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.
     """