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This is taken in exclusive mode whenever we prune the repo. Anything that is not protected against object disappearing from the repo can take the lock in a shared mode to avoid running at the same time as a prune operation. The two operations the are problematic in this respect is Pull: * During the pull we see that some object we require is already available and doesn't need downloading. We can't have it be removed before we commit the transaction. * During the transaction commit we're moving the object to the repo, and they risk being pruned as unreachable until we have updated the ref. Deploy: * Once we start checking out a particular ref we assume all the object from it is reachable. If the ref is updated in parallel some object can become unreachable and removed. So, we take shared locks in these operations. In the prune operation we take the block non-blocking, and skip the prune entirely if some other operation is outstanding, because we don't want to block a long time, and its likely that due to the other operation we will run prune shortly anyway. Note: Nothing protects the system-helper case when we download to a separate repo. However, if there is a race we will get an error when importing this to the system repo, so we'll never end up in an inconsistent state.
Flatpak is a system for building, distributing and running sandboxed desktop applications on Linux.
See http://flatpak.org/ for more information.
Read documentation for the flatpak commandline tools and for the libflatpak library API.
INSTALLATION
Flatpak uses a traditional autoconf-style build mechanism. To build just do
./configure [args]
make
make install
Most configure arguments are documented in ./configure --help. However,
there are some options that are a bit more complicated.
Flatpak relies on a project called
Bubblewrap for the
low-level sandboxing. By default, an in-tree copy of this is built
(distributed in the tarball or using git submodules in the git
tree). This will build a helper called flatpak-bwrap. If your system
has a recent enough version of Bubblewrap already, you can use
--with-system-bubblewrap to use that instead.
Bubblewrap can run in two modes, either using unprivileged user
namespaces or setuid mode. This requires that the kernel supports this,
which some distributions disable. For instance, Arch completely
disables user namespaces, while Debian supports unprivileged user
namespaces, but only if you turn on the
kernel.unprivileged_userns_clone sysctl.
If unprivileged user namespaces are not available, then Bubblewrap must be built as setuid root. This is believed to be safe, as it is designed to do this. Any build of Bubblewrap supports both unprivileged and setuid mode, you just need to set the setuid bit for it to change mode.
However, this does complicate the installation a bit. If you pass
--with-priv-mode=setuid to configure (of Flatpak or Bubblewrap) then
make install will try to set the setuid bit. However that means you
have to run make install as root. Alternatively, you can pass
--enable-sudo to configure and it will call sudo when setting the
setuid bit. Alternatively you can enable setuid completely outside of
the installation, which is common for example when packaging Bubblewrap
in a .deb or .rpm.
There are some complications when building Flatpak to a different
prefix than the system-installed version. First of all, the newly
built Flatpak will look for system-installed flatpaks in
$PREFIX/var/lib/flatpak, which will not match existing installed
flatpaks. You can use --with-system-install-dir=/var/lib/flatpak
to make both installations use the same location.
Secondly, Flatpak ships with a root-privileged policykit helper for
system-installation, called flatpak-system-helper. This is dbus
activated (on the system-bus) and if you install in a non-standard
location it is likely that this will not be found by dbus and
policykit. However, if the system installation is synchronized,
you can often use the system installed helper instead - at least
if the two versions are close in versions.