Make BlockAffinity a raw string type (#301)

Compatible with old golang

lib/backend/model/block_affinity.go

@@ -27,7 +27,24 @@ import (
 
 var (
 	matchBlockAffinity = regexp.MustCompile("^/?calico/ipam/v2/host/([^/]+)/ipv./block/([^/]+)$")
-	typeBlockAff       = reflect.TypeOf(BlockAffinity{})
+
+	// The BlockAffinity is stored as a raw string type.  It currently does
+	// not contain any information since all required information is stored
+	// in the key (hostname and block CIDR).  If we end up needing to store
+	// data in the BlockAffinity then care will need to be taken to ensure
+	// existing versions of the value (an empty string) can be successfully
+	// unmarshalled.
+	// - The Python version of IPAM wrote an empty string, but can handle
+	//   any value written into the data.
+	// - The original golang port of IPAM wrote "{}" into the data (the JSON
+	//   value for an empty dict).  It was unable to handle reading of an
+	//   empty string written out by the Python IPAM.
+	// - The current version of the golang port now has the BlockAffinity
+	//   as a raw-string type so that it can handle reading in any value.
+	//   We write in a fixed value of "{}" so that we are compatible with
+	//   both the Python and the original golang port.
+	BlockAffinityValue = "{}"
+	typeBlockAff       = rawStringType
 )
 
 type BlockAffinityKey struct {
@@ -94,6 +111,3 @@ func (options BlockAffinityListOptions) KeyFromDefaultPath(path string) Key {
 	}
 	return BlockAffinityKey{CIDR: *cidr, Host: host}
 }
-
-type BlockAffinity struct {
-}

lib/client/ipam_block_reader_writer.go

@@ -141,11 +141,12 @@ func (rw blockReaderWriter) claimNewAffineBlock(
 }
 
 func (rw blockReaderWriter) claimBlockAffinity(subnet cnet.IPNet, host string, config IPAMConfig) error {
-	// Claim the block affinity for this host.
+	// Claim the block affinity for this host.  See model.BlockAffinityValue
+	// for details on the hard-coded value that is used.
 	log.Infof("Host %s claiming block affinity for %s", host, subnet)
 	obj := model.KVPair{
 		Key:   model.BlockAffinityKey{Host: host, CIDR: subnet},
-		Value: &model.BlockAffinity{},
+		Value: model.BlockAffinityValue,
 	}
 	_, err := rw.client.backend.Create(&obj)