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Installing, Removing, and Updating Software Packages
This chapter describes how the IPS client works internally when installing,
updating and removing the software installed in an image.
Understanding basically how |pkg| works will help administrators and developers
better understand the various errors that can occur, and allow them to more
quickly resolve package dependency problems.
How package changes are performed
The following steps are executed when |pkg| is invoked to modify the software
installed on the machine:
* Check input for errors
* Determine the system end-state
* Run basic checks
* Run the solver
* Optimize the solver results
* Evaluate actions
* Download content
* Execute actions
* Process actuators
When operating on the global zone, during execution of the steps above, |pkg|
can execute operations on any non-global zones on the machine, for example
to ensure that dependencies are correct between the global and non-global zones,
or to download content or execute actions for non-global zones. *Chapter 12*
has more detail about zones.
In the following sections, we'll describe each of these steps.
Check Input for Errors
We perform basic error checking on the options presented on the command line.
Determine the System End State
A description of the desired end state of the system is
constructed. In the case of updating all packages in the image this
might be something like *"all the packages currently installed, or
newer versions of them"*. In the case of package removal, it would
be *"all the packages currently installed without this one"*.
IPS tries hard to determine what the user intends this end state to look
like. In some cases, IPS might determine an end state that is not what the
user intended, even though that end state does match what the user requested.
When troubleshooting, it is best to be as *specific* as possible. The
following command is not specific::
# pkg update
If this command fails with a message such as ``No updates available for this
image``, then you might want to try a more specific command such as the
# pkg update "*@latest"
This command defines the end state more exactly, and can produce more
directed error messages.
Run Basic Checks
The desired end state of the system is reviewed to make sure that a
solution appears possible. During this basic review, |pkg| checks that a
plausible version exists of all dependencies, and that desired packages
do not exclude each other.
If an obvious error exists, then |pkg| will print an appropriate error
message and exit.
Run the Solver
The solver forms the core of the computation engine used by |pkg5|
to determine the packages that can be installed, updated or removed,
given the constraints in the image and constraints introduced by any
new packages for installation.
This problem is an example of a *Boolean satisfiability problem*,
and can be solved by a |SAT solver|.
The various possible choices for all the packages are assigned
boolean variables, and all the dependencies between those packages,
any required packages, etc. are cast as boolean expressions in
conjunctive normal form.
The set of expressions generated is passed to |MiniSAT|. If MiniSAT
cannot find any solution, the error handling code attempts to walk
the set of installed packages and the attempted operation, and print
the reasons that each possible choice was eliminated.
If the currently installed set of packages meet the requirements but
no other does, |pkg| will report that there is nothing to do.
As mentioned in a previous section, the error message generation and
specificity is determined by the inputs to |pkg|. Being as specific as
possible in commands issued to |pkg| will produce the most useful error
If on the other hand MiniSAT finds a possible solution, we begin
Optimize the Solver Results
The optimization phase is necessary because there is no way of
describing some solutions as more desirable than others to a SAT
Instead, once a solution is found, IPS adds constraints
to the problem to separate less desirable choices,
and to separate the current solution as well. We then repeatedly
invoke MiniSAT and repeat the above operation until no more
solutions are found. The last successful solution is taken as the
Clearly, the difficulty of finding a solution is proportional to
the number of possible solutions. Being more specific about the desired
result will produce solutions more quickly.
Once the set of package FMRIs that best satisfy the posed
problem is found, the evaluation phase begins.
In this phase, we compare the packages currently installed on the
system with the end state, and compare package manifests
of old and new packages to determine three lists:
* Actions that are being removed
* Actions that are being added
* Actions that are being updated
The action lists are then updated so that:
* directory and link actions are reference counted, mediated link
processing is done
* hardlinks are marked for repair if their target file is updated.
This is done because updating a target of a hardlink in a manner that
is safe for currently executing processes breaks the hard links.
* editable files moving between packages are correctly handled
so that any user edits are not lost.
* the action lists are sorted so that removals, additions and
updates occur in the correct order.
All the currently installed packages are then cross-checked to
make sure that no packages conflict. That is, ensuring that two
packages do not attempt to deliver a file to the same location,
ensuring that directory attributes for the same directory agree
between packages, etc.
If conflicts exist, these are reported and |pkg| exits with an error message.
Finally, the action lists are scanned to determine if any
SMF services need to be restarted if this operation is performed,
whether or not this change can be applied to a running system,
whether the boot archive needs to be rebuilt and whether the
amount of space required is available, etc.
If |pkg| is running without the ``-n`` flag, processing continues
to the download phase.
For each action that requires content, we download any required
files by hash and cache them. This step can take some time if
the amount of content to be retrieved is large.
Once downloading is complete, if the change is to be applied to
a live system (image is rooted at '/') and a reboot is required,
the running system is cloned and the target image is switched to
Executing actions involves actually performing the install or
remove methods specific to each action type on the image.
Execution begins with all the removal actions being executed. If
any unexpected content is found in directories being removed from
the system, that content is placed in ``/var/pkg/lost+found``.
Execution then proceeds to install and update actions. Note that all
the actions have been blended across all packages. Thus all the
changes in a single package operation are applied to the system at once
rather than package by package. This permits packages to depend on each
other, exchange content, etc. safely. For details on how files
are updated, see the description of the ``file`` action in *Chapter 3*.
If we're updating a live system, any pending actuators are executed
at this point. These are typically SMF service restarts and refreshes.
Once these are launched, we update the local search indicies. We discuss
actuators in more detail in *Chapter 9*
Lastly, if needed, we update the boot archive.