Bus: Expose shared memory on Unix systems

On Windows, the shared memory handle is closed on Destroy() and
therefore is accessible at runtime. On Unixes the shared memory was
unlinked right after its creation, effectively making the shared memory
unavailable from the outside world.
This commit brings feature parity to Unix systems.
Allows the shared memory to be accessed at runtime from an external
process for RAM inspection.

src/common/memmap.cpp

@@ -63,8 +63,11 @@ void* MemMap::CreateSharedMemory(const char* name, size_t size, Error* error)
   return mapping;
 }
 
-void MemMap::DestroySharedMemory(void* ptr)
+void MemMap::DestroySharedMemory(void* ptr, const char* name)
 {
+  // Unused
+  (void)name;
+
   CloseHandle(static_cast<HANDLE>(ptr));
 }
 
@@ -319,8 +322,11 @@ void* MemMap::CreateSharedMemory(const char* name, size_t size, Error* error)
   return reinterpret_cast<void*>(static_cast<uintptr_t>(port));
 }
 
-void MemMap::DestroySharedMemory(void* ptr)
+void MemMap::DestroySharedMemory(void* ptr, const char* name)
 {
+  // Unused
+  (void)name;
+
   mach_port_deallocate(mach_task_self(), static_cast<mach_port_t>(reinterpret_cast<uintptr_t>(ptr)));
 }
 
@@ -487,9 +493,6 @@ void* MemMap::CreateSharedMemory(const char* name, size_t size, Error* error)
     return nullptr;
   }
 
-  // we're not going to be opening this mapping in other processes, so remove the file
-  shm_unlink(name);
-
   // use fallocate() to ensure we don't SIGBUS later on.
 #ifdef __linux__
   if (fallocate(fd, 0, 0, static_cast<off_t>(size)) < 0)
@@ -509,8 +512,9 @@ void* MemMap::CreateSharedMemory(const char* name, size_t size, Error* error)
   return reinterpret_cast<void*>(static_cast<intptr_t>(fd));
 }
 
-void MemMap::DestroySharedMemory(void* ptr)
+void MemMap::DestroySharedMemory(void* ptr, const char* name)
 {
+  shm_unlink(name);
   close(static_cast<int>(reinterpret_cast<intptr_t>(ptr)));
 }
 

src/common/memmap.h

@@ -53,7 +53,7 @@ class Error;
 namespace MemMap {
 std::string GetFileMappingName(const char* prefix);
 void* CreateSharedMemory(const char* name, size_t size, Error* error);
-void DestroySharedMemory(void* ptr);
+void DestroySharedMemory(void* ptr, const char* name);
 void* MapSharedMemory(void* handle, size_t offset, void* baseaddr, size_t size, PageProtect mode);
 void UnmapSharedMemory(void* baseaddr, size_t size);
 bool MemProtect(void* baseaddr, size_t size, PageProtect mode);

src/core/bus.cpp

@@ -105,6 +105,7 @@ union MEMCTRL
 } // namespace
 
 static void* s_shmem_handle = nullptr;
+static std::string s_shmem_name = MemMap::GetFileMappingName("duckstation");
 
 std::bitset<RAM_8MB_CODE_PAGE_COUNT> g_ram_code_bits{};
 u8* g_ram = nullptr;
@@ -177,7 +178,7 @@ static constexpr size_t TOTAL_SIZE = LUT_OFFSET + LUT_SIZE;
 bool Bus::AllocateMemory(Error* error)
 {
   s_shmem_handle =
-    MemMap::CreateSharedMemory(MemMap::GetFileMappingName("duckstation").c_str(), MemoryMap::TOTAL_SIZE, error);
+    MemMap::CreateSharedMemory(s_shmem_name.c_str(), MemoryMap::TOTAL_SIZE, error);
   if (!s_shmem_handle)
   {
 #ifndef __linux__
@@ -287,7 +288,7 @@ void Bus::ReleaseMemory()
 
   if (s_shmem_handle)
   {
-    MemMap::DestroySharedMemory(s_shmem_handle);
+    MemMap::DestroySharedMemory(s_shmem_handle, s_shmem_name.c_str());
     s_shmem_handle = nullptr;
   }
 }