Core: remove support for writing MemFile undo steps as .blend files

In many modes, Blender uses the `MemFile` undo step, which serializes all DNA
data in RAM almost as if writing a .blend file. For auto-save, Blender used to
write the last `MemFile` undo step to disk because that was faster serializing
all of DNA again. Furthermore, saving the `quit.blend` file when closing Blender
also used this.

This functionality is now removed in preparate for supporting implicit sharing
in the undo system (#106903). Auto-save and saving the quit.blend file now use
regular file saving.

The removal of this feature and its implications have also been discussed here:
https://devtalk.blender.org/t/remove-support-for-saving-memfile-undo-steps-as-blend-files-proposal/33544

source/blender/blenloader/BLO_undofile.hh

@@ -94,11 +94,5 @@ void BLO_memfile_clear_future(MemFile *memfile);
 /* Utilities. */
 
 Main *BLO_memfile_main_get(MemFile *memfile, Main *bmain, Scene **r_scene);
-/**
- * Saves .blend using undo buffer.
- *
- * \return success.
- */
-bool BLO_memfile_write_file(MemFile *memfile, const char *filepath);
 
 FileReader *BLO_memfile_new_filereader(MemFile *memfile, int undo_direction);

source/blender/blenloader/intern/undofile.cc

@@ -175,61 +175,6 @@ Main *BLO_memfile_main_get(MemFile *memfile, Main *bmain, Scene **r_scene)
   return bmain_undo;
 }
 
-bool BLO_memfile_write_file(MemFile *memfile, const char *filepath)
-{
-  MemFileChunk *chunk;
-  int file, oflags;
-
-  /* NOTE: This is currently used for auto-save and `quit.blend`,
-   * where _not_ following symbolic-links is OK,
-   * however if this is ever executed explicitly by the user,
-   * we may want to allow writing to symbolic-links. */
-
-  oflags = O_BINARY | O_WRONLY | O_CREAT | O_TRUNC;
-#ifdef O_NOFOLLOW
-  /* use O_NOFOLLOW to avoid writing to a symlink - use 'O_EXCL' (CVE-2008-1103) */
-  oflags |= O_NOFOLLOW;
-#else
-  /* TODO(sergey): How to deal with symlinks on windows? */
-#  ifndef _MSC_VER
-#    warning "Symbolic links will be followed on undo save, possibly causing CVE-2008-1103"
-#  endif
-#endif
-  file = BLI_open(filepath, oflags, 0666);
-
-  if (file == -1) {
-    fprintf(stderr,
-            "Unable to save '%s': %s\n",
-            filepath,
-            errno ? strerror(errno) : "Unknown error opening file");
-    return false;
-  }
-
-  for (chunk = static_cast<MemFileChunk *>(memfile->chunks.first); chunk;
-       chunk = static_cast<MemFileChunk *>(chunk->next))
-  {
-#ifdef _WIN32
-    if (size_t(write(file, chunk->buf, uint(chunk->size))) != chunk->size)
-#else
-    if (size_t(write(file, chunk->buf, chunk->size)) != chunk->size)
-#endif
-    {
-      break;
-    }
-  }
-
-  close(file);
-
-  if (chunk) {
-    fprintf(stderr,
-            "Unable to save '%s': %s\n",
-            filepath,
-            errno ? strerror(errno) : "Unknown error writing file");
-    return false;
-  }
-  return true;
-}
-
 static int64_t undo_read(FileReader *reader, void *buffer, size_t size)
 {
   UndoReader *undo = (UndoReader *)reader;

source/blender/windowmanager/intern/wm_files.cc

@@ -2115,9 +2115,12 @@ static bool wm_autosave_write_try(Main *bmain, wmWindowManager *wm)
 
   wm_autosave_location(filepath);
 
-  if (MemFile *memfile = ED_undosys_stack_memfile_get_if_active(wm->undo_stack)) {
-    /* Fast save of last undo-buffer, now with UI. */
-    BLO_memfile_write_file(memfile, filepath);
+  /* Technically, we could always just save here, but that would cause performance regressions
+   * compared to when the #MemFile undo step was used for saving undo-steps. So for now just skip
+   * auto-save when we are in a mode where auto-save wouldn't have worked previously anyway. This
+   * check can be removed once the performance regressions have been solved. */
+  if (ED_undosys_stack_memfile_get_if_active(wm->undo_stack) != nullptr) {
+    WM_autosave_write(wm, bmain);
     return true;
   }
   if ((U.uiflag & USER_GLOBALUNDO) == 0) {

source/blender/windowmanager/intern/wm_init_exit.cc

@@ -471,26 +471,19 @@ void WM_exit_ex(bContext *C, const bool do_python_exit, const bool do_user_exit_
   /* NOTE: same code copied in `wm_files.cc`. */
   if (C && wm) {
     if (do_user_exit_actions) {
-      /* save the undo state as quit.blend */
+      /* Save quit.blend. */
       Main *bmain = CTX_data_main(C);
       char filepath[FILE_MAX];
-      BLI_path_join(filepath, sizeof(filepath), BKE_tempdir_base(), BLENDER_QUIT_FILE);
-      MemFile *undo_memfile = wm->undo_stack ?
-                                  ED_undosys_stack_memfile_get_if_active(wm->undo_stack) :
-                                  nullptr;
-      /* When true, the `undo_memfile` doesn't contain all information necessary
-       * for writing and up to date blend file. */
-      const bool is_memfile_outdated = ED_editors_flush_edits(bmain);
+      const int fileflags = G.fileflags & ~G_FILE_COMPRESS;
 
-      if (undo_memfile && !is_memfile_outdated) {
-        BLO_memfile_write_file(undo_memfile, filepath);
+      BLI_path_join(filepath, sizeof(filepath), BKE_tempdir_base(), BLENDER_QUIT_FILE);
+
+      ED_editors_flush_edits(bmain);
+
+      BlendFileWriteParams blend_file_write_params{};
+      if (BLO_write_file(bmain, filepath, fileflags, &blend_file_write_params, nullptr)) {
+        printf("Saved session recovery to \"%s\"\n", filepath);
       }
-      else {
-        const int fileflags = G.fileflags & ~G_FILE_COMPRESS;
-        BlendFileWriteParams blend_file_write_params{};
-        BLO_write_file(bmain, filepath, fileflags, &blend_file_write_params, nullptr);
-      }
-      printf("Saved session recovery to \"%s\"\n", filepath);
     }
 
     WM_jobs_kill_all(wm);