Review feedback: rename readlock, add and update comments

src/xrpld/nodestore/detail/DatabaseRotatingImp.cpp

@@ -50,8 +50,10 @@ DatabaseRotatingImp::rotateWithLock(
     // This function should be the only one taking any kind of unique/write
     // lock, and should only be called once at a time by its syncronous caller.
 
-    boost::upgrade_lock readLock(mutex_, boost::defer_lock);
-    if (!readLock.try_lock())
+    // The upgradable lock will NOT block shared locks, but will block other
+    // upgrade locks and unique/exclusive locks.
+    boost::upgrade_lock upgradeableLock(mutex_, boost::defer_lock);
+    if (!upgradeableLock.try_lock())
     {
         // If anything other than a unit test gets here, something has gone very
         // wrong
@@ -66,7 +68,7 @@ DatabaseRotatingImp::rotateWithLock(
     // boost::upgrade_mutex guarantees that only one thread can have "upgrade
     // ownership" at a time, so this is 100% safe, and guaranteed to avoid
     // deadlock.
-    boost::upgrade_to_unique_lock writeLock(readLock);
+    boost::upgrade_to_unique_lock writeLock(upgradeableLock);
 
     archiveBackend_->setDeletePath();
     archiveBackend_ = std::move(writableBackend_);

src/xrpld/nodestore/detail/DatabaseRotatingImp.h

@@ -88,7 +88,7 @@ class DatabaseRotatingImp : public DatabaseRotating
     bool const unitTest_;
 
     // Implements the "UpgradeLockable" concept
-    // https://www.boost.org/doc/libs/1_86_0/doc/html/thread/synchronization.html#thread.synchronization.mutex_concepts.upgrade_lockable
+    // https://www.boost.org/doc/libs/release/doc/html/thread/synchronization.html#thread.synchronization.mutex_concepts.upgrade_lockable
     // In short: Many threads can have shared ownership. One thread can have
     // upgradable ownership at the same time as others have shared ownership.
     // The upgradeable ownership can be upgraded to exclusive ownership,