x/sync -- Add proto for P2P messages

proto/sync/sync.proto

@@ -4,26 +4,90 @@ package sync;
 
 option go_package = "github.com/ava-labs/avalanchego/proto/pb/sync";
 
+// Request represents a request for information during syncing.
 message Request {
   oneof message {
     RangeProofRequest range_proof_request = 1;
     ChangeProofRequest change_proof_request = 2;
   }
 }
 
+// A RangeProofRequest requests the key-value pairs in a given key range
+// at a specific revision.
 message RangeProofRequest {
-  bytes root = 1;
-  bytes start = 2;
-  bytes end = 3;
+  bytes root_hash = 1;
+  bytes start_key = 2;
+  bytes end_key = 3;
   uint32 key_limit = 4;
   uint32 bytes_limit = 5;
 }
 
+// A ChangeProofRequest requests the changes between two revisions.
 message ChangeProofRequest {
-  bytes start_root = 1;
-  bytes end_root = 2;
-  bytes start = 3;
-  bytes end = 4;
+  bytes start_root_hash = 1;
+  bytes end_root_hash = 2;
+  bytes start_key = 3;
+  bytes end_key = 4;
   uint32 key_limit = 5;
   uint32 bytes_limit = 6;
 }
+
+// KeyValue represents a single key and its value.
+message KeyValue {
+  bytes key = 1;
+  bytes value = 2;
+}
+
+// KeyChange is a change for a key from one revision to another.
+// If the value is None, the key was deleted.
+message KeyChange {
+  bytes key = 1;
+  MaybeBytes value = 2;
+}
+
+// SerializedPath is the serialized representation of a path.
+message SerializedPath {
+  uint32 nibble_length = 1;
+  bytes value = 2;
+}
+
+// MaybeBytes is an option wrapping bytes.
+message MaybeBytes {
+  bytes value = 1;
+  // If false, this is None.
+  bool is_nothing = 2;
+}
+
+// ProofNode is a node in a merkle proof.
+message ProofNode {
+  SerializedPath key = 1;
+  MaybeBytes value_or_hash = 2;
+  map<uint32, bytes> children = 3;
+}
+
+// RangeProof is the response to a RangeProofRequest.
+message RangeProof {
+  repeated ProofNode start = 1;
+  repeated ProofNode end = 2;
+  repeated KeyValue key_values = 3;
+}
+
+// ChangeProof is a possible response to a ChangeProofRequest.
+// It only consists of a proof of the smallest changed key,
+// the highest changed key, and the keys that have changed 
+// between those. Some keys may be deleted (hence
+// the use of KeyChange instead of KeyValue).
+message ChangeProof {
+  bool had_roots_in_history = 1; // TODO remove
+  repeated ProofNode start_proof = 2;
+  repeated ProofNode end_proof = 3;
+  repeated KeyChange key_changes = 4;
+}
+
+// ChangeProofResponse is the response for a ChangeProofRequest.
+message ChangeProofResponse {
+  oneof response {
+    ChangeProof change_proof = 1;
+    RangeProof range_proof = 2;
+  }
+}