The following sequence results in a constraint violation in the scanner code and the only way to be able to perform a rescan again afterwards is to restart the server.

1. Rename a music file (test1.mp3 -> test2.mp3)
2. Perform a new/changed rescan and let it finish
3. Rename the music file again back to the original name (test2.mp3 -> test1.mp3)
4. Perform a new/changed rescan

The problem only happens if you change back the file to the original name, so it probably doesn't affect a lot of users, but I thought I'd register it since it's pretty serious if this happens.

The sequence was tested in svn "onebrowser branch" 32501

In point 2, the log correctly reports:
======================
[11-06-06 08:46:19.2885] Slim::Utils::Scanner::Local::rescan (163) Discovering files in /mnt/mp3music_small
[11-06-06 08:46:19.4885] Slim::Utils::Scanner::Local::__ANON__ (235) Removing deleted files (1)
[11-06-06 08:46:19.4928] Slim::Utils::Scanner::Local::__ANON__ (291) Scanning new files (1)
[11-06-06 08:46:19.4955] Slim::Utils::Scanner::Local::__ANON__ (347) Rescanning changed files (0)
[11-06-06 08:46:19.4967] Slim::Utils::Scanner::Local::deleted (477) Handling deleted track file:///mnt/mp3music_small/mp3/testar/test1.mp3
[11-06-06 08:46:19.5712] Slim::Music::Artwork::precacheAllArtwork (435) Starting precacheArtwork for 1 albums
[11-06-06 08:46:19.6350] Slim::Music::Artwork::__ANON__ (528) precacheArtwork finished in 0.0631330013275146


The problem happens in point 4, where the log reports the following and the scanner is locked in "in progress" mode so the only way to get back is to restart the server. The "Rescan" button is disabled until you have restarted the server.
=======================
[11-06-06 08:46:47.0781] Slim::Utils::Scanner::Local::rescan (163) Discovering files in /mnt/mp3music_small
[11-06-06 08:46:47.2771] Slim::Utils::Scanner::Local::__ANON__ (235) Removing deleted files (1)
[11-06-06 08:46:47.2806] Slim::Utils::Scanner::Local::__ANON__ (291) Scanning new files (1)
[11-06-06 08:46:47.2836] Slim::Utils::Scanner::Local::__ANON__ (347) Rescanning changed files (0)
[11-06-06 08:46:47.2845] Slim::Utils::Scanner::Local::deleted (477) Handling deleted track file:///mnt/mp3music_small/mp3/testar/test2.mp3
[11-06-06 08:46:47.3012] Slim::Schema::Storage::throw_exception (119) Error: DBI Exception: DBD::SQLite::st execute failed: constraint failed
foreign key constraint failed [for Statement "INSERT INTO tracks (tagversion,bitrate,audio_offset,drm,audio_size,block_alignment,vbr_scale,remote,audio,titlesort,cover,timestamp,url,lossless,urlmd5,channels,primary_artist,samplerate,secs,filesize,album,content_type,tracknum,titlesearch,title,disc,musicbrainz_id,year) VALUES (?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?)"]
[11-06-06 08:46:47.3020] Slim::Schema::Storage::throw_exception (119) Backtrace:

   frame 0: Slim::Utils::Log::logBacktrace (/media/disk2/squeezecenter/7.6/server_smd_onebrowser/Slim/Schema/Storage.pm line 119)
   frame 1: Slim::Schema::Storage::throw_exception (/media/disk2/squeezecenter/7.6/server_smd_onebrowser/CPAN/DBIx/Class/Storage/DBI.pm line 1006)
   frame 2: DBIx::Class::Storage::DBI::__ANON__ (/media/disk2/squeezecenter/7.6/server_smd_onebrowser/Slim/Schema.pm line 2949)
   frame 3: Slim::Schema::_insertHash (/media/disk2/squeezecenter/7.6/server_smd_onebrowser/Slim/Schema.pm line 1342)
   frame 4: Slim::Schema::_createTrack (/media/disk2/squeezecenter/7.6/server_smd_onebrowser/Slim/Schema.pm line 1601)
   frame 5: Slim::Schema::_newTrack (/media/disk2/squeezecenter/7.6/server_smd_onebrowser/Slim/Schema.pm line 1799)
   frame 6: Slim::Schema::updateOrCreateBase (/media/disk2/squeezecenter/7.6/server_smd_onebrowser/Slim/Utils/Scanner/Local.pm line 688)
   frame 7: Slim::Utils::Scanner::Local::__ANON__ (/media/disk2/squeezecenter/7.6/server_smd_onebrowser/CPAN/DBIx/Class/Storage/DBI.pm line 638)
   frame 8: (eval) (/media/disk2/squeezecenter/7.6/server_smd_onebrowser/CPAN/DBIx/Class/Storage/DBI.pm line 627)
   frame 9: DBIx::Class::Storage::DBI::txn_do (/media/disk2/squeezecenter/7.6/server_smd_onebrowser/lib/DBIx/Class/Schema.pm line 650)
   frame 10: DBIx::Class::Schema::txn_do (/media/disk2/squeezecenter/7.6/server_smd_onebrowser/Slim/Utils/Scanner/Local.pm line 748)
   frame 11: Slim::Utils::Scanner::Local::new (/media/disk2/squeezecenter/7.6/server_smd_onebrowser/Slim/Utils/Scanner/Local.pm line 321)
   frame 12: Slim::Utils::Scanner::Local::__ANON__ (/media/disk2/squeezecenter/7.6/server_smd_onebrowser/Slim/Utils/Scheduler.pm line 150)
   frame 13: (eval) (/media/disk2/squeezecenter/7.6/server_smd_onebrowser/Slim/Utils/Scheduler.pm line 150)
   frame 14: Slim::Utils::Scheduler::run_tasks (/opt/squeezecenter76_smd_onebrowser/slimserver.pl line 676)
   frame 15: main::idle (/opt/squeezecenter76_smd_onebrowser/slimserver.pl line 643)
   frame 16: main::main (/opt/squeezecenter76_smd_onebrowser/slimserver.pl line 1138)

[11-06-06 08:46:47.3034] Slim::Schema::Storage::throw_exception (119) Error: Carp::Clan::__ANON__(): DBI Exception: DBD::SQLite::st execute failed: constraint failed
foreign key constraint failed [for Statement "INSERT INTO tracks (tagversion,bitrate,audio_offset,drm,audio_size,block_alignment,vbr_scale,remote,audio,titlesort,cover,timestamp,url,lossless,urlmd5,channels,primary_artist,samplerate,secs,filesize,album,content_type,tracknum,titlesearch,title,disc,musicbrainz_id,year) VALUES (?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?)"] at /media/disk2/squeezecenter/7.6/server_smd_onebrowser/Slim/Schema/Storage.pm line 123
[11-06-06 08:46:47.3039] Slim::Schema::Storage::throw_exception (119) Backtrace:

   frame 0: Slim::Utils::Log::logBacktrace (/media/disk2/squeezecenter/7.6/server_smd_onebrowser/Slim/Schema/Storage.pm line 119)
   frame 1: Slim::Schema::Storage::throw_exception (/media/disk2/squeezecenter/7.6/server_smd_onebrowser/CPAN/DBIx/Class/Storage/DBI.pm line 660)
   frame 2: DBIx::Class::Storage::DBI::txn_do (/media/disk2/squeezecenter/7.6/server_smd_onebrowser/lib/DBIx/Class/Schema.pm line 650)
   frame 3: DBIx::Class::Schema::txn_do (/media/disk2/squeezecenter/7.6/server_smd_onebrowser/Slim/Utils/Scanner/Local.pm line 748)
   frame 4: Slim::Utils::Scanner::Local::new (/media/disk2/squeezecenter/7.6/server_smd_onebrowser/Slim/Utils/Scanner/Local.pm line 321)
   frame 5: Slim::Utils::Scanner::Local::__ANON__ (/media/disk2/squeezecenter/7.6/server_smd_onebrowser/Slim/Utils/Scheduler.pm line 150)
   frame 6: (eval) (/media/disk2/squeezecenter/7.6/server_smd_onebrowser/Slim/Utils/Scheduler.pm line 150)
   frame 7: Slim::Utils::Scheduler::run_tasks (/opt/squeezecenter76_smd_onebrowser/slimserver.pl line 676)
   frame 8: main::idle (/opt/squeezecenter76_smd_onebrowser/slimserver.pl line 643)
   frame 9: main::main (/opt/squeezecenter76_smd_onebrowser/slimserver.pl line 1138)

[11-06-06 08:46:47.3059] Slim::Utils::Scheduler::run_tasks (157) Error: Scheduler task failed: Carp::Clan::__ANON__(): Carp::Clan::__ANON__(): DBI Exception: DBD::SQLite::st execute failed: constraint failed
foreign key constraint failed [for Statement "INSERT INTO tracks (tagversion,bitrate,audio_offset,drm,audio_size,block_alignment,vbr_scale,remote,audio,titlesort,cover,timestamp,url,lossless,urlmd5,channels,primary_artist,samplerate,secs,filesize,album,content_type,tracknum,titlesearch,title,disc,musicbrainz_id,year) VALUES (?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?)"] at /media/disk2/squeezecenter/7.6/server_smd_onebrowser/Slim/Schema/Storage.pm line 123