allocator: zero limit in SetExiting for backward compatibility

When deploying to existing clusters, old allocators don't understand the Exiting
field and need to see a zeroed limit to drain items from exiting members. Revert
this commit once the cluster is fully upgraded.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: williamhbaker

allocator/allocator_key_space_test.go

@@ -361,7 +361,8 @@ type testMember struct {
 
 func (m testMember) ItemLimit() int   { return m.R }
 func (m testMember) Validate() error  { return nil }
-func (m *testMember) SetExiting()     { m.E = true }
+// TODO(whb): Zero'ing R is for backward compatibility; remove once deployment is complete.
+func (m *testMember) SetExiting()     { m.E = true; m.R = 0 }
 func (m testMember) IsExiting() bool  { return m.E }
 
 func (m *testMember) MarshalString() string {

allocator/announce_test.go

@@ -127,7 +127,8 @@ func (s *AnnounceSuite) TestBasicSessionStart(c *gc.C) {
 	close(sigCh)
 
 	c.Check(args.Tasks.Wait(), gc.IsNil) // All tasks have exited.
-	c.Check(spec.E, gc.Equals, true)     // Member was marked as exiting.
+	c.Check(spec.E, gc.Equals, true) // Member was marked as exiting.
+	c.Check(spec.R, gc.Equals, 0)   // TODO(whb): Remove once backward compat is removed.
 
 	leasesResp, err := etcd.Leases(context.Background())
 	c.Check(err, gc.IsNil)

broker/protocol/broker_spec_extensions.go

@@ -56,7 +56,9 @@ func (m *BrokerSpec) ItemLimit() int { return int(m.JournalLimit) }
 func (m *BrokerSpec) IsExiting() bool { return m.Exiting }
 
 // SetExiting marks this BrokerSpec as exiting.
-func (m *BrokerSpec) SetExiting() { m.Exiting = true }
+// TODO(whb): Zero'ing JournalLimit is for backward compatibility; remove once
+// deployment is complete.
+func (m *BrokerSpec) SetExiting() { m.Exiting = true; m.JournalLimit = 0 }
 
 const (
 	minZoneLen            = 1

consumer/protocol/shard_spec_extensions.go

@@ -289,7 +289,9 @@ func (m *ConsumerSpec) ItemLimit() int { return int(m.ShardLimit) }
 func (m *ConsumerSpec) IsExiting() bool { return m.Exiting }
 
 // SetExiting marks this ConsumerSpec as exiting.
-func (m *ConsumerSpec) SetExiting() { m.Exiting = true }
+// TODO(whb): Zero'ing ShardLimit is for backward compatibility; remove once
+// deployment is complete.
+func (m *ConsumerSpec) SetExiting() { m.Exiting = true; m.ShardLimit = 0 }
 
 // Reduce folds another ReplicaStatus into this one.
 func (m *ReplicaStatus) Reduce(other *ReplicaStatus) {