Add hash check on document read/write (#890)

* Add sha256sum function
* Add hash check to document class
* Make sha256sum test more thorough
* Handle exceptions in the sha256sum function directly
* Update test coverage
* Clarify title on dialog box
* Don't write blank lines in makeAlert

novelwriter/gui/doceditor.py

@@ -453,9 +453,23 @@ class GuiDocEditor(QTextEdit):
 
         self.saveCursorPosition()
         if not self._nwDocument.writeDocument(docText):
-            self.theParent.makeAlert([
-                self.tr("Could not save document."), self._nwDocument.getError()
-            ], nwAlert.ERROR)
+            saveOk = False
+            if self._nwDocument._currHash != self._nwDocument._prevHash:
+                msgYes = self.theParent.askQuestion(
+                    self.tr("File Changed on Disk"),
+                    self.tr(
+                        "This document has been changed outside of novelWriter "
+                        "while it was open. Overvrite the file on disk?"
+                    )
+                )
+                if msgYes:
+                    saveOk = self._nwDocument.writeDocument(docText, forceWrite=True)
+
+            if not saveOk:
+                self.theParent.makeAlert([
+                    self.tr("Could not save document."), self._nwDocument.getError()
+                ], nwAlert.ERROR)
+
             return False
 
         self.setDocumentChanged(False)