[TD-591]fix alter table tag value bugs

src/client/src/tscSQLParser.c

@@ -4479,7 +4479,7 @@ int32_t setAlterTableInfo(SSqlObj* pSql, struct SSqlInfo* pInfo) {
     if (pTagsSchema->type != TSDB_DATA_TYPE_BINARY && pTagsSchema->type != TSDB_DATA_TYPE_NCHAR) {
       len = tDataTypeDesc[pTagsSchema->type].nSize;
     } else {
-      len = varDataLen(pUpdateMsg->data);
+      len = varDataTLen(pUpdateMsg->data);
     }
     
     pUpdateMsg->tagValLen = htonl(len);  // length may be changed after dump data

src/query/src/tvariant.c

@@ -800,12 +800,13 @@ int32_t tVariantDump(tVariant *pVariant, char *payload, int16_t type, bool inclu
       break;
     }
     case TSDB_DATA_TYPE_NCHAR: {
+      int32_t newlen = 0;
       if (!includeLengthPrefix) {
         if (pVariant->nType == TSDB_DATA_TYPE_NULL) {
           *(uint32_t *)payload = TSDB_DATA_NCHAR_NULL;
         } else {
           if (pVariant->nType != TSDB_DATA_TYPE_NCHAR) {
-            toNchar(pVariant, &payload, &pVariant->nLen);
+            toNchar(pVariant, &payload, &newlen);
           } else {
             wcsncpy((wchar_t *)payload, pVariant->wpz, pVariant->nLen);
           }
@@ -817,12 +818,13 @@ int32_t tVariantDump(tVariant *pVariant, char *payload, int16_t type, bool inclu
           char *p = varDataVal(payload);
 
           if (pVariant->nType != TSDB_DATA_TYPE_NCHAR) {
-            toNchar(pVariant, &p, &pVariant->nLen);
+            toNchar(pVariant, &p, &newlen);
           } else {
             wcsncpy((wchar_t *)p, pVariant->wpz, pVariant->nLen);
+            newlen = pVariant->nLen;
           }
 
-          varDataSetLen(payload, pVariant->nLen);  // the length may be changed after toNchar function called
+          varDataSetLen(payload, newlen);  // the length may be changed after toNchar function called
           assert(p == varDataVal(payload));
         }
       }