fix "check" for repos that have incomplete chunks, fixes #364

added try/finally (the code in between was just indented, no
other code changes) to make sure it sets self.index back to None,
even if the code crashes e.g. due to an IntegrityError caused
by an incomplete segment caused by a disk full condition.

also, in prepare_txn, create an empty in-memory index if transaction_id
is None, which is required by the Repository.check code to work correctly.
If the index is not empty there, it will miscalculate segment usage
(self.segments).

borg/repository.py

@@ -175,7 +175,7 @@ class Repository:
             # the repository instance lives on - even if exceptions happened.
             self._active_txn = False
             raise
-        if not self.index:
+        if not self.index or transaction_id is None:
             self.index = self.open_index(transaction_id)
         if transaction_id is None:
             self.segments = {}
@@ -237,38 +237,41 @@ class Repository:
 
     def replay_segments(self, index_transaction_id, segments_transaction_id):
         self.prepare_txn(index_transaction_id, do_cleanup=False)
-        for segment, filename in self.io.segment_iterator():
-            if index_transaction_id is not None and segment <= index_transaction_id:
-                continue
-            if segment > segments_transaction_id:
-                break
-            self.segments[segment] = 0
-            for tag, key, offset in self.io.iter_objects(segment):
-                if tag == TAG_PUT:
-                    try:
-                        s, _ = self.index[key]
-                        self.compact.add(s)
-                        self.segments[s] -= 1
-                    except KeyError:
-                        pass
-                    self.index[key] = segment, offset
-                    self.segments[segment] += 1
-                elif tag == TAG_DELETE:
-                    try:
-                        s, _ = self.index.pop(key)
-                        self.segments[s] -= 1
-                        self.compact.add(s)
-                    except KeyError:
-                        pass
-                    self.compact.add(segment)
-                elif tag == TAG_COMMIT:
+        try:
+            for segment, filename in self.io.segment_iterator():
+                if index_transaction_id is not None and segment <= index_transaction_id:
                     continue
-                else:
-                    raise self.CheckNeeded(self.path)
-            if self.segments[segment] == 0:
-                self.compact.add(segment)
-        self.write_index()
-        self.rollback()
+                if segment > segments_transaction_id:
+                    break
+                # code duplication below?? vvv (see similar code in check())
+                self.segments[segment] = 0
+                for tag, key, offset in self.io.iter_objects(segment):
+                    if tag == TAG_PUT:
+                        try:
+                            s, _ = self.index[key]
+                            self.compact.add(s)
+                            self.segments[s] -= 1
+                        except KeyError:
+                            pass
+                        self.index[key] = segment, offset
+                        self.segments[segment] += 1
+                    elif tag == TAG_DELETE:
+                        try:
+                            s, _ = self.index.pop(key)
+                            self.segments[s] -= 1
+                            self.compact.add(s)
+                        except KeyError:
+                            pass
+                        self.compact.add(segment)
+                    elif tag == TAG_COMMIT:
+                        continue
+                    else:
+                        raise self.CheckNeeded(self.path)
+                if self.segments[segment] == 0:
+                    self.compact.add(segment)
+            self.write_index()
+        finally:
+            self.rollback()
 
     def check(self, repair=False):
         """Check repository consistency
@@ -297,7 +300,7 @@ class Repository:
         if repair:
             self.io.cleanup(transaction_id)
         segments_transaction_id = self.io.get_segments_transaction_id()
-        self.prepare_txn(None)
+        self.prepare_txn(None)  # self.index, self.compact, self.segments all empty now!
         for segment, filename in self.io.segment_iterator():
             if segment > transaction_id:
                 continue