Singleton backend/device-watcher

src/CMakeLists.txt

@@ -103,6 +103,7 @@ target_sources(${LRS_TARGET}
         "${CMAKE_CURRENT_LIST_DIR}/api.h"
         "${CMAKE_CURRENT_LIST_DIR}/archive.h"
         "${CMAKE_CURRENT_LIST_DIR}/backend.h"
+        "${CMAKE_CURRENT_LIST_DIR}/backend-device.h"
         "${CMAKE_CURRENT_LIST_DIR}/platform/backend-device-group.h"
         "${CMAKE_CURRENT_LIST_DIR}/platform/platform-device-info.h"
         "${CMAKE_CURRENT_LIST_DIR}/platform/device-watcher.h"

src/backend-device-factory.cpp

@@ -7,14 +7,24 @@
 #include "platform/platform-device-info.h"
 #include "platform/device-watcher.h"
 
+#include "backend-device.h"
 #include "ds/d400/d400-info.h"
 #include "ds/d500/d500-info.h"
 #include "fw-update/fw-update-factory.h"
 #include "platform-camera.h"
 
+#include <rsutils/shared-ptr-singleton.h>
+#include <rsutils/signal.h>
 #include <rsutils/json.h>
 
 
+namespace librealsense {
+namespace platform {
+std::shared_ptr< backend > create_backend();
+}  // namespace platform
+}  // namespace librealsense
+
+
 namespace {
 
 
@@ -77,56 +87,122 @@ subtract_sets( const std::vector< std::shared_ptr< librealsense::platform::platf
 namespace librealsense {
 
 
-backend_device_factory::backend_device_factory( context & ctx, callback && cb )
-    : _device_watcher( ctx.get_backend().create_device_watcher() )
-    , _context( ctx )
+// This singleton creates the actual backend; as long as someone holds it, the backend will stay alive.
+// The instance is held below by the device-watcher. I.e., the device-watcher triggers creation of the
+// backend!
+//
+class backend_singleton
 {
-    assert( _device_watcher->is_stopped() );
-    _device_watcher->start(
-        [this, cb = std::move( cb )]( platform::backend_device_group const & old,
-                                      platform::backend_device_group const & curr )
-        {
-            auto old_list = create_devices_from_group( old, RS2_PRODUCT_LINE_ANY );
-            auto new_list = create_devices_from_group( curr, RS2_PRODUCT_LINE_ANY );
+    std::shared_ptr< platform::backend > const _backend;
 
-            std::vector< rs2_device_info > devices_removed;
-            for( auto & device_removed : subtract_sets( old_list, new_list ) )
-            {
-                devices_removed.push_back( { _context.shared_from_this(), device_removed } );
-                LOG_DEBUG( "Device disconnected: " << device_removed->get_address() );
-            }
+public:
+    backend_singleton()
+        : _backend( platform::create_backend() )
+    {
+    }
 
-            std::vector< rs2_device_info > devices_added;
-            for( auto & device_added : subtract_sets( new_list, old_list ) )
-            {
-                devices_added.push_back( { _context.shared_from_this(), device_added } );
-                LOG_DEBUG( "Device connected: " << device_added->get_address() );
-            }
+    std::shared_ptr< platform::backend > get() const { return _backend; }
+};
 
-            if( devices_removed.size() + devices_added.size() )
-            {
-                cb( devices_removed, devices_added );
-            }
-        } );
+
+static rsutils::shared_ptr_singleton< backend_singleton > the_backend;
+
+
+// The device-watcher is also a singleton: we don't need multiple agents of notifications. It is held alive by the
+// device-factory below, which is held per context. I.e., as long as the context is alive, we'll stay alive and the
+// backend-singleton will stay alive.
+// 
+// We are responsible for exposing the single notification from the platform-device-watcher to several subscribers:
+// one device-watcher, but many contexts, each with further subscriptions.
+//
+class device_watcher_singleton
+{
+    // The device-watcher keeps a direct pointer to the backend instance, so we have to make sure it stays alive!
+    std::shared_ptr< backend_singleton > const _backend;
+    std::shared_ptr< platform::device_watcher > const _device_watcher;
+    rsutils::signal< platform::backend_device_group const &, platform::backend_device_group const & > _callbacks;
+
+public:
+    device_watcher_singleton()
+        : _backend( the_backend.instance() )
+        , _device_watcher( _backend->get()->create_device_watcher() )
+    {
+        assert( _device_watcher->is_stopped() );
+        _device_watcher->start(
+            [this]( platform::backend_device_group const & old, platform::backend_device_group const & curr )
+            { _callbacks.raise( old, curr ); } );
+    }
+
+    rsutils::subscription subscribe( platform::device_changed_callback && cb )
+    {
+        return _callbacks.subscribe( std::move( cb ) );
+    }
+
+    std::shared_ptr< platform::backend > const get_backend() const { return _backend->get(); }
+};
+
+
+static rsutils::shared_ptr_singleton< device_watcher_singleton > backend_device_watcher;
+
+
+std::shared_ptr< platform::backend > backend_device::get_backend()
+{
+    auto singleton = the_backend.get();
+    if( ! singleton )
+        // Whoever is calling us, they are expecting a backend to exist, but it does not!
+        throw std::runtime_error( "backend not created yet!" );
+
+    return singleton->get();
+}
+
+
+backend_device_factory::backend_device_factory( context & ctx, callback && cb )
+    : _context( ctx )
+    , _device_watcher( backend_device_watcher.instance() )
+    , _dtor( _device_watcher->subscribe(
+          [this, cb = std::move( cb )]( platform::backend_device_group const & old,
+                                        platform::backend_device_group const & curr )
+          {
+              auto old_list = create_devices_from_group( old, RS2_PRODUCT_LINE_ANY );
+              auto new_list = create_devices_from_group( curr, RS2_PRODUCT_LINE_ANY );
+
+              std::vector< rs2_device_info > devices_removed;
+              for( auto & device_removed : subtract_sets( old_list, new_list ) )
+              {
+                  devices_removed.push_back( { _context.shared_from_this(), device_removed } );
+                  LOG_DEBUG( "Device disconnected: " << device_removed->get_address() );
+              }
+
+              std::vector< rs2_device_info > devices_added;
+              for( auto & device_added : subtract_sets( new_list, old_list ) )
+              {
+                  devices_added.push_back( { _context.shared_from_this(), device_added } );
+                  LOG_DEBUG( "Device connected: " << device_added->get_address() );
+              }
+
+              if( devices_removed.size() + devices_added.size() )
+              {
+                  cb( devices_removed, devices_added );
+              }
+          } ) )
+{
 }
 
 
 backend_device_factory::~backend_device_factory()
 {
-    if( _device_watcher )
-        _device_watcher->stop();
 }
 
 
 std::vector< std::shared_ptr< device_info > > backend_device_factory::query_devices( unsigned requested_mask ) const
 {
-    if( (requested_mask & RS2_PRODUCT_LINE_SW_ONLY) || (_context.get_device_mask() & RS2_PRODUCT_LINE_SW_ONLY) )
+    if( ( requested_mask & RS2_PRODUCT_LINE_SW_ONLY ) || ( _context.get_device_mask() & RS2_PRODUCT_LINE_SW_ONLY ) )
         return {};  // We don't carry any software devices
 
-    auto & backend = _context.get_backend();
-    platform::backend_device_group group( backend.query_uvc_devices(),
-                                          backend.query_usb_devices(),
-                                          backend.query_hid_devices() );
+    auto backend = _device_watcher->get_backend();
+    platform::backend_device_group group( backend->query_uvc_devices(),
+                                          backend->query_usb_devices(),
+                                          backend->query_hid_devices() );
     auto devices = create_devices_from_group( group, requested_mask );
     return { devices.begin(), devices.end() };
 }
@@ -169,4 +245,4 @@ backend_device_factory::create_devices_from_group( platform::backend_device_grou
 }
 
 
-}  // namespace librealsense
+}  // namespace librealsense

src/backend-device-factory.h

@@ -3,7 +3,7 @@
 
 #pragma once
 
-#include <functional>
+#include <rsutils/subscription.h>
 #include <memory>
 #include <vector>
 
@@ -16,10 +16,10 @@ namespace librealsense {
 
 class device_info;
 class context;
+class device_watcher_singleton;
 
 
 namespace platform {
-class device_watcher;
 struct backend_device_group;
 class platform_device_info;
 }  // namespace platform
@@ -38,12 +38,13 @@ class platform_device_info;
 class backend_device_factory
 {
     context & _context;
-    std::shared_ptr< platform::device_watcher > const _device_watcher;
+    std::shared_ptr< device_watcher_singleton > const _device_watcher;
+    rsutils::subscription const _dtor;  // raii generic code, used to automatically unsubscribe our callback
 
+public:
     using callback = std::function< void( std::vector< rs2_device_info > & rs2_devices_info_removed,
                                           std::vector< rs2_device_info > & rs2_devices_info_added ) >;
 
-public:
     backend_device_factory( context &, callback && );
     ~backend_device_factory();
 

src/ds/ds-device.h → src/backend-device.h

@@ -4,25 +4,32 @@
 #pragma once
 
 #include <src/device.h>
+#include <memory>
 
 
 namespace librealsense {
 
 
-// Common base class for all Stereo devices
+namespace platform {
+class backend;
+}
+
+
+// Common base class for all backend devices (i.e., those that require a platform backend)
 //
-class ds_device : public virtual device
+class backend_device : public virtual device
 {
     typedef device super;
 
 protected:
-    ds_device( std::shared_ptr< const device_info > const & dev_info, bool device_changed_notifications = true )
+    backend_device( std::shared_ptr< const device_info > const & dev_info, bool device_changed_notifications = true )
         : super( dev_info, device_changed_notifications )
     {
     }
 
 public:
     uint16_t get_pid() const { return _pid; }
+    std::shared_ptr< platform::backend > get_backend();
 
 protected:
     uint16_t _pid = 0;

src/backend.h

@@ -53,8 +53,6 @@ namespace librealsense
             virtual ~backend() = default;
         };
 
-        std::shared_ptr<backend> create_backend();
-
     }
 
     double monotonic_to_realtime(double monotonic);

src/context.cpp

@@ -42,14 +42,13 @@ using json = nlohmann::json;
 namespace librealsense
 {
     context::context( json const & settings )
-        : _backend( platform::create_backend() )
-        , _settings( settings )
+        : _settings( settings )
         , _device_mask( rsutils::json::get< unsigned >( settings, "device-mask", RS2_PRODUCT_LINE_ANY ) )
         , _devices_changed_callback( nullptr, []( rs2_devices_changed_callback * ) {} )
         , _backend_device_factory(
-            *this,
-            [this]( std::vector< rs2_device_info > & removed, std::vector< rs2_device_info > & added )
-            { invoke_devices_changed_callbacks( removed, added ); } )
+              *this,
+              [this]( std::vector< rs2_device_info > & removed, std::vector< rs2_device_info > & added )
+              { invoke_devices_changed_callbacks( removed, added ); } )
     {
         static bool version_logged = false;
         if( ! version_logged )
@@ -58,7 +57,6 @@ namespace librealsense
             LOG_DEBUG( "Librealsense VERSION: " << RS2_API_VERSION_STR );
         }
 
-
 #ifdef BUILD_WITH_DDS
         nlohmann::json dds_settings
             = rsutils::json::get< nlohmann::json >( settings, std::string( "dds", 3 ), nlohmann::json::object() );
@@ -116,7 +114,7 @@ namespace librealsense
         // The normal bits enable, so enable only those that are on
         mask &= mask_in_settings & ~RS2_PRODUCT_LINE_SW_ONLY;
         // But the above turned off the SW-only bits, so turn them back on again
-        if( (mask_in_settings & RS2_PRODUCT_LINE_SW_ONLY) || (requested_mask & RS2_PRODUCT_LINE_SW_ONLY) )
+        if( ( mask_in_settings & RS2_PRODUCT_LINE_SW_ONLY ) || ( requested_mask & RS2_PRODUCT_LINE_SW_ONLY ) )
             mask |= RS2_PRODUCT_LINE_SW_ONLY;
         return mask;
     }

src/context.h

@@ -4,8 +4,7 @@
 #pragma once
 
 #include "backend-device-factory.h"
-#include "device-info.h"
-#include "types.h"
+#include "types.h"  // devices_changed_callback_ptr
 
 #include <rsutils/lazy.h>
 #include <nlohmann/json.hpp>
@@ -49,19 +48,11 @@ namespace realdds {
 
 namespace librealsense
 {
-    class device;
-    class context;
     class playback_device_info;
     class stream_interface;
 
-    namespace platform {
-        class backend;
-        class device_watcher;
-    }
-
     class context : public std::enable_shared_from_this<context>
     {
-        context();
     public:
         explicit context( nlohmann::json const & );
         explicit context( char const * json_settings );
@@ -81,7 +72,6 @@ namespace librealsense
         static unsigned combine_device_masks( unsigned requested_mask, unsigned mask_in_settings );
 
         std::vector<std::shared_ptr<device_info>> query_devices(int mask) const;
-        const platform::backend& get_backend() const { return *_backend; }
 
         uint64_t register_internal_device_callback(devices_changed_callback_ptr callback);
         void unregister_internal_device_callback(uint64_t cb_id);
@@ -101,8 +91,6 @@ namespace librealsense
                                                std::vector<rs2_device_info> & rs2_devices_info_added );
         void raise_devices_changed(const std::vector<rs2_device_info>& removed, const std::vector<rs2_device_info>& added);
 
-        std::shared_ptr<platform::backend> _backend;
-
         std::map<std::string, std::weak_ptr<device_info>> _playback_devices;
         std::map<uint64_t, devices_changed_callback_ptr> _devices_changed_callbacks;
 #ifdef BUILD_WITH_DDS

src/dds/rs-dds-device-info.h

@@ -3,7 +3,8 @@
 
 #pragma once
 
-#include <src/context.h>  // device_info, context
+#include <src/context.h>
+#include <src/device-info.h>
 
 #include <memory>
 

src/ds/CMakeLists.txt

@@ -18,7 +18,6 @@ target_sources(${LRS_TARGET}
         "${CMAKE_CURRENT_LIST_DIR}/advanced_mode/advanced_mode.cpp"
         "${CMAKE_CURRENT_LIST_DIR}/ds-calib-parsers.cpp"
         "${CMAKE_CURRENT_LIST_DIR}/ds-device-common.h"
-        "${CMAKE_CURRENT_LIST_DIR}/ds-device.h"
         "${CMAKE_CURRENT_LIST_DIR}/ds-motion-common.h"
         "${CMAKE_CURRENT_LIST_DIR}/ds-color-common.h"
         "${CMAKE_CURRENT_LIST_DIR}/ds-active-common.h"

src/ds/d400/d400-color.cpp

@@ -43,7 +43,6 @@ namespace librealsense
     void d400_color::create_color_device(std::shared_ptr<context> ctx, const platform::backend_device_group& group)
     {
         using namespace ds;
-        auto&& backend = ctx->get_backend();
 
         _color_calib_table_raw = [this]()
         {
@@ -82,7 +81,7 @@ namespace librealsense
                 info = color_devs_info[1];
             else
                 info = color_devs_info.front();
-            auto uvcd = backend.create_uvc_device(info);
+            auto uvcd = get_backend()->create_uvc_device( info );
             //auto ftr = std::unique_ptr<frame_timestamp_reader>(new global_timestamp_reader(std::move(d400_timestamp_reader_metadata), _tf_keeper, enable_global_time_option));
             auto raw_color_ep = std::make_shared<uvc_sensor>("Raw RGB Camera",
                 uvcd,