Screensaver: exclude content when excluding cover, random image as fallback (#9912)

Exclude some screensaver option showing book content
(title, page image) when "Exclude this book cover" is checked. 
Random image as fallback, KOReader logo as 2nd-order fallback.

frontend/apps/reader/modules/readermenu.lua

@@ -250,10 +250,14 @@ function ReaderMenu:setUpdateItemTable()
 
     if Device:supportsScreensaver() then
         local ss_book_settings = {
-            text = _("Exclude this book's cover from screensaver"),
+            text = _("Exclude this book's content and cover from screensaver"),
             enabled_func = function()
-                return not (self.ui == nil or self.ui.document == nil)
-                    and G_reader_settings:readSetting("screensaver_type") == "cover"
+                if self.ui and self.ui.document then
+                    local screensaverType = G_reader_settings:readSetting("screensaver_type")
+                    return screensaverType == "cover" or screensaverType == "disable"
+                else
+                    return false
+                end
             end,
             checked_func = function()
                 return self.ui and self.ui.doc_settings and self.ui.doc_settings:isTrue("exclude_screensaver")

frontend/ui/screensaver.lua

@@ -568,40 +568,33 @@ function Screensaver:setup(event, event_message)
         end
     end
     if self.screensaver_type == "bookstatus" then
-        if lastfile and lfs.attributes(lastfile, "mode") == "file" then
-            if not ui then
-                self.screensaver_type = "disable"
-                self.show_message = true
-            end
-        else
-            self.screensaver_type = "disable"
-            self.show_message = true
-        end
-    end
-    if self.screensaver_type == "random_image" then
-        local screensaver_dir = G_reader_settings:readSetting(self.prefix .. "screensaver_dir")
-                             or G_reader_settings:readSetting("screensaver_dir")
-        self.image_file = self:_getRandomImage(screensaver_dir)
-        if self.image_file == nil then
-            self.screensaver_type = "disable"
-            self.show_message = true
+        if not ui or not lastfile or lfs.attributes(lastfile, "mode") ~= "file" or (ui.doc_settings and ui.doc_settings:isTrue("exclude_screensaver")) then
+            self.screensaver_type = "random_image"
         end
     end
     if self.screensaver_type == "image_file" then
         self.image_file = G_reader_settings:readSetting(self.prefix .. "screensaver_image")
                        or G_reader_settings:readSetting("screensaver_image")
         if self.image_file == nil or lfs.attributes(self.image_file, "mode") ~= "file" then
-            self.screensaver_type = "disable"
-            self.show_message = true
+            self.screensaver_type = "random_image"
         end
     end
     if self.screensaver_type == "readingprogress" then
         -- This is implemented by the Statistics plugin
         if Screensaver.getReaderProgress == nil then
-            self.screensaver_type = "disable"
-            self.show_message = true
+            self.screensaver_type = "random_image"
+        end
+    end
+    if self.screensaver_type == "disable" then
+        if ui and ui.doc_settings and ui.doc_settings:isTrue("exclude_screensaver") then
+            self.screensaver_type = "random_image"
         end
     end
+    if self.screensaver_type == "random_image" then
+        local screensaver_dir = G_reader_settings:readSetting(self.prefix .. "screensaver_dir")
+                             or G_reader_settings:readSetting("screensaver_dir")
+        self.image_file = self:_getRandomImage(screensaver_dir) or "resources/koreader.png" -- Fallback image
+    end
 
     -- Use the right background setting depending on the effective mode, now that fallbacks have kicked in.
     if self:modeIsImage() then