mir::SharedLibrary: Add a stable Handle for SharedLibrary

It can be useful to know if two `SharedLibrary` instances refer to the same DSO.
The new `SharedLibrary::Handle` implements such a comparable handle. This works
because `dlopen` guarantees that loading an already-loaded DSO returns the same
handle.

include/common/mir/shared_library.h

@@ -17,6 +17,7 @@
 #ifndef MIR_SHARED_LIBRARY_H_
 #define MIR_SHARED_LIBRARY_H_
 
+#include <compare>
 #include <string>
 
 namespace mir
@@ -49,6 +50,21 @@ class SharedLibrary
         (void*&)result = load_symbol(function_name.c_str(), version.c_str());
         return result;
     }
+
+    class Handle
+    {
+    public:    
+        auto operator<=>(Handle const& rhs) const -> std::strong_ordering;
+        auto operator==(Handle const& rhs) const -> bool = default;
+        auto operator!=(Handle const& rhs) const -> bool = default;
+    private:
+        friend class SharedLibrary;
+
+        Handle(void* handle);
+        void* handle;
+    };
+
+    auto get_handle() const -> Handle;
 private:
     void* const so;
     void* load_symbol(char const* function_name) const;
@@ -58,5 +74,4 @@ class SharedLibrary
 };
 }
 
-
 #endif /* MIR_SHARED_LIBRARY_H_ */

src/common/sharedlibrary/shared_library.cpp

@@ -15,6 +15,7 @@
  */
 
 #include "mir/shared_library.h"
+#include <compare>
 
 #ifdef MIR_DONT_USE_DLVSYM
 #include <mir/log.h>
@@ -74,3 +75,18 @@ void* mir::SharedLibrary::load_symbol(char const* function_name, char const* ver
     }
 #endif
 }
+
+auto mir::SharedLibrary::get_handle() const -> Handle
+{
+    return Handle{so};
+}
+
+mir::SharedLibrary::Handle::Handle(void* handle)
+    : handle{handle}
+{
+}
+
+auto mir::SharedLibrary::Handle::operator<=>(Handle const& rhs) const -> std::strong_ordering
+{
+    return handle <=> rhs.handle;
+}

src/common/symbols.map

@@ -325,6 +325,9 @@ global:
     mir::SharedLibrary::?SharedLibrary*;
     mir::SharedLibrary::SharedLibrary*;
     mir::SharedLibrary::load_symbol*;
+    mir::SharedLibrary::get_handle*;
+    mir::SharedLibrary::Handle::?Handle*;
+    mir::SharedLibrary::Handle::operator*;
     mir::SharedLibraryProberReport::?SharedLibraryProberReport*;
     mir::SharedLibraryProberReport::SharedLibraryProberReport*;
     mir::SharedLibraryProberReport::operator*;
@@ -500,6 +503,7 @@ global:
     typeinfo?for?mir::NonBlockingExecutor;
     typeinfo?for?mir::PosixRWMutex;
     typeinfo?for?mir::SharedLibrary;
+    typeinfo?for?mir::SharedLibrary::Handle;
     typeinfo?for?mir::SharedLibraryProberReport;
     typeinfo?for?mir::Signal;
     typeinfo?for?mir::ThreadPoolExecutor;
@@ -695,4 +699,4 @@ MIR_COMMON_2.18 {
     MirTouchpadConfig::disable_with_external_mouse*;
     mir::event_type_to_c_str*;
   };
-} MIR_COMMON_2.17;
+} MIR_COMMON_2.17;