Hvorfor overskriver schroot konfigurationsfiler i chroot'en?

By default, schroot copies over the system NSS databases (‘passwd’, ‘shadow’, ‘group’, ‘gshadow’, ‘services’, ‘protocols’, ‘networks’, and ‘hosts’, etc.) into the chroot. The reason for this is that the chroot environment is not a completely separate system, and it copying them over keeps them synchronised. However, this is not always desirable, particularly if installing a package in the chroot creates system users and groups which are not present on the host, since these will disappear next time the databases are copied over.

The suggested workaround here is to disable the copying. This may be achieved by setting the setup.nssdatabases key to be empty in schroot.conf. In prior schroot releases, this was done by commenting out the NSSDATABASES file for the chroot (/etc/schroot/default/config by default). The database list may also be customised by editing the file containing the database list (/etc/schroot/default/nssdatabases by default).

In the future, we will be working on a better scheme for keeping the host and chroot databases in sync which can merge entries rather than overwriting the entire database, which would preserve chroot-specific changes.

Should I use the plain or directory chroot type?

These two chroot types are basically equivalent, since they are both just directories in the filesystem. plain is very simple and does not perform any setup tasks; the only reason you would want to use it is if you're upgrading from a program such as dchroot(1) or chroot(8) which don't do anything other than running a command or shell in a directory. On the other hand, directory chroots do run setup scripts, which can mount additional filesystems and do other setup tasks.

What are snapshots and unions?

Some chroot types support cloning. This means when you start a session, you get a copy of the chroot which lasts just for the lifetime of the session. This is useful when you want a temporary clean copy of a system for a single task, which is then automatically deleted when you're done with it. For example, the Debian package build dæmons run sbuild(1) to build Debian packages, and this program uses schroot to create a clean build environment for each package. Without snapshotting, the chroot would need to be reset to its initial state at the end of each build to make it ready for the next one, and any debris left over from package removals or earlier builds could interfere with the next build.

The most commonly-used snapshotting method is to use LVM snapshots (chroot type ‘lvm-snapshot’). In this case the chroot must exist on an LVM logical volume (LV); snapshots of an LV may then be made with lvcreate(8) during chroot session setup. However, these use up a lot of disk space. A newer method is to use Btrfs snapshots which use up much less disk space (chroot type ‘btrfs-snapshot’), and may be more reliable than LVM snapshots. Btrfs is however still experimental, but it is hoped that it will become the recommended method as it matures.

Unions are an alternative to snapshots. In this situation, instead of creating a copy of the chroot filesystem, we overlay a read-write temporary filesystem on top of the chroot filesystem so that any modifications are stored in the overlay, leaving the original chroot filesystem untouched. The Linux kernel has yet to integrate support for union filesystems such as aufs and unionfs, so LVM snapshots are still the recommended method at present.

Kan jeg køre en dæmons i en chroot?

A common problem is trying to run a dæmon in a chroot, and finding that this doesn't work. Typically, the dæmon is killed shortly after it starts up.

When schroot runs, it begins a session, runs the specified command or shell, waits for the command or shell to exit, and then it ends the session. For a normal command or shell, this works just fine. However, dæmons normally start up by running in the background and detaching from the controlling terminal. They do this by forking twice and letting the parent processes exit. Unfortunately, this means schroot detects that the program exited (the dæmon is a orphaned grandchild of this process) and it then ends the session. Part of ending the session is killing all processes running inside the chroot, which means the dæmon is killed as the session ends.

In consequence, it's not possible to run a dæmon directly with schroot. You can however do it if you create a session with --begin-session and then run the dæmon with --run-session. It's your responsibility to end the session with --end-session when the dæmon has terminated or you no longer need it.

Hvordan rydder jeg manuelt op i en ødelagt session?

Occasionally, it may be necessary to manually clean up sessions. If something changes on your system which causes the setup scripts to fail when ending a session, for example removal of a needed file or directory, it may not be possible for schroot to clean everything up automatically. For each of the session directories listed in the “Session directories” section in schroot(1), any files with the name of the session ID need deleting, and any directories with the name of the session ID need umounting (if there are any filesystems mounted under it), and then also removing.

For eksempel, for at fjerne en session navngivet min-session manuelt:

•

Fjern sessionskonfigurationsfilen

% rm /var/lib/schroot/session/min-session↵

•

Kontroller for monterede filsystemer

% /usr/lib/schroot/schroot-listmounts -m \
  /var/run/schroot/mount/min-session↵

•

Afmonter alle monterede filsystemer

•

Fjern /var/run/schroot/mount/min-session

•

Gentag for andre mapper såsom /var/lib/schroot/union/underlay, /var/lib/schroot/union/overlay og /var/lib/schroot/unpack

NOTE: Fjern ikke mapper uden at kontroller om der er filsystemer monteret i dem, da filsystemer såsom /home stadig kan være bind-monteret. Dette kan medføre tab af data!

Hvordan bruger jeg sessioner?

Under normal brug kan kørsel af en kommando se således ud:

% schroot -c squeeze -- kommando↵

Som vil køre kommandoen kommando i squeeze-chroot'en. Selv om det ikke er indlysende at en seesion bruges her, så udfører schroot faktisk de følgende trin:

•

Oprettelse af en session med brug af squeeze-chroot'en. Denne vil automatisk får et unikt navn, såsom squeeze-57a69547-e014-4f5d-a98b-f4f35a005307, selvom du ikke nødvendigvis kender til dette

•

Opsætningsskripter køres for at oprette sessionschroot'en og konfigurere den for dig.

•

Kommandoen kommando køres inden i sessionschroot'en

•

Opsætningsskripter køres for at rydde op i sessionschroot'en

•

Sessionen slettes

Now, if you wanted to run more than one command, you could run a shell and run them interactively, or you could put them into shell script and run that instead. But you might want to do something in between, such as running arbitrary commands from a program or script where you don't know which commands to run in advance. You might also want to preseve the chroot state in between commands, where the normal automatic session creation would reset the state in between each command. This is what sessions are for: once created, the session is persistent and won't be automatically removed. With a session, you can run as many commands as you like, but you need to create and delete the session by hand since schroot can't know by itself when you're done with it unlike in the single command case above. This is quite easy:

% schroot --begin-session -c squeeze↵
squeeze-57a69547-e014-4f5d-a98b-f4f35a005307

This created a new session based upon the squeeze chroot. The unique name for the session, the session ID, was printed to standard output, so we could also save it as a shell variable at the same time like so:

% SESSION=$(schroot --begin-session -c squeeze)↵
% echo $SESSION↵
squeeze-57a69547-e014-4f5d-a98b-f4f35a005307

Nu vi har oprettet sessionen og fået sessions-id'et, så kan vi køre kommandoer i den via brug af sessions-id'et:

% schroot --run-session -c squeeze-57a69547-e014-4f5d-a98b-f4f35a005307 \
  -- kommando1↵

eller

% schroot --run-session -c "$SESSION" -- kommando1↵

og så det antal kommandoer vi ønsker

% schroot --run-session -c "$SESSION" -- kommando2↵
% schroot --run-session -c "$SESSION" -- kommando3↵
% schroot --run-session -c "$SESSION" -- kommando4↵

etc.

Når vi er færdig med sessionen, så kan vi fjerne den med --end-session:

% schroot --end-session -c squeeze-57a69547-e014-4f5d-a98b-f4f35a005307↵

eller

% schroot --end-session -c "$SESSION"↵

Da de automatisk oprettede sessionsnavne kan være lange og uhåndterlige, giver tilvalget --session-name dig mulighed for at angive dit eget navn:

% schroot --begin-session -c squeeze --session-name mit-navn↵
mit-navn

Få hjælp og bliv involveret

Postlisten <buildd-tools-devel@lists.alioth.debian.org> bruges til både brugerhjælp og udviklingsdiskussion. Listen kan tilmeldes på projektes hjemmeside https://alioth.debian.org/projects/buildd-tools/ eller via Mailmans listegrænseflade http://lists.alioth.debian.org/mailman/listinfo/buildd-tools-devel.

Indsendelse af fejlrapporter

På Debiansystemer kan fejl rapporteres via værktøjet reportbug(1) eller alternativt med post <submit@bugs.debian.org> (se http://bugs.debian.org for detaljer om hvordan dette gøres.

Hent de seneste kilder

schroot vedligeholdes i versionskontrolsystemet git. Du kan hente de seneste kilder fra git://git.debian.org/git/buildd-tools/schroot.

% git clone git://git.debian.org/git/buildd-tools/schroot↵

Hovedgrenen (master) indeholder den nuværende udviklingsudgivelse. Stabile udgivelser findes i andre grene, for eksempel serien 1.4 i grenen schroot-1.4.