credocument: deal with loadDocument() failures (#3570)

cre:loadDocument() may fail in recognizing the document format, and
koreader would previously keep calling other methods on it, which would
make crengine segfaults. We now check loadDocument success at the
various places it is called, and try to deal the best way we can when it fails.

frontend/apps/reader/readerui.lua

@@ -263,7 +263,9 @@ function ReaderUI:init()
     else
         -- make sure we render document first before calling any callback
         self:registerPostInitCallback(function()
-            self.document:loadDocument()
+            if not self.document:loadDocument() then
+                self:dealWithLoadDocumentFailure()
+            end
 
             -- used to read additional settings after the document has been
             -- loaded (but not rendered yet)
@@ -561,4 +563,34 @@ function ReaderUI:onClose()
     end
 end
 
+function ReaderUI:dealWithLoadDocumentFailure()
+    -- Sadly, we had to delay loadDocument() to about now, so we only
+    -- know now this document is not valid or recognized.
+    -- We can't do much more than crash properly here (still better than
+    -- going on and segfaulting when calling other methods on unitiliazed
+    -- _document)
+    -- We must still remove it from lastfile and history (as it has
+    -- already been added there) so that koreader don't crash again
+    -- at next launch...
+    local readhistory = require("readhistory")
+    readhistory:removeItemByPath(self.document.file)
+    if G_reader_settings:readSetting("lastfile") == self.document.file then
+        G_reader_settings:saveSetting("lastfile", #readhistory.hist > 0 and readhistory.hist[1].file or nil)
+    end
+    -- As we are in a coroutine, we can pause and show an InfoMessage before exiting
+    local _coroutine = coroutine.running()
+    if coroutine then
+        logger.warn("crengine failed recognizing or parsing this file: unsupported or invalid document")
+        UIManager:show(InfoMessage:new{
+            text = _("Failed recognizing or parsing this file: unsupported or invalid document.\nKOReader will exit now."),
+            dismiss_callback = function()
+                coroutine.resume(_coroutine, false)
+            end,
+        })
+        coroutine.yield() -- pause till InfoMessage is dismissed
+    end
+    -- We have to error and exit the coroutine anyway to avoid any segfault
+    error("crengine failed recognizing or parsing this file: unsupported or invalid document")
+end
+
 return ReaderUI