -90,5 +96,36 @@ class Rules
     operator!=(Rules const& other) const;
 };
 
+std::optional<Rules> const&
+getCurrentTransactionRules();
+
+void
+setCurrentTransactionRules(std::optional<Rules> r);
+
+/** RAII class to set and restore the current transaction rules
+ */
+class CurrentTransactionRulesGuard
+{
+public:
+    explicit CurrentTransactionRulesGuard(Rules r)
+        : saved_(getCurrentTransactionRules())
+    {
+        setCurrentTransactionRules(std::move(r));
+    }
+
+    ~CurrentTransactionRulesGuard()
+    {
+        setCurrentTransactionRules(saved_);
+    }
+
+    CurrentTransactionRulesGuard() = delete;
+    CurrentTransactionRulesGuard(CurrentTransactionRulesGuard const&) = delete;
+    CurrentTransactionRulesGuard&
+    operator=(CurrentTransactionRulesGuard const&) = delete;
+
+private:
+    std::optional<Rules> saved_;
+};
+
 }  // namespace ripple
 #endif

src/ripple/protocol/impl/Rules.cpp

@@ -17,11 +17,36 @@
 */
 //==============================================================================
 
+#include <ripple/basics/LocalValue.h>
 #include <ripple/protocol/Feature.h>
 #include <ripple/protocol/Rules.h>
 
+#include <optional>
+
 namespace ripple {
 
+namespace {
+// Use a static inisde a function to help prevent order-of-initialization issues
+LocalValue<std::optional<Rules>>&
+getCurrentTransactionRulesRef()
+{
+    static LocalValue<std::optional<Rules>> r;
+    return r;
+}
+}  // namespace
+
+std::optional<Rules> const&
+getCurrentTransactionRules()
+{
+    return *getCurrentTransactionRulesRef();
+}
+
+void
+setCurrentTransactionRules(std::optional<Rules> r)
+{
+    *getCurrentTransactionRulesRef() = std::move(r);
+}
+
 class Rules::Impl
 {
 private: