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/common.py

@@ -24,6 +24,7 @@ along with this program. If not, see <https://www.gnu.org/licenses/>.
 """
 
 import json
+import hashlib
 import logging
 
 from datetime import datetime
@@ -411,7 +412,7 @@ def jsonEncode(data, n=0, nmax=0):
 
 
 # =============================================================================================== #
-#  Other Functions
+#  File and File System Functions
 # =============================================================================================== #
 
 def makeFileNameSafe(theText):
@@ -424,6 +425,29 @@ def makeFileNameSafe(theText):
     return cleanName
 
 
+def sha256sum(filePath):
+    """Make a shasum of a file using a buffer.
+    Based on: https://stackoverflow.com/a/44873382/5825851
+    """
+    hDigest = hashlib.sha256()
+    bData = bytearray(65536)
+    mData = memoryview(bData)
+    try:
+        with open(filePath, mode="rb", buffering=0) as inFile:
+            for n in iter(lambda: inFile.readinto(mData), 0):
+                hDigest.update(mData[:n])
+    except Exception:
+        logger.error("Could not read sha256sum of: %s", filePath)
+        logException()
+        return None
+
+    return hDigest.hexdigest()
+
+
+# =============================================================================================== #
+#  Other Functions
+# =============================================================================================== #
+
 def getGuiItem(theName):
     """Returns a QtWidget based on its objectName.
     """

novelwriter/core/document.py

@@ -27,7 +27,7 @@ import os
 import logging
 
 from novelwriter.enum import nwItemLayout, nwItemClass
-from novelwriter.common import isHandle
+from novelwriter.common import isHandle, sha256sum
 
 logger = logging.getLogger(__name__)
 
@@ -44,6 +44,8 @@ class NWDoc():
         self._fileLoc   = None  # The file location of the currently open item
         self._docMeta   = {}    # The meta data of the currently open item
         self._docError  = ""    # The latest encountered IO error
+        self._prevHash  = None  # Previous sha256sum of the document file
+        self._currHash  = None  # Latest sha256sum of the document file
 
         if isHandle(theHandle):
             self._docHandle = theHandle
@@ -80,6 +82,8 @@ class NWDoc():
 
         theText = ""
         self._docMeta = {}
+        self._prevHash = sha256sum(docPath)
+
         if os.path.isfile(docPath):
             try:
                 with open(docPath, mode="r", encoding="utf-8") as inFile:
@@ -108,7 +112,7 @@ class NWDoc():
 
         return theText
 
-    def writeDocument(self, docText):
+    def writeDocument(self, docText, forceWrite=False):
         """Write the document. The file is saved via a temp file in case
         of save failure. Returns True if successful, False if not.
         """
@@ -125,7 +129,13 @@ class NWDoc():
         docPath = os.path.join(self.theProject.projContent, docFile)
         docTemp = os.path.join(self.theProject.projContent, docFile+"~")
 
-        # DocMeta line
+        if self._prevHash is not None and not forceWrite:
+            self._currHash = sha256sum(docPath)
+            if self._currHash is not None and self._currHash != self._prevHash:
+                logger.error("File has been altered on disk since opened")
+                return False
+
+        # DocMeta Line
         if self._theItem is None:
             docMeta = ""
         else:
@@ -149,6 +159,9 @@ class NWDoc():
             os.unlink(docPath)
         os.rename(docTemp, docPath)
 
+        self._prevHash = sha256sum(docPath)
+        self._currHash = self._prevHash
+
         return True
 
     def deleteDocument(self):

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)

novelwriter/guimain.py

@@ -1121,6 +1121,7 @@ class GuiMain(QMainWindow):
         can be either a string or an array of strings.
         """
         if isinstance(theMessage, list):
+            theMessage = list(filter(None, theMessage))
             popMsg = "<br>".join(theMessage)
             logMsg = theMessage
         else: