Warn about .partial files in the WAL archive during recovery

Updates _xlog_copy so that special handling is only used for `.partial`
files in the streaming directory. Any `.partial` files found in the
WAL archive are handled the same way Barman would previously handle
them, i.e. they are copied across to the recovery WAL destination
(decompressing if necessary) and *not* renamed. Because this is a
situation which requires some manual intervention a warning message is
now logged to tell the user about each `.partial` WAL found in the
archive.

The user must then determine whether the `.partial` WAL in the archive
is the result of a recovery from backup or a standby promotion and, if
so, can rename the file to omit its `.partial` extension.

There is a clear opportunity for Barman to handle this itself however
this is deliberately not tackled by this patch and will instead be
added at a later date when we improve the behaviour of Barman in a
clustered environment.

Relates to #393.

barman/recovery_executor.py

@@ -785,9 +785,27 @@ def _xlog_copy(self, required_xlog_files, wal_dest, remote_command):
         compression_manager = self.backup_manager.compression_manager
         # Fill xlogs and compressors maps from required_xlog_files
         for wal_info in required_xlog_files:
-            if xlog.is_partial_file(wal_info.name):
+            if (
+                xlog.is_partial_file(wal_info.name)
+                and hasattr(wal_info, "orig_filename")
+                and (
+                    os.path.dirname(wal_info.orig_filename)
+                    == self.config.streaming_wals_directory
+                )
+            ):
+                # If a .partial file is in the `streaming` directory then add it to a
+                # separate list so it can be renamed and transferred in a separate
+                # Rsync operation.
                 partial_xlogs.append(wal_info)
             else:
+                if xlog.is_partial_file(wal_info.name):
+                    # If a .partial file is in the main WAL archive then either a
+                    # standby was promoted or a primary was recovered from a backup.
+                    # In either case we do not attempt to handle it and instead warn
+                    # the user.
+                    output.warning(
+                        ".partial WAL file '%s' found in WAL archive", wal_info.name
+                    )
                 hashdir = xlog.hash_dir(wal_info.name)
                 xlogs[hashdir].append(wal_info)
                 # If a compressor is required, make sure it exists in the cache