Historically, daemon.RegistryHosts() has returned a docker.RegistryHosts
callback function which closes over a point-in-time snapshot of the
daemon configuration. When constructing the BuildKit builder at daemon
startup, the return value of daemon.RegistryHosts() has been used.
Therefore the BuildKit builder would use the registry configuration as
it was at daemon startup for the life of the process, even if the
registry configuration is changed and the configuration reloaded.
Provide BuildKit with a RegistryHosts callback which reflects the
live daemon configuration after reloads so that registry operations
performed by BuildKit always use the same configuration as the rest of
the daemon.

Signed-off-by: Cory Snider <[email protected]>

Config reloading has interleaved validations and other fallible
operations with mutating the live daemon configuration. The daemon
configuration could be left in a partially-reloaded state if any of the
operations returns an error. Mutating a copy of the configuration and
atomically swapping the config struct on success is not currently an
option as config values are not copyable due to the presence of
sync.Mutex fields. Introduce a two-phase commit protocol to defer any
mutations of the daemon state until after all fallible operations have
succeeded.

Reload transactions are not yet entirely hermetic. The platform
reloading logic for custom runtimes on *nix could still leave the
directory of generated runtime wrapper scripts in an indeterminate state
if an error is encountered.

Signed-off-by: Cory Snider <[email protected]>

Ensure data-race-free access to the daemon configuration without
locking by mutating a deep copy of the config and atomically storing
a pointer to the copy into the daemon-wide configStore value. Any
operations which need to read from the daemon config must capture the
configStore value only once and pass it around to guarantee a consistent
view of the config.

Signed-off-by: Cory Snider <[email protected]>

The existing runtimes reload logic went to great lengths to replace the
directory containing runtime wrapper scripts as atomically as possible
within the limitations of the Linux filesystem ABI. Trouble is,
atomically swapping the wrapper scripts directory solves the wrong
problem! The runtime configuration is "locked in" when a container is
started, including the path to the runC binary. If a container is
started with a runtime which requires a daemon-managed wrapper script
and then the daemon is reloaded with a config which no longer requires
the wrapper script (i.e. some args -> no args, or the runtime is dropped
from the config), that container would become unmanageable. Any attempts
to stop, exec or otherwise perform lifecycle management operations on
the container are likely to fail due to the wrapper script no longer
existing at its original path.

Atomically swapping the wrapper scripts is also incompatible with the
read-copy-update paradigm for reloading configuration. A handler in the
daemon could retain a reference to the pre-reload configuration for an
indeterminate amount of time after the daemon configuration has been
reloaded and updated. It is possible for the daemon to attempt to start
a container using a deleted wrapper script if a request to run a
container races a reload.

Solve the problem of deleting referenced wrapper scripts by ensuring
that all wrapper scripts are *immutable* for the lifetime of the daemon
process. Any given runtime wrapper script must always exist with the
same contents, no matter how many times the daemon config is reloaded,
or what changes are made to the config. This is accomplished by using
everyone's favourite design pattern: content-addressable storage. Each
wrapper script file name is suffixed with the SHA-256 digest of its
contents to (probabilistically) guarantee immutability without needing
any concurrency control. Stale runtime wrapper scripts are only cleaned
up on the next daemon restart.

Split the derived runtimes configuration from the user-supplied
configuration to have a place to store derived state without mutating
the user-supplied configuration or exposing daemon internals in API
struct types. Hold the derived state and the user-supplied configuration
in a single struct value so that they can be updated as an atomic unit.

Signed-off-by: Cory Snider <[email protected]>

The daemon has made a habit of mutating the DefaultRuntime and Runtimes
values in the Config struct to merge defaults. This would be fine if it
was a part of the regular configuration loading and merging process,
as is done with other config options. The trouble is it does so in
surprising places, such as in functions with 'verify' or 'validate' in
their name. It has been necessary in order to validate that the user has
not defined a custom runtime named "runc" which would shadow the
built-in runtime of the same name. Other daemon code depends on the
runtime named "runc" always being defined in the config, but merging it
with the user config at the same time as the other defaults are merged
would trip the validation. The root of the issue is that the daemon has
used the same config values for both validating the daemon runtime
configuration as supplied by the user and for keeping track of which
runtimes have been set up by the daemon. Now that a completely separate
value is used for the latter purpose, surprising contortions are no
longer required to make the validation work as intended.

Consolidate the validation of the runtimes config and merging of the
built-in runtimes into the daemon.setupRuntimes() function. Set the
result of merging the built-in runtimes config and default default
runtime on the returned runtimes struct, without back-propagating it
onto the config.Config argument.

Signed-off-by: Cory Snider <[email protected]>