librbd, tools: migrate from boost::variant to std::variant

Complete migration started in commit 017f333, replacing boost::variant with
std::variant throughout the librbd codebase. This change is part of our ongoing
effort to reduce third-party dependencies by leveraging C++ standard library
alternatives where possible.

Benefits include:
- Improved code readability and maintainability
- Reduced external dependency surface
- More consistent API usage with other components

Implementation note: Unlike Boost.variant, std::variant lacks built-in
operator<< support. This commit implements the necessary operator<< for
AttributeValue, our specific std::variant instantiation, to preserve the
existing behavior.

Also, despite that `apply_visit()` calls can be replaced with `visit()`
without being qualified with `std::` because of ADL, we are taking this
opportunity to adding the `std::` prefix for better readability.

Signed-off-by: Kefu Chai <[email protected]>

Author: tchaikov

src/librbd/MirroringWatcher.cc

@@ -108,8 +108,8 @@ void MirroringWatcher<I>::handle_notify(uint64_t notify_id, uint64_t handle,
     return;
   }
 
-  apply_visitor(watcher::util::HandlePayloadVisitor<MirroringWatcher<I>>(
-                  this, notify_id, handle), notify_message.payload);
+  std::visit(watcher::util::HandlePayloadVisitor<MirroringWatcher<I>>(
+      this, notify_id, handle), notify_message.payload);
 }
 
 template <typename I>

src/librbd/TrashWatcher.cc

@@ -83,7 +83,7 @@ void TrashWatcher<I>::handle_notify(uint64_t notify_id, uint64_t handle,
     return;
   }
 
-  apply_visitor(watcher::util::HandlePayloadVisitor<TrashWatcher<I>>(
+  std::visit(watcher::util::HandlePayloadVisitor<TrashWatcher<I>>(
     this, notify_id, handle), notify_message.payload);
 }
 

src/librbd/WatchNotifyTypes.h

@@ -12,7 +12,6 @@
 #include <list>
 #include <memory>
 #include <string>
-#include <boost/variant.hpp>
 
 namespace ceph {
 class Formatter;

src/librbd/api/Snapshot.cc

@@ -13,8 +13,6 @@
 #include "include/Context.h"
 #include "common/Cond.h"
 
-#include <boost/variant.hpp>
-
 #include <shared_mutex> // for std::shared_lock
 
 #define dout_subsys ceph_subsys_rbd

src/librbd/io/ImageDispatchSpec.cc

@@ -6,7 +6,6 @@
 #include "librbd/io/AioCompletion.h"
 #include "librbd/io/ImageRequest.h"
 #include "librbd/io/ImageDispatcherInterface.h"
-#include <boost/variant.hpp>
 
 namespace librbd {
 namespace io {

src/librbd/io/ObjectDispatchSpec.cc

@@ -4,7 +4,6 @@
 #include "librbd/io/ObjectDispatchSpec.h"
 #include "include/Context.h"
 #include "librbd/io/ObjectDispatcherInterface.h"
-#include <boost/variant.hpp>
 
 namespace librbd {
 namespace io {

src/librbd/io/ObjectDispatchSpec.h

@@ -11,7 +11,7 @@
 #include "common/zipkin_trace.h"
 #include "librbd/Types.h"
 #include "librbd/io/Types.h"
-#include <boost/variant/variant.hpp>
+#include <variant>
 
 namespace librbd {
 namespace io {
@@ -144,13 +144,13 @@ struct ObjectDispatchSpec {
     }
   };
 
-  typedef boost::variant<ReadRequest,
-                         DiscardRequest,
-                         WriteRequest,
-                         WriteSameRequest,
-                         CompareAndWriteRequest,
-                         FlushRequest,
-                         ListSnapsRequest> Request;
+  typedef std::variant<ReadRequest,
+		       DiscardRequest,
+		       WriteRequest,
+		       WriteSameRequest,
+		       CompareAndWriteRequest,
+		       FlushRequest,
+		       ListSnapsRequest> Request;
 
   C_Dispatcher dispatcher_ctx;
 

src/librbd/io/ObjectDispatcher.cc

@@ -10,7 +10,6 @@
 #include "librbd/asio/ContextWQ.h"
 #include "librbd/io/ObjectDispatch.h"
 #include "librbd/io/ObjectDispatchSpec.h"
-#include <boost/variant.hpp>
 
 #include <shared_mutex> // for std::shared_lock
 
@@ -35,7 +34,7 @@ struct ObjectDispatcher<I>::C_ResetExistenceCache : public C_LayerIterator {
 };
 
 template <typename I>
-struct ObjectDispatcher<I>::SendVisitor : public boost::static_visitor<bool> {
+struct ObjectDispatcher<I>::SendVisitor {
   ObjectDispatchInterface* object_dispatch;
   ObjectDispatchSpec* object_dispatch_spec;
 
@@ -199,7 +198,7 @@ template <typename I>
 bool ObjectDispatcher<I>::send_dispatch(
     ObjectDispatchInterface* object_dispatch,
     ObjectDispatchSpec* object_dispatch_spec) {
-  return boost::apply_visitor(
+  return std::visit(
     SendVisitor{object_dispatch, object_dispatch_spec},
     object_dispatch_spec->request);
 }

src/librbd/io/ReadResult.cc

@@ -6,8 +6,6 @@
 #include "common/dout.h"
 #include "librbd/io/AioCompletion.h"
 #include "librbd/io/Utils.h"
-#include <boost/variant/apply_visitor.hpp>
-#include <boost/variant/static_visitor.hpp>
 
 #define dout_subsys ceph_subsys_rbd
 #undef dout_prefix
@@ -17,7 +15,7 @@
 namespace librbd {
 namespace io {
 
-struct ReadResult::SetImageExtentsVisitor : public boost::static_visitor<void> {
+struct ReadResult::SetImageExtentsVisitor {
   Extents image_extents;
 
   explicit SetImageExtentsVisitor(const Extents& image_extents)
@@ -40,15 +38,15 @@ struct ReadResult::SetImageExtentsVisitor : public boost::static_visitor<void> {
   }
 };
 
-struct ReadResult::AssembleResultVisitor : public boost::static_visitor<void> {
+struct ReadResult::AssembleResultVisitor {
   CephContext *cct;
   Striper::StripedReadResult &destriper;
 
   AssembleResultVisitor(CephContext *cct, Striper::StripedReadResult &destriper)
     : cct(cct), destriper(destriper) {
   }
 
-  void operator()(Empty &empty) const {
+  void operator()(std::monostate &empty) const {
     ldout(cct, 20) << "dropping read result" << dendl;
   }
 
@@ -230,9 +228,6 @@ void ReadResult::C_ObjectReadMergedExtents::finish(int r) {
   on_finish->complete(r);
 }
 
-ReadResult::ReadResult() : m_buffer(Empty()) {
-}
-
 ReadResult::ReadResult(char *buf, size_t buf_len)
   : m_buffer(Linear(buf, buf_len)) {
 }
@@ -250,11 +245,11 @@ ReadResult::ReadResult(Extents* extent_map, ceph::bufferlist* bl)
 }
 
 void ReadResult::set_image_extents(const Extents& image_extents) {
-  boost::apply_visitor(SetImageExtentsVisitor(image_extents), m_buffer);
+  std::visit(SetImageExtentsVisitor(image_extents), m_buffer);
 }
 
 void ReadResult::assemble_result(CephContext *cct) {
-  boost::apply_visitor(AssembleResultVisitor(cct, m_destriper), m_buffer);
+  std::visit(AssembleResultVisitor(cct, m_destriper), m_buffer);
 }
 
 } // namespace io

src/librbd/io/ReadResult.h

@@ -11,7 +11,7 @@
 #include "librbd/io/Types.h"
 #include "osdc/Striper.h"
 #include <sys/uio.h>
-#include <boost/variant/variant.hpp>
+#include <variant>
 
 
 namespace librbd {
@@ -60,7 +60,7 @@ class ReadResult {
       void finish(int r) override;
   };
 
-  ReadResult();
+  ReadResult() = default;
   ReadResult(char *buf, size_t buf_len);
   ReadResult(const struct iovec *iov, int iov_count);
   ReadResult(ceph::bufferlist *bl);
@@ -71,9 +71,6 @@ class ReadResult {
   void assemble_result(CephContext *cct);
 
 private:
-  struct Empty {
-  };
-
   struct Linear {
     char *buf;
     size_t buf_len;
@@ -109,11 +106,11 @@ class ReadResult {
     }
   };
 
-  typedef boost::variant<Empty,
-                         Linear,
-                         Vector,
-                         Bufferlist,
-                         SparseBufferlist> Buffer;
+  typedef std::variant<std::monostate,
+		       Linear,
+		       Vector,
+		       Bufferlist,
+		       SparseBufferlist> Buffer;
   struct SetImageExtentsVisitor;
   struct AssembleResultVisitor;