fix: retry transactions who fail due to deadlocks

In my opinion, this kind of handling of transactions is pretty ugly.
The being said, we have issues with running into deadlocks on aur.al,
so this commit works against that immediate bug.

An ideal solution would be to deal with retrying transactions through
the `db.begin()` scope, so we wouldn't have to explicitly annotate
functions as "retry functions," which is what this commit does.

Closes #376

Signed-off-by: Kevin Morris <kevr@0cost.org>

test/test_db.py

@@ -5,6 +5,7 @@ import tempfile
 from unittest import mock
 
 import pytest
+from sqlalchemy.exc import OperationalError
 
 import aurweb.config
 import aurweb.initdb
@@ -226,3 +227,22 @@ def test_name_without_pytest_current_test():
     with mock.patch.dict("os.environ", {}, clear=True):
         dbname = aurweb.db.name()
     assert dbname == aurweb.config.get("database", "name")
+
+
+def test_retry_deadlock():
+    @db.retry_deadlock
+    def func():
+        raise OperationalError("Deadlock found", tuple(), "")
+
+    with pytest.raises(OperationalError):
+        func()
+
+
+@pytest.mark.asyncio
+async def test_async_retry_deadlock():
+    @db.async_retry_deadlock
+    async def func():
+        raise OperationalError("Deadlock found", tuple(), "")
+
+    with pytest.raises(OperationalError):
+        await func()