Use SQL savepoints to improve @transaction.

I didn't know about these! Now we don't have to roll all the way
back in case of problems. This allows for raising exceptions as
control flow since they won't undo unrelated stuff.

README.md

@@ -79,7 +79,6 @@ If you are interested in helping, please raise an issue before making any pull r
 - Currently, the Jinja templates are having a tangling influence on the backend objects, because Jinja cannot import my other modules like bytestring, but it can access the methods of the objects I pass into the template. As a result, the objects have excess helper methods. Consider making them into Jinja filters instead. Which is also kind of ugly but will move that pollution out of the backend at least.
 - Perhaps instead of actually deleting objects, they should just have a `deleted` flag, to make easy restoration possible. Also consider regrouping the children of restored Groupables if those children haven't already been reassigned somewhere else.
 - Add a new table to store permanent history of add/remove of tags on photos, so that accidents or trolling can be reversed.
-- Improve transaction rollbacking. I'm not satisfied with the @transaction decorator because sometimes I want to use exceptions as control flow without them rolling things back. Context managers are good but it's a matter of how abstracted they should be.
 - Photo thumbnail paths should be relative to the data_dir, they are currently one level up. Or maybe should remove the paths entirely and just recalculate it by the ID. Can't think of any reason to have a thumbnail point elsewhere.
 - Fix album size cache when photo reload metadata and generally improve that validation.
 - Better bookmark url validation.

etiquette/decorators.py

@@ -5,24 +5,27 @@ import warnings
 from . import exceptions
 
 
+def _get_relevant_photodb(instance):
+    from . import objects
+    if isinstance(instance, objects.ObjectBase):
+        photodb = instance.photodb
+    else:
+        photodb = instance
+    return photodb
+
 def required_feature(features):
     '''
     Declare that the photodb or object method requires certain 'enable_*'
     fields in the config.
     '''
-    from . import objects
     if isinstance(features, str):
         features = [features]
 
     def wrapper(function):
         @functools.wraps(function)
         def wrapped(self, *args, **kwargs):
-            if isinstance(self, objects.ObjectBase):
-                config = self.photodb.config
-            else:
-                config = self.config
-
-            config = config['enable_feature']
+            photodb = _get_relevant_photodb(self)
+            config = photodb.config['enable_feature']
 
             # Using the received string like "photo.new", try to navigate the
             # config and wind up at a True.
@@ -62,13 +65,19 @@ def time_me(function):
     return timed_function
 
 def transaction(method):
+    '''
+    Open a savepoint before running the method.
+    If the method fails, roll back to that savepoint.
+    '''
     @functools.wraps(method)
     def wrapped(self, *args, **kwargs):
+        photodb = _get_relevant_photodb(self)
+        photodb.savepoint()
         try:
-            ret = method(self, *args, **kwargs)
-            return ret
+            result = method(self, *args, **kwargs)
         except Exception as e:
-            self.log.debug('Rolling back')
-            self.sql.rollback()
+            photodb.rollback()
             raise
+        else:
+            return result
     return wrapped

etiquette/photodb.py

@@ -685,6 +685,7 @@ class PDBSQLMixin:
     def __init__(self):
         super().__init__()
         self.on_commit_queue = []
+        self.savepoints = []
 
     def close(self):
         self.sql.close()
@@ -698,8 +699,34 @@ class PDBSQLMixin:
             args = task.get('args', [])
             kwargs = task.get('kwargs', {})
             task['action'](*args, **kwargs)
+        self.savepoints.clear()
         self.sql.commit()
 
+    def rollback(self):
+        if len(self.savepoints) == 0:
+            self.log.debug('Nothing to rollback.')
+            return
+
+        if len(self.savepoints) == 1:
+            self.log.debug('Final rollback.')
+            self.sql.rollback()
+            self.savepoints.clear()
+            return
+
+        cur = self.sql.cursor()
+        restore_to = self.savepoints.pop(-1)
+        self.log.debug('Rolling back to %s', restore_to)
+        query = 'ROLLBACK TO "%s"' % restore_to
+        cur.execute(query)
+
+    def savepoint(self):
+        savepoint_id = helpers.random_hex(length=16)
+        self.log.debug('Savepoint %s.', savepoint_id)
+        query = 'SAVEPOINT "%s"' % savepoint_id
+        self.sql.execute(query)
+        self.savepoints.append(savepoint_id)
+        return savepoint_id
+
     def sql_delete(self, table, pairs, *, commit=False):
         cur = self.sql.cursor()