Introduce new EXIST_IN_DB migration code

dbs/migrate.go

@@ -47,11 +47,13 @@ DBS migration status codes:
         1=IN PROGRESS
         2=COMPLETED
         3=FAILED (will be retried)
+		4=EXIST_IN_DB
         9=Terminally FAILED
         status change:
         0 -> 1
         1 -> 2
         1 -> 3
+		1 -> 4
         1 -> 9
         are only allowed changes for working through migration.
         3 -> 1 is allowed for retrying and retry count +1.
@@ -63,6 +65,7 @@ const (
 	IN_PROGRESS
 	COMPLETED
 	FAILED
+	EXIST_IN_DB
 	TERM_FAILED = 9
 )
 
@@ -497,6 +500,8 @@ func statusString(status int64) string {
 		s = "FAILED"
 	} else if status == TERM_FAILED {
 		s = "TERMINATED"
+	} else if status == EXIST_IN_DB {
+		s = "EXIST_IN_DB"
 	}
 	return s
 }
@@ -610,6 +615,8 @@ func startMigrationRequest(rec MigrationRequest) ([]MigrationReport, error) {
 
 	// if no migration blocks found to process return immediately
 	if len(migBlocks) == 0 {
+		rec.MIGRATION_STATUS = EXIST_IN_DB
+		updateMigrationStatus(rec, EXIST_IN_DB)
 		msg = fmt.Sprintf("%s is already fulfilled, no blocks found for migration", mstr)
 		log.Println(msg)
 		return []MigrationReport{migrationReport(rec, msg, status, err)}, nil

docs/MigrationServer.md

@@ -32,17 +32,20 @@ from underlying DB backend on periodic basis
   - 2 migration request has successfully completed
   - 3 migration request has failed, it will be retried according to DB
     migration server settings (by default 3 times)
+  - 4 migration request is already exist in DB, i.e. the requested block or
+    dataset is already found in database
   - 9 migration request termindated, this can happen in two scenarios
     - migration request has been cancelled explicitly by user
     - migration request failed N times and will no longer be retried
       automatically
 The migration request goes throught the followin cycle:
 ```
 status change:
-0 -> 1
-1 -> 2
+0 -> 1, request in progress
+1 -> 2, request is completed successfully
 1 -> 3, and if failed 3->1
-1 -> 9
+1 -> 4, if request is alaready in DB
+1 -> 9, request is terminated
 ```
 
 ### Examples