bug fix : correct file extension derivation

[rraymondgh]

path of the form movie...mkv would derive an extension of null

• changed regexp to derive extension from [^/.]\.([a-z0-9]+)$ to .\.([a-z0-9]+)$
• previous regexp would allow any character except period before period extension delimiter. Changed to any character
• go code used by classifier / protobuf has been made consistent. Unit tests created for this function

closes issue #414

[mgdigital]

Thanks for this. It needs some thought as the migration may take a very long time (for large DBs) for what is a fairly minor fix...

[rraymondgh]

migration may take a very long time (for large DBs)

I did consider dropping extension columns completely and move to database access layer. This has fringe benefits

• guarantees consistency - not coded multiple times, once in go and twice in psql
• a disk space saving - columns and indexes. I haven't conducted a full impact assessment of dropping indexes, at first review I didn't find and queries using indexes of extension columns

[mgdigital]

All the proposed solutions entail a long migration for a minor fix so this will be a difficult one to move forward.
Postgres 17 allows updating generated columns so that's a possible path forward but we'd have to drop support for postgres <17, and so this fix might have to wait a while.
The index should be getting used by the file type filter.

[rraymondgh]

Postgres 17 allows updating generated columns so that's a possible path forward

I've tried Postgres 17 syntax (have been using 17 for quite a while now). I didn't expect it performs any better on changing definition of a generated column versus doing in three steps. For reference, here are timings I get below.

bitmagnet_dev=# \timing
Timing is on.
bitmagnet_dev=# alter table "torrent_files" alter column "extension" set expression as 
    (substring(lower(path) from '[^/.]\.([a-z0-9]+)$')) ;
ALTER TABLE
Time: 104993.464 ms (01:44.993)
bitmagnet_dev=# alter table "torrent_files" drop column "extension";
alter table "torrent_files" add column "extension" text generated always as 
    (substring(lower(path) from '[^/.]\.([a-z0-9]+)$')) stored;
create index on torrent_files (extension);
ALTER TABLE
Time: 18.209 ms
ALTER TABLE
Time: 98311.817 ms (01:38.312)
CREATE INDEX
Time: 9606.450 ms (00:09.606)
bitmagnet_dev=# select count(*) from torrent_files;
  count  
---------
 5948038
(1 row)

Time: 282.374 ms

Closing PR - but not deleting branch. If there is a user where filenames that have a period before extension separator is important they can clone repo and merge branch associated with this closed PR.