Don't read the magic number of all objects to show a backup description (#4610)

Now we lazily initialize the description of each backup on each
session. This means that only when we have to display the item on the
screen (onPaint) we'll ask for the description/doc
info (width/height/color mode, etc.). We've also removed the check of
all magic numbers of every single object in the backup when we only
need the doc description.

src/app/crash/read_document.cpp

@@ -91,7 +91,12 @@ public:
       if (!id || !ver)
         continue;               // Error converting strings to ID/ver
 
-      if (!check_magic_number(base::join_path(m_dir, fn))) {
+      // Checking for the magic number of each file takes a long time,
+      // we can guess that all files are valid when there is no
+      // m_taskToken, i.e. when we have to just show the description
+      // of the doc in the list of backups.
+      if (m_taskToken &&
+          !check_magic_number(base::join_path(m_dir, fn))) {
         RECO_TRACE("RECO: Ignoring invalid file %s (no magic number)\n", fn.c_str());
         continue;
       }

src/app/crash/session.cpp

@@ -44,22 +44,24 @@ static const char* kOpenFilename = "open"; // File that indicates if the documen
 Session::Backup::Backup(const std::string& dir)
   : m_dir(dir)
 {
-  DocumentInfo info;
-  read_document_info(dir, info);
-
-  m_fn = info.filename;
-  m_desc =
-    fmt::format("{} Sprite {}x{}, {} {}",
-                info.mode == ColorMode::RGB ? "RGB":
-                info.mode == ColorMode::GRAYSCALE ? "Grayscale":
-                info.mode == ColorMode::INDEXED ? "Indexed":
-                info.mode == ColorMode::BITMAP ? "Bitmap": "Unknown",
-                info.width, info.height, info.frames,
-                info.frames == 1 ? "frame": "frames");
 }
 
 std::string Session::Backup::description(const bool withFullPath) const
 {
+  // Lazy initialize description and filename.
+  if (m_desc.empty()) {
+    DocumentInfo info;
+    read_document_info(m_dir, info);
+    m_fn = info.filename;
+    m_desc =
+      fmt::format("{} Sprite {}x{}, {} {}",
+                  info.mode == ColorMode::RGB ? "RGB":
+                  info.mode == ColorMode::GRAYSCALE ? "Grayscale":
+                  info.mode == ColorMode::INDEXED ? "Indexed":
+                  info.mode == ColorMode::BITMAP ? "Bitmap": "Unknown",
+                  info.width, info.height, info.frames,
+                  info.frames == 1 ? "frame": "frames");
+  }
   return fmt::format("{}: {}",
                      m_desc,
                      withFullPath ? m_fn:

src/app/crash/session.h

@@ -35,8 +35,8 @@ namespace crash {
       std::string description(const bool withFullPath) const;
     private:
       std::string m_dir;
-      std::string m_desc;
+      mutable std::string m_desc;
-      std::string m_fn;
+      mutable std::string m_fn;
     };
     using BackupPtr = std::shared_ptr<Backup>;
     using Backups = std::vector<BackupPtr>;

src/app/ui/data_recovery_view.cpp

@@ -58,7 +58,6 @@ public:
     : m_session(session)
     , m_backup(backup)
     , m_task(nullptr) {
-    updateText();
   }
 
   crash::Session* session() const { return m_session; }
@@ -150,6 +149,15 @@ public:
   }
 
 private:
+  void onPaint(PaintEvent& ev) override {
+    // The text is lazily initialized. So we read the backup data only
+    // when we have to show its information.
+    if (text().empty()) {
+      updateText();
+    }
+    ListItem::onPaint(ev);
+  }
+
   void onSizeHint(SizeHintEvent& ev) override {
     ListItem::onSizeHint(ev);
     gfx::Size sz = ev.sizeHint();