Public bug reported:
While working on mysql-5.6 package we met unusual (maybe even partially
incorrect) Alias option in systemd unit config:
Alias=mysql.service
It is unusual because mysql.service is the original name of the service which
can be found inside /lib/systemd/system/<...> This Alias option doesn't make
any sense (mysql service will be accessible as mysql.service even without this
alias) and my assumption is that it was created because the author was not sure
if it's okay not to have any aliases at all or you need at least one. Anyhow
this seems to be legal to do that (i was not able to find any documents which
state that this is incorrect) even if it's something useless.
Installation went well and the following alias symlink has been created:
/etc/systemd/system/mysq.service --> /lib/systemd/system/mysql.service
Service was accessible via systemctl and everything was fine.
When you try to remove/purge the package you get the following error:
/usr/bin/deb-systemd-helper: error: unable to link
/etc/systemd/system/mysql.service to /dev/null: File exists
This happens because deb-systemd-helper tries to 'mask' service the following
way:
sub mask_service {
my ($scriptname, $service_path) = @_;
my $mask_link = '/etc/systemd/system/' . basename($service_path);
if (-e $mask_link) {
# If the link already exists, don’t do anything.
return if -l $mask_link && readlink($mask_link) eq '/dev/null';
debug "$mask_link already exists, not masking.";
return;
}
make_path(dirname($mask_link));
symlink('/dev/null', $mask_link) or
error("unable to link $mask_link to /dev/null: $!");
<...>
}
Check for '-e $mask_link' FAILS because symlink's TARGET
(/lib/systemd/system/mysql.service) doesn't exists while doing postrm
and '-e' FOLLOWS symlinks. That's why we go to symlink(...) call
directly which fails because symlink already exists.
WORKAROUND: If apt-get remove/purge crashes with the error mentioned
above you need to manually remove symlink inside /etc/systemd/system
(/etc/systemd/system/mysql.service in our case) and rerun apt-get
remove/purge.
I'm not sure if we want to fix that.
On one hand, this situation is pretty rare.
On other hand though, such Alias=<original-name> situation is somewhat legal
and we may want to handle it correctly.
This issue with Alias= has been fixed (by removing this useless Alias)
in mysql-5.6 so that's not a blocker for us.
** Affects: init-system-helpers (Ubuntu)
Importance: Undecided
Status: New
** Description changed:
While working on mysql-5.6 package we met unusual (maybe even partially
incorrect) Alias option in systemd unit config:
Alias=mysql.service
It is unusual because mysql.service is the original name of the service which
can be found inside /lib/systemd/system/<...> This Alias option doesn't make
any sense (mysql service will be accessible as mysql.service even without this
alias) and my assumption is that it was created because the author was not sure
if it's okay not to have any aliases at all or you need at least one. Anyhow
this seems to be legal to do that (i was not able to find any documents which
state that this is incorrect) even if it's something useless.
Installation went well and the following alias symlink has been created:
/etc/systemd/system/mysq.service --> /lib/systemd/system/mysql.service
Service was accessible via systemctl and everything was fine.
When you try to remove/purge the package you get the following error:
/usr/bin/deb-systemd-helper: error: unable to link
/etc/systemd/system/mysql.service to /dev/null: File exists
This happens because deb-systemd-helper tries to 'mask' service the following
way:
sub mask_service {
- my ($scriptname, $service_path) = @_;
+ my ($scriptname, $service_path) = @_;
- my $mask_link = '/etc/systemd/system/' . basename($service_path);
+ my $mask_link = '/etc/systemd/system/' . basename($service_path);
- if (-e $mask_link) {
- # If the link already exists, don’t do anything.
- return if -l $mask_link && readlink($mask_link) eq '/dev/null';
+ if (-e $mask_link) {
+ # If the link already exists, don’t do anything.
+ return if -l $mask_link && readlink($mask_link) eq '/dev/null';
- # If the file already exists, the user most likely copied the .service
- # file to /etc/ to change it in some way. In this case we don’t need
to
- # mask the .service in the first place, since it will not be removed
by
- # dpkg.
- debug "$mask_link already exists, not masking.";
- return;
- }
+ debug "$mask_link already exists, not masking.";
+ return;
+ }
- make_path(dirname($mask_link));
- symlink('/dev/null', $mask_link) or
- error("unable to link $mask_link to /dev/null: $!");
+ make_path(dirname($mask_link));
+ symlink('/dev/null', $mask_link) or
+ error("unable to link $mask_link to /dev/null: $!");
<...>
}
Check for '-e $mask_link' FAILS because symlink's TARGET
(/lib/systemd/system/mysql.service) doesn't exists while doing postrm
and '-e' FOLLOWS symlinks. That's why we go to symlink(...) call
directly which fails because symlink already exists.
WORKAROUND: If apt-get remove/purge crashes with the error mentioned
above you need to manually remove symlink inside /etc/systemd/system
(/etc/systemd/system/mysql.service in our case) and rerun apt-get
remove/purge.
I'm not sure if we want to fix that.
On one hand, this situation is pretty rare.
On other hand though, such Alias=<original-name> situation is somewhat legal
and we may want to handle it correctly.
This issue with Alias= has been fixed (by removing this useless Alias)
in mysql-5.6 so that's not a blocker for us.
--
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Server Team, which is subscribed to init-system-helpers in Ubuntu.
https://bugs.launchpad.net/bugs/1439793
Title:
deb-systemd-helper crashes while removing packages with unusual (but
legal) Alias in unit config
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https://bugs.launchpad.net/ubuntu/+source/init-system-helpers/+bug/1439793/+subscriptions
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