Append-only mode for repositories

New repository config item, repository.append_only, causes Borg to never
delete or append to existing data files. Hints and indices are handled
as before, old ones are deleted, because they can get quite large, but
are automatically reconstructed: no need to keep them.

When append_only is activated a file /path/to/repo/transactions/<NUMBER>
will be created for every commit.
Deleting all segments <NUMBER+1> and higher will rollback to that commit.

Note that this only influences Borg behaviour. Since repository config
can't be altered remotely (except for repository.key) this can't be
disabled when accessed remotely over SSH with "borg serve" as the
forced command.

This is only a feature to support the use case, and does not replace
appropriate file system permissions or monitoring.

Resolves #809

borg/repository.py

@@ -84,6 +84,7 @@ class Repository:
         config.set('repository', 'version', '1')
         config.set('repository', 'segments_per_dir', str(self.DEFAULT_SEGMENTS_PER_DIR))
         config.set('repository', 'max_segment_size', str(self.DEFAULT_MAX_SEGMENT_SIZE))
+        config.set('repository', 'append_only', '0')
         config.set('repository', 'id', hexlify(os.urandom(32)).decode('ascii'))
         self.save_config(path, config)
 
@@ -105,6 +106,8 @@ class Repository:
     def destroy(self):
         """Destroy the repository at `self.path`
         """
+        if self.append_only:
+            raise ValueError(self.path + " is in append-only mode")
         self.close()
         os.remove(os.path.join(self.path, 'config'))  # kill config first
         shutil.rmtree(self.path)
@@ -148,6 +151,7 @@ class Repository:
             raise self.InvalidRepository(path)
         self.max_segment_size = self.config.getint('repository', 'max_segment_size')
         self.segments_per_dir = self.config.getint('repository', 'segments_per_dir')
+        self.append_only = self.config.getboolean('repository', 'append_only', fallback=False)
         self.id = unhexlify(self.config.get('repository', 'id').strip())
         self.io = LoggedIO(self.path, self.max_segment_size, self.segments_per_dir)
 
@@ -163,7 +167,8 @@ class Repository:
         """Commit transaction
         """
         self.io.write_commit()
-        self.compact_segments(save_space=save_space)
+        if not self.append_only:
+            self.compact_segments(save_space=save_space)
         self.write_index()
         self.rollback()
 
@@ -211,6 +216,11 @@ class Repository:
         self.index.write(os.path.join(self.path, 'index.tmp'))
         os.rename(os.path.join(self.path, 'index.tmp'),
                   os.path.join(self.path, 'index.%d' % transaction_id))
+        if self.append_only:
+            transaction_log = os.path.join(self.path, 'transactions')
+            if not os.path.exists(transaction_log):
+                os.mkdir(transaction_log)
+            open(os.path.join(transaction_log, str(transaction_id)), 'w').close()
         # Remove old indices
         current = '.%d' % transaction_id
         for name in os.listdir(self.path):

borg/testsuite/repository.py

@@ -187,6 +187,34 @@ class RepositoryCommitTestCase(RepositoryTestCaseBase):
         self.assert_equal(len(self.repository), 3)
 
 
+class RepositoryAppendOnlyTestCase(RepositoryTestCaseBase):
+    def test_destroy_append_only(self):
+        # Can't destroy append only repo (via the API)
+        self.repository.append_only = True
+        with self.assert_raises(ValueError):
+            self.repository.destroy()
+
+    def test_append_only(self):
+        def segments_in_repository():
+            return len(list(self.repository.io.segment_iterator()))
+        self.repository.put(b'00000000000000000000000000000000', b'foo')
+        self.repository.commit()
+
+        self.repository.append_only = False
+        assert segments_in_repository() == 1
+        self.repository.put(b'00000000000000000000000000000000', b'foo')
+        self.repository.commit()
+        # normal: compact squashes the data together, only one segment
+        assert segments_in_repository() == 1
+
+        self.repository.append_only = True
+        assert segments_in_repository() == 1
+        self.repository.put(b'00000000000000000000000000000000', b'foo')
+        self.repository.commit()
+        # append only: does not compact, only new segments written
+        assert segments_in_repository() == 2
+
+
 class RepositoryCheckTestCase(RepositoryTestCaseBase):
 
     def list_indices(self):