fix #1351 how to deal with local checksums and arbitrary.

Need to check against local file age, not message age to
understand whether to use local file xattr storage.

Also, if you don't have a stored xattr, and you have arbitrary,
you can't calculate it, so return without it.

sarracenia/flow/__init__.py

@@ -1427,7 +1427,25 @@ def write_inline_file(self, msg) -> bool:
 
         return True
 
-    def compute_local_checksum(self, msg) -> None:
+    def compute_local_checksum(self, msg, lstat=None) -> None:
+        """
+            For a file whose path is given by the msg, calculate 'local_identity' field.
+
+            when checksums for files are stored in extended attributes, it's ideal to retrieve them,
+            rather than having to read the entire file again and re-calculate.
+
+            The extended attributes have a field:
+              * 'identity' ... the field from the message when the file was written.
+              * 'mtime' ... the mtime of the file when it was written.
+
+            The 'identity' extended attribute should be correct/good/useful if:
+               * the mtime of the file on disk is not newer than the current file mtime.
+
+            If the file has been over-written afterwards, the mtime will be different,
+            and the local checksum must be re-calculated from scratch.
+
+            If the checksum method is arbitrary, no local recalculation is possible.
+        """
 
         if sarracenia.filemetadata.supports_extended_attributes:
             try:
@@ -1436,7 +1454,7 @@ def compute_local_checksum(self, msg) -> None:
 
                 if s:
                     metadata_cached_mtime = x.get('mtime')
-                    if ((metadata_cached_mtime >= msg['mtime'])):
+                    if (lstat and (metadata_cached_mtime >= lstat.st_mtime)):
                         # file has not been modified since checksum value was stored.
 
                         if (( 'identity' in msg ) and ( 'method' in msg['identity']  ) and \
@@ -1453,12 +1471,13 @@ def compute_local_checksum(self, msg) -> None:
             except:
                 pass
 
+        # no local recalculation possible.
+        if msg['identity']['method'] in [ 'arbitrary' ]:
+            return
+
         local_identity = sarracenia.identity.Identity.factory(
             msg['identity']['method'])
 
-        if msg['identity']['method'] == 'arbitrary':
-            local_identity.value = msg['identity']['value']
-
         local_identity.update_file(msg['new_path'])
         msg['local_identity'] = {
             'method': msg['identity']['method'],
@@ -1468,7 +1487,7 @@ def compute_local_checksum(self, msg) -> None:
 
     def file_should_be_downloaded(self, msg) -> bool:
         """
-          determine whether a comparison of local_file and message metadata indicates that it is new enough
+          Determine whether a comparison of local_file and message metadata indicates that it is new enough
           that writing the file locally is warranted.
 
           return True to say downloading is warranted.
@@ -1555,16 +1574,19 @@ def file_should_be_downloaded(self, msg) -> bool:
             return True
 
         try:
-            self.compute_local_checksum(msg)
+            self.compute_local_checksum(msg,lstat)
         except:
             logger.debug(
                 "something went wrong when computing local checksum... considered different"
             )
             return True
 
-        logger.debug( f"checksum in message: {msg['identity']} vs. local: {msg['local_identity']}" )
+        if 'local_identity' in msg:
+           logger.debug( f"checksum in message: {msg['identity']} vs. local: {msg['local_identity']}" )
+        else:
+           logger.debug( f"checksum in message: {msg['identity']} vs. local: None" )
 
-        if msg['local_identity'] == msg['identity']:
+        if 'local_identity' in msg and msg['local_identity'] == msg['identity']:
             self.reject(msg, 304, f"same checksum {msg['new_path']}" )
             return False
         else: