Ignore shared B-tree lookup errors during backup

Because of inherent race conditions in Obnam's chosen locking
model, the lookup may fail due to another Obnam instance updating
the B-trees while we are looking up (or after we last opened
the B-tree). This should not be a fatal error, so we ignore such
errors.

1 files changed, 10 insertions, 2 deletions

diff --git a/obnamlib/plugins/backup_plugin.py b/obnamlib/plugins/backup_plugin.py
index 74aa74b5..81fb2237 100644
--- a/obnamlib/plugins/backup_plugin.py
+++ b/obnamlib/plugins/backup_plugin.py
@@ -603,8 +603,16 @@ class BackupPlugin(obnamlib.ObnamPlugin):
         '''Back up a chunk of data by putting it into the repository.'''
 
         def find():
-            return (self.repo.find_chunks(checksum) + 
-                     self.chunkid_pool.get(checksum))
+            # We ignore lookup errors here intentionally. We're reading
+            # the checksum trees without a lock, so another Obnam may be
+            # modifying them, which can lead to spurious NodeMissing
+            # exceptions, and other errors. We don't care: we'll just
+            # pretend no chunk with the checksum exists yet.
+            try:
+                in_tree = self.repo.find_chunks(checksum)
+            except larch.Error:
+                in_tree = []
+            return in_tree + self.chunkid_pool.get(checksum)
 
         def get(chunkid):
             return self.repo.get_chunk(chunkid)