Remove transaction support from sharding API

The size of Zookeeper transactions is limited by the `jute.maxbuffer`
setting. It is the same setting that also defines the maximum allowed
size of a Znode.

This means that it doesn't make sense to use Zookeeper transactions
in combination with sharding.

Change-Id: I828bca32219b07b064d967f46379568c4d94c151

zuul/zk/sharding.py

@@ -17,7 +17,6 @@ from contextlib import suppress
 import zlib
 
 from kazoo.exceptions import NoNodeError
-from kazoo.client import TransactionRequest
 
 # The default size limit for a node in Zookeeper is ~1MiB. However, as this
 # also includes the size of the key we can not use all of it for data.
@@ -29,7 +28,6 @@ class RawShardIO(io.RawIOBase):
     def __init__(self, client, path):
         self.client = client
         self.shard_base = path
-        self.is_transaction = isinstance(client, TransactionRequest)
 
     def readable(self):
         return True
@@ -40,8 +38,6 @@ class RawShardIO(io.RawIOBase):
     def truncate(self, size=None):
         if size != 0:
             raise ValueError("Can only truncate to 0")
-        if self.is_transaction:
-            raise Exception("Truncate within transactions unimplemented")
         with suppress(NoNodeError):
             self.client.delete(self.shard_base, recursive=True)
 
@@ -69,13 +65,11 @@ class RawShardIO(io.RawIOBase):
         shard_bytes = bytes(shard_data[0:NODE_BYTE_SIZE_LIMIT])
         shard_bytes = zlib.compress(shard_bytes)
         assert(len(shard_bytes) < NODE_BYTE_SIZE_LIMIT)
-        kw = dict(sequence=True)
-        if not self.is_transaction:
-            kw['makepath'] = True
         self.client.create(
             "{}/".format(self.shard_base),
             shard_bytes,
-            **kw
+            sequence=True,
+            makepath=True,
         )
         return min(byte_count, NODE_BYTE_SIZE_LIMIT)