Move code into Movie::SignalDiscChange

DVDInterface shouldn't need to know anything about
the DTM format's 40-character limitation.

Also replacing "filename" in variable names with "path"
to make it clearer which variables contain the whole path
and which ones only contain the filename.

Source/Core/Core/HW/DVDInterface.cpp

@@ -460,48 +460,40 @@ static void EjectDiscCallback(u64 userdata, s64 cyclesLate)
 
 static void InsertDiscCallback(u64 userdata, s64 cyclesLate)
 {
-  std::string& SavedFileName = SConfig::GetInstance().m_strFilename;
-  std::string* _FileName = (std::string*)userdata;
+  const std::string& old_path = SConfig::GetInstance().m_strFilename;
+  std::string* new_path = reinterpret_cast<std::string*>(userdata);
 
-  if (!SetVolumeName(*_FileName))
+  if (!SetVolumeName(*new_path))
   {
     // Put back the old one
-    SetVolumeName(SavedFileName);
+    SetVolumeName(old_path);
     PanicAlertT("Invalid file");
   }
   SetDiscInside(VolumeIsValid());
-  delete _FileName;
+  delete new_path;
 }
 
 // Can only be called by the host thread
-void ChangeDiscAsHost(const std::string& newFileName)
+void ChangeDiscAsHost(const std::string& new_path)
 {
   bool was_unpaused = Core::PauseAndLock(true);
 
   // The host thread is now temporarily the CPU thread
-  ChangeDiscAsCPU(newFileName);
+  ChangeDiscAsCPU(new_path);
 
   Core::PauseAndLock(false, was_unpaused);
 }
 
 // Can only be called by the CPU thread
-void ChangeDiscAsCPU(const std::string& newFileName)
+void ChangeDiscAsCPU(const std::string& new_path)
 {
-  std::string* _FileName = new std::string(newFileName);
+  // TODO: This is bad. Pointers in CoreTiming userdata require
+  // manual memory management and aren't savestate-safe.
+  u64 new_path_pointer = reinterpret_cast<u64>(new std::string(new_path));
   CoreTiming::ScheduleEvent(0, s_eject_disc);
-  CoreTiming::ScheduleEvent(SystemTimers::GetTicksPerSecond(), s_insert_disc, (u64)_FileName);
-  if (Movie::IsRecordingInput())
-  {
-    std::string fileName = newFileName;
-    auto sizeofpath = fileName.find_last_of("/\\") + 1;
-    if (fileName.substr(sizeofpath).length() > 40)
-    {
-      PanicAlertT("The disc change to \"%s\" could not be saved in the .dtm file.\n"
-                  "The filename of the disc image must not be longer than 40 characters.",
-                  newFileName.c_str());
-    }
-    Movie::SignalDiscChange(fileName.substr(sizeofpath));
-  }
+  CoreTiming::ScheduleEvent(SystemTimers::GetTicksPerSecond(), s_insert_disc, new_path_pointer);
+
+  Movie::SignalDiscChange(new_path);
 }
 
 void SetLidOpen(bool open)

Source/Core/Core/HW/DVDInterface.h

@@ -108,8 +108,8 @@ bool VolumeIsValid();
 // Disc detection and swapping
 void SetDiscInside(bool _DiscInside);
 bool IsDiscInside();
-void ChangeDiscAsHost(const std::string& path);  // Can only be called by the host thread
-void ChangeDiscAsCPU(const std::string& path);   // Can only be called by the CPU thread
+void ChangeDiscAsHost(const std::string& new_path);  // Can only be called by the host thread
+void ChangeDiscAsCPU(const std::string& new_path);   // Can only be called by the CPU thread
 
 // DVD Access Functions
 bool ChangePartition(u64 offset);

Source/Core/Core/Movie.cpp

@@ -415,10 +415,22 @@ void SetClearSave(bool enabled)
   s_bClearSave = enabled;
 }
 
-void SignalDiscChange(const std::string& new_disc_filename)
+void SignalDiscChange(const std::string& new_path)
 {
-  s_discChange = new_disc_filename;
-  s_bDiscChange = true;
+  if (Movie::IsRecordingInput())
+  {
+    size_t size_of_path_without_filename = new_path.find_last_of("/\\") + 1;
+    std::string filename = new_path.substr(size_of_path_without_filename);
+    constexpr size_t maximum_length = sizeof(DTMHeader::discChange);
+    if (filename.length() > maximum_length)
+    {
+      PanicAlertT("The disc change to \"%s\" could not be saved in the .dtm file.\n"
+                  "The filename of the disc image must not be longer than 40 characters.",
+                  filename.c_str());
+    }
+    s_discChange = filename;
+    s_bDiscChange = true;
+  }
 }
 
 void SetReset(bool reset)

Source/Core/Core/Movie.h

@@ -129,7 +129,7 @@ u64 GetCurrentLagCount();
 u64 GetTotalLagCount();
 
 void SetClearSave(bool enabled);
-void SignalDiscChange(const std::string& new_disc_filename);
+void SignalDiscChange(const std::string& new_path);
 void SetReset(bool reset);
 void SetTitleId(u64 title_id);