avoid orphan content chunks on BackupOSErrors

src/borg/archive.py

@@ -1300,13 +1300,31 @@ def chunk_processor(chunk):
         # to get rid of .chunks_healthy, as it might not correspond to .chunks any more.
         if self.rechunkify and "chunks_healthy" in item:
             del item.chunks_healthy
-        for chunk in chunk_iter:
-            cle = chunk_processor(chunk)
-            item.chunks.append(cle)
-            self.current_volume += cle[1]
-            if show_progress:
-                stats.show_progress(item=item, dt=0.2)
-            self.maybe_checkpoint(item)
+        try:
+            for chunk in chunk_iter:
+                cle = chunk_processor(chunk)
+                item.chunks.append(cle)
+                self.current_volume += cle[1]
+                if show_progress:
+                    stats.show_progress(item=item, dt=0.2)
+                self.maybe_checkpoint(item)
+        except BackupOSError:
+            # something went wrong (e.g. an I/O error while reading a source file), try to avoid orphan content chunks:
+            # case A: "no checkpoint archive has been created yet":
+            # we have incref'd (written) some chunks, no commit yet, no file item for these chunks yet.
+            # -> item.chunks has a list of orphaned content chunks, we need to decref them.
+            # case B: "some checkpoint archives have been created already":
+            # at the time we commit them, everything is fine and consistent:
+            # we have incref'd (written) some chunks, created a part file item referencing them, committed.
+            # directly after commit, we have removed the part file item, but kept chunks in the repo, kept refcounts.
+            # maybe we have incref'd (written) some more chunks after the commit, no file item for these chunks yet.
+            # -> item.chunks has a list of orphaned content chunks, we need to decref them.
+            # So, cases A and B need same treatment.
+            for chunk in item.chunks:
+                cache.chunk_decref(chunk.id, stats, wait=False)
+            # now that we have cleaned up the chunk references, we can re-raise the exception
+            # this will skip THIS processing of this file, but continue with the next one.
+            raise
 
 
 class FilesystemObjectProcessors: