Avoid writting highlights into read-only PDFs (#5889)

frontend/apps/reader/modules/readerhighlight.lua

@@ -1167,8 +1167,20 @@ end
 --]]
 
 function ReaderHighlight:exportToDocument(page, item)
+    local setting = G_reader_settings:readSetting("save_document")
+    if setting == "disable" then return end
     logger.dbg("export highlight to document", item)
-    self.ui.document:saveHighlight(page, item)
+    local can_write = self.ui.document:saveHighlight(page, item)
+    if can_write == false and not self.warned_once then
+        self.warned_once = true
+        UIManager:show(InfoMessage:new{
+            text = _([[
+Highlights in this document will be saved in the settings file, but they won't be written in the document itself because the file is in a read-only location.
+
+If you wish your highlights to be saved in the document, just move it to a writable directory first.]]),
+            timeout = 5,
+        })
+    end
 end
 
 function ReaderHighlight:addNote()

frontend/document/pdfdocument.lua

@@ -175,6 +175,14 @@ function PdfDocument:saveHighlight(pageno, item)
     local suffix = util.getFileNameSuffix(self.file)
     if string.lower(suffix) ~= "pdf" then return end
 
+    if self.is_writable == nil then
+        local handle = io.open(self.file, 'r+b')
+        self.is_writable = handle ~= nil
+        if handle then handle:close() end
+    end
+    if self.is_writable == false then
+        return false
+    end
     self.is_edited = true
     -- will also need mupdf_h.lua to be evaluated once
     -- but this is guaranteed at this point