Push down precondition to TiKV

proto/kvrpcpb.proto

@@ -18,11 +18,23 @@ message LockInfo {
     uint64 lock_ttl = 4;
 }
 
+message AlreadyExist {
+    bytes key = 1;
+}
+
+message NotEqualTo {
+    bytes key = 1;
+    bytes value = 2;
+    bytes should_equal_to = 3;
+}
+
 message KeyError {
     LockInfo locked = 1; // Client should backoff or cleanup the lock then retry.
     string retryable = 2; // Client may restart the txn. e.g write conflict.
     string abort = 3; // Client should abort the txn.
     WriteConflict conflict = 4; // Write conflict is moved from retryable to here.
+    AlreadyExist already_exist = 5; // Key already exists
+    NotEqualTo not_equal_to = 6; // Value not equal to specified value
 }
 
 message WriteConflict {
@@ -121,10 +133,19 @@ enum Op {
     Rollback = 3;
 }
 
+message Precondition {
+    // Should not exist
+    bool should_not_exist = 1;
+    // Value must equal to
+    bytes equal_to = 2;
+}
+
 message Mutation {
     Op op = 1;
     bytes key = 2;
     bytes value = 3;
+    // Write only when the precondition satisfied
+    Precondition precondition = 4;
 }
 
 message PrewriteRequest {