Submitted by: @romansimakov

In case of error after phase 3 of create_relation in dfw the engine have filled RDB$PAGES in system transaction. Changes of RDB$RELATIONS and RDB$DATABASE are doing in user transaction and they rolled back. As result we have a couple of records in RDB$PAGES. Later if some attachment will scan pages it creates an object jrd_rel in att_relations and marks it by flag REL_check_existance and later REL_deleted.
If we will try to create a table in this attachment we can do it. But at least we will have duplicated records in RDB$PAGES for the relation. Then if we will try to create an index for that table MET_lookup_relation_id will check REL_deleted and returns NULL. Index creation will fails.
Reconnect can helps but duplicates of RDB$PAGES is not good in any case.

Here is a scenario to reproduce it by 2 terminals (1: and 2:) on Classic arcitecture:
2: create database '/tmp/d.fdb';
2: set transaction read committed;
1: gdb --args ./isql /tmp/d.fdb
1: r
1: Ctrl+C
1: break check_not_null if (phase==3)
1: c
1: create table tn(i integer);
1: insert into tn values(NULL);
1: commit;
1: set autoddl off;
1: set transaction read committed;
1: create table terr(i integer);
1: alter table tn alter i set not null;
1: commit;
1: will break in breakpoint
2: select * from rdb$pages; // to force write RDB$PAGES via AST
1: c
1: ERROR: Cannot make field I of table TN NOT NULL because there are NULLs present
1+2: Ctrl+D to exit

Now we can connect to the database and check:
SQL> select * from rdb$pages where rdb$relation_id=129;

RDB$PAGE_NUMBER RDB$RELATION_ID RDB$PAGE_SEQUENCE RDB$PAGE_TYPE
=============== =============== ================= =============
186 129 0 4
187 129 0 6

SQL> select * from rdb$relations where rdb$relation_id=129;
SQL> select rdb$relation_id from rdb$database;

RDB$RELATION_ID

        129 

The idea of fix is to cleanup RDB$PAGES and release physically located pages at phase 0 of create_relation. I'll prepare PR soon.

Commits: 9afef19 bfb008f 3f62ad0 caa21df

====== Test Details ======

See test for CORE2284