fix: NULL RWDB and mutex locks

include/xrpl/basics/ReaderPreferringSharedMutex.h

@@ -0,0 +1,106 @@
+#pragma once
+
+#include <shared_mutex>
+
+// On Linux (glibc), std::shared_mutex wraps pthread_rwlock_t initialised
+// with PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP.  This means a
+// pending exclusive lock() blocks new shared (reader) acquisitions,
+// causing reader starvation when writers contend frequently.
+//
+// On macOS / ARM (libc++), std::shared_mutex is already reader-preferring,
+// so the same code behaves differently across platforms.
+//
+// This header provides reader_preferring_shared_mutex:
+//   - On Linux it wraps pthread_rwlock_t initialised with
+//     PTHREAD_RWLOCK_PREFER_READER_NP, matching macOS semantics.
+//   - On all other platforms it is a type alias for std::shared_mutex.
+//
+// The interface is identical to std::shared_mutex, so it works with
+// std::shared_lock and std::unique_lock.
+
+#if defined(__linux__)
+
+#include <cerrno>
+#include <pthread.h>
+#include <stdexcept>
+
+namespace ripple {
+
+class reader_preferring_shared_mutex
+{
+    pthread_rwlock_t rwlock_;
+
+public:
+    reader_preferring_shared_mutex()
+    {
+        pthread_rwlockattr_t attr;
+        pthread_rwlockattr_init(&attr);
+        pthread_rwlockattr_setkind_np(&attr, PTHREAD_RWLOCK_PREFER_READER_NP);
+        int rc = pthread_rwlock_init(&rwlock_, &attr);
+        pthread_rwlockattr_destroy(&attr);
+        if (rc != 0)
+            throw std::system_error(
+                rc, std::system_category(), "pthread_rwlock_init");
+    }
+
+    ~reader_preferring_shared_mutex()
+    {
+        pthread_rwlock_destroy(&rwlock_);
+    }
+
+    reader_preferring_shared_mutex(reader_preferring_shared_mutex const&) =
+        delete;
+    reader_preferring_shared_mutex&
+    operator=(reader_preferring_shared_mutex const&) = delete;
+
+    // Exclusive (writer) locking
+    void
+    lock()
+    {
+        pthread_rwlock_wrlock(&rwlock_);
+    }
+
+    bool
+    try_lock()
+    {
+        return pthread_rwlock_trywrlock(&rwlock_) == 0;
+    }
+
+    void
+    unlock()
+    {
+        pthread_rwlock_unlock(&rwlock_);
+    }
+
+    // Shared (reader) locking
+    void
+    lock_shared()
+    {
+        pthread_rwlock_rdlock(&rwlock_);
+    }
+
+    bool
+    try_lock_shared()
+    {
+        return pthread_rwlock_tryrdlock(&rwlock_) == 0;
+    }
+
+    void
+    unlock_shared()
+    {
+        pthread_rwlock_unlock(&rwlock_);
+    }
+};
+
+}  // namespace ripple
+
+#else  // !__linux__
+
+namespace ripple {
+
+// macOS, Windows, etc. — std::shared_mutex is already reader-preferring.
+using reader_preferring_shared_mutex = std::shared_mutex;
+
+}  // namespace ripple
+
+#endif

include/xrpl/protocol/IOUAmount.h

@@ -97,6 +97,12 @@ class IOUAmount : private boost::totally_ordered<IOUAmount>,
 
     static IOUAmount
     minPositiveAmount();
+
+    friend std::ostream&
+    operator<<(std::ostream& os, IOUAmount const& x)
+    {
+        return os << to_string(x);
+    }
 };
 
 inline IOUAmount::IOUAmount(beast::Zero)

include/xrpl/protocol/Rules.h

@@ -28,6 +28,9 @@
 
 namespace ripple {
 
+bool
+isFeatureEnabled(uint256 const& feature);
+
 class DigestAwareReadView;
 
 /** Rules controlling protocol behavior. */

src/libxrpl/protocol/Rules.cpp

@@ -153,4 +153,12 @@ Rules::operator!=(Rules const& other) const
 {
     return !(*this == other);
 }
+
+bool
+isFeatureEnabled(uint256 const& feature)
+{
+    auto const& rules = getCurrentTransactionRules();
+    return rules && rules->enabled(feature);
+}
+
 }  // namespace ripple