Firebird 2.5 ‘DEFINE GENERATOR failed’ seemingly due to reaching database generator limit, but actual amount nowhere near that limit - firebird

Question

I’m using Firebird 2.5 with FlameRobin and ran into a strange issue yesterday when creating a simple sequence / generator with the following SQL:
CREATE GENERATOR MY_GEN_NAME_HERE;
This gave the following error message:
Error: *** IBPP::SQLException ***
Context: Statement::Execute( CREATE GENERATOR MY_GEN_NAME_HERE)
Message: isc_dsql_execute2 failed
SQL Message : -607
This operation is not defined for system tables.
Engine Code : 335544351
Engine Message :
unsuccessful metadata update
DEFINE GENERATOR failed
arithmetic exception, numeric overflow, or string truncation
numeric value is out of range
At trigger 'RDB$TRIGGER_6'
According to the Firebird FAQ this means that the maximum number of generators in the database has been reached. The database only contains ~250 actual generators however, and according to the manual there should be 32767 available.
The FAQ suggests that a backup and restore will fix the issue, and this did indeed work, but ideally I’d like to understand why it happened so I can prevent it next time.
I’m aware that even failed generator creations can increment the counter, so I believe this must be the problem. It’s highly unlikely to be ‘manual’ failed generator creation statements as the database is not in production use yet, and there are only two of us working with it for development. I think it must be something attempting to create generators programmatically therefore, although nothing we've written should be doing this as far as I can see. I can’t rule out the industry ERP system we’re using with the database, and we have raised it with the supplier, but I’d be highly surprised if it’s that either.
Has anyone run into this issue before, is there anything else which can affect the generator counter?

Answer 1

A sequence (generator) has a 'slot' on the generator data page(s) that stores its current value. This slot number (RDB$GENERATOR_ID) is assigned when the generator is created (using an internal sequence).
When you drop a sequence, the slot numbers will only increase, until the maximum number of slots have been assigned (and possibly dropped).
In Firebird 2.1 and earlier, this would be the end: having created (and dropped) 32757 sequences would mean you could no longer create sequences. So, if your application is creating (and dropping) a lot of sequences, you will eventually run out of slots, even if you only have 250 'live' sequences.
The only way to reclaim those slots, is by backing up and restoring the database. During the restore, the sequences will be created anew (with the start value from the backup) and get a new slot assigned. These slots will be assigned contiguously, so previously existing gaps disappear, and you will then have unassigned slots available.
However, this was changed Firebird 2.5 with CORE-1544, Firebird will now automatically recycle unused slots. This change will only work with ODS 11.2 or higher databases (ODS = On-Disk Structure). ODS 11.2 is the on-disk structure for databases created with Firebird 2.5.
If you get this error, then probably your database is (was) still ODS 11.1 (the Firebird 2.1 on-disk structure) or earlier. Firebird 2.5 can read earlier on-disk structures. Upgrading the ODS of a database is a matter of backing up and restoring the database. Given you already did this, I assume your database is now ODS 11.2, and the error should no longer occur (unless you actually have 32767 sequences in your database).