morphlib: Add SystemMetadataDir class

This provides access to the /baserock directory as if it were a dict,
abstracting away the details of how to get data out of it.

The abstraction is useful since it is easier to use than accessing
/baserock yourself, and allows the storage format to be changed more
easily.

Keys with / in may be supported in the future. since there have been
discussions about allowing morphologies to be placed in subdirectories.

Adding this support would require creating and removing directory
components when values are set and deleted respectively.

Iterating would require using os.walk instead of glob.iglob, since
python doesn't support ** in globs.

1 files changed, 87 insertions, 0 deletions

diff --git a/morphlib/systemmetadatadir.py b/morphlib/systemmetadatadir.py
new file mode 100644
index 00000000..eac5b446
--- /dev/null
+++ b/morphlib/systemmetadatadir.py
@@ -0,0 +1,87 @@
+# Copyright (C) 2013  Codethink Limited
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; version 2 of the License.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License along
+# with this program; if not, write to the Free Software Foundation, Inc.,
+# 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+#
+# =*= License: GPL-2 =*=
+
+
+import collections
+import glob
+import json
+import os
+
+
+class SystemMetadataDir(collections.MutableMapping):
+
+    '''An abstraction over the /baserock metadata directory.
+
+    This allows methods of iterating over it, and accessing it like
+    a dict.
+
+    The /baserock metadata directory contains information about all of
+    the chunks in a built system. It exists to provide traceability from
+    the input sources to the output.
+
+    If you create the object with smd = SystemMetadataDir('/baserock')
+    data = smd['key'] will read /baserock/key.meta and return its JSON
+    encoded contents as native python objects.
+
+    smd['key'] = data will write data to /baserock/key.meta as JSON
+
+    The key may not have '\0' characters in it since the underlying
+    system calls don't support embedded NUL bytes.
+
+    The key may not have '/' characters in it since we do not support
+    morphologies with slashes in their names.
+
+    '''
+
+    def __init__(self, metadata_path):
+        collections.MutableMapping.__init__(self)
+        self._metadata_path = metadata_path
+
+    def _join_path(self, *args):
+        return os.path.join(self._metadata_path, *args)
+
+    def _raw_path_iter(self):
+        return glob.iglob(self._join_path('*.meta'))
+
+    @staticmethod
+    def _check_key(key):
+        if any(c in key for c in "\0/"):
+            raise KeyError(key)
+
+    def __getitem__(self, key):
+        self._check_key(key)
+        try:
+            with open(self._join_path('%s.meta' % key), 'r') as f:
+                return json.load(f)
+        except IOError:
+            raise KeyError(key)
+
+    def __setitem__(self, key, value):
+        self._check_key(key)
+        with open(self._join_path('%s.meta' % key), 'w') as f:
+            json.dump(value, f, indent=4, sort_keys=True)
+
+    def __delitem__(self, key):
+        self._check_key(key)
+        os.unlink(self._join_path('%s.meta' % key))
+
+    def __iter__(self):
+        return (os.path.basename(fn)[:-len('.meta')]
+                for fn in self._raw_path_iter())
+
+    def __len__(self):
+        return len(list(self._raw_path_iter()))