persistent-nodemap: properly ignore non-existent `.nd` data file This code was meant to handle the case of a nodemap docket file pointing to a nodemap data file that doesn’t exist (anymore), but most likely caused an `UnboundLocalError` exception instead when `data` was used on the next line without being defined. This case is theoretically possible with a race condition between two hg processes, but is hard to reproduce or test: * Process A reads a docket file and finds a UID in it that points to a given data file name. * Process B decides that this same data file needs compacting. It writes a new one with a different UID, overwrites the docket file, then removes the old data file. * Only then process A tries to a open a file that doesn’t exist anymore. Differential Revision: https://phab.mercurial-scm.org/D9533

--- a/mercurial/revlogutils/nodemap.py	Wed Dec 09 18:51:52 2020 -0800
+++ b/mercurial/revlogutils/nodemap.py	Mon Dec 07 18:06:53 2020 +0100
@@ -58,7 +58,9 @@
             else:
                 data = fd.read(data_length)
     except OSError as e:
-        if e.errno != errno.ENOENT:
+        if e.errno == errno.ENOENT:
+            return None
+        else:
             raise
     if len(data) < data_length:
         return None