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APT External Installation Planner Protocol (EIPP) - version 0.1

This document describes the communication protocol between APT and external installation planner. The protocol is called APT EIPP, for “APT External Installation Planner Protocol”.

Terminology

In the following we use the term architecture qualified package name (or arch-qualified package names for short) to refer to package identifiers of the form “package:arch” where “package” is a package name and “arch” a dpkg architecture.

Components

  • APT: we know this one.
  • APT is equipped with its own internal planner for the order of package installation (and removal) which is identified by the string internal.
  • External planner: an external software component able to plan an installation on behalf of APT.

At each interaction with APT, a single planner is in use. When there is a total of 2 or more planners, internals or externals, the user can choose which one to use.

Each planner is identified by an unique string, the planner name. Planner names must be formed using only alphanumeric ASCII characters, dashes, and underscores; planner names must start with a lowercase ASCII letter. The special name internal denotes APT’s internal planner, is reserved, and cannot be used by external planners.

Installation

Each external planner is installed as a file under Dir::Bin::Planners (see below), which defaults to /usr/lib/apt/planners. We will assume in the remainder of this section that such a default value is in effect.

The naming scheme is /usr/lib/apt/planners/NAME, where NAME is the name of the external planner.

Each file under /usr/lib/apt/planners corresponding to an external planner must be executable.

No non-planner files must be installed under /usr/lib/apt/planners, so that an index of available external planners can be obtained by listing the content of that directory.

Configuration

Several APT options can be used to affect installation planing in APT. An overview of them is given below. Please refer to proper APT configuration documentation for more, and more up to date, information.

  • APT::Planner: the name of the planner to be used for dependency solving. Defaults to internal

  • Dir::Bin::Planners: absolute path of the directory where to look for external solvers. Defaults to /usr/lib/apt/planners.

Protocol

When configured to use an external planner, APT will resort to it to decide in which order packages should be installed, configured and removed.

The interaction happens in batch: APT will invoke the external planner passing the current status of (half-)installed packages and of packages which should be installed, as well as a request denoting the packages to install, reinstall, remove and purge. The external planner will compute a valid plan of when and how to call the low-level package manager (like dpkg) with each package to satisfy the request.

External planners are invoked by executing them. Communications happens via the file descriptors: stdin (standard input) and stdout (standard output). stderr is not used by the EIPP protocol. Planners can therefore use stderr to dump debugging information that could be inspected separately.

After invocation, the protocol passes through a sequence of phases:

  1. APT invokes the external planner
  2. APT send to the planner an installation planner scenario
  3. The planner calculates the order. During this phase the planner may send, repeatedly, progress information to APT.
  4. The planner sends back to APT an answer, i.e. either a solution or an error report.
  5. The external planner exits

Scenario

A scenario is a text file encoded in a format very similar to the “Deb 822” format (AKA “the format used by Debian Packages files”). A scenario consists of two distinct parts: a request and a package universe, occurring in that order. The request consists of a single Deb 822 stanza, while the package universe consists of several such stanzas. All stanzas occurring in a scenario are separated by an empty line.

Request

Within an installation planner scenario, a request represents the action on packages requested by the user explicitly as well as potentially additions calculated by a dependency resolver which the user has accepted.

An installation planner is not allowed to suggest the modification of package states (e.g. removing additional packages) even if it can’t calculate a solution otherwise – the planner must error out in such a case. An exception is made for scenarios which contain packages which aren’t completely installed (like half-installed or trigger-awaiting): Solvers are free to move these packages to a fully installed state (but are still forbidden to remove them).

A request is a single Deb 822 stanza opened by a mandatory Request field and followed by a mixture of action, preference, and global configuration fields.

The value of the Request: field is a string describing the EIPP protocol which will be used to communicate and especially which answers APT will understand. At present, the string must be EIPP 0.1. Request fields are mainly used to identify the beginning of a request stanza; their actual values are otherwise not used by the EIPP protocol.

The following configuration fields are supported in request stanzas:

  • Architecture: (mandatory) The name of the native architecture on the user machine (see also: dpkg --print-architecture)

  • Architectures: (optional, defaults to the native architecture) A space separated list of all architectures known to APT (this is roughly equivalent to the union of dpkg --print-architecture and dpkg --print-foreign-architectures)

The following action fields are supported in request stanzas:

  • Install: (optional, defaults to the empty string) A space separated list of arch-qualified package names, with no version attached, to install. This field denotes a list of packages that the user wants to install, usually via an APT install request.

  • Remove: (optional, defaults to the empty string) Same syntax of Install. This field denotes a list of packages that the user wants to remove, usually via APT remove or purge requests.

  • ReInstall: (optional, defaults to the empty string) Same syntax of Install. This field denotes a list of packages which are installed, but should be reinstalled again e.g. because files shipped by that package were removed or corrupted accidentally, usually requested via an APT install request with the --reinstall flag.

The following preference fields are supported in request stanzas:

  • Planner: (optional, defaults to the empty string) a purely informational string specifying to which planner this request was send initially.

  • Immediate-Configuration: (option, unset by default) A boolean value defining if the planner should try to configure all packages as quickly as possible (true) or shouldn’t perform any kind of immediate configuration at all (false). If not explicitly set with this field the planner is free to pick either mode or implementing e.g. a mode which configures only packages immediately if they are flagged as Essential (or are dependencies of packages marked as Essential).

  • Allow-Temporary-Remove-of-Essentials (optional, defaults to no). A boolean value allowing the planner (if set to yes) to temporarily remove an essential package. Associated with the APT::Force-LoopBreak configuration option its main use is highlighting that planners who do temporary removes must take special care in terms of essentials. Legit uses of this option by users is very uncommon, traditionally a situation in which it is needed indicates a packaging error.

Package universe

A package universe is a list of Deb 822 stanzas, one per package, called package stanzas. Each package stanzas starts with a Package field. The following fields are supported in package stanzas:

  • The fields Package, Version, Architecture (all mandatory) and Multi-Arch, Pre-Depends, Depends, Conflicts, Breaks, Essential (optional) as they are contained in the dpkg database (see the manpage dpkg-query (1)).

  • Status: (optional, defaults to uninstalled). Allowed values are the “package status” names as listed in dpkg-query (1) and visible e.g. in the dpkg database as the second value in the space separated list of values in the Status field there. In other words: Neither desired action nor error flags are present in this field in EIPP!

  • APT-ID: (mandatory). Unique package identifier, according to APT.

Answer

An answer from the external planner to APT is either a solution or an error.

The following invariant on exit codes must hold true. When the external planner is able to find a solution, it will write the solution to standard output and then exit with an exit code of 0. When the external planner is unable to find a solution (and is aware of that), it will write an error to standard output and then exit with an exit code of 0. An exit code other than 0 will be interpreted as a planner crash with no meaningful error about dependency resolution to convey to the user.

Solution

A solution is a list of Deb 822 stanzas. Each of them could be an:

  • unpack stanza to cause the extraction of a package to the disk

  • configure stanza to cause an unpacked package to be configured and therefore the installation to be completed

  • remove stanza to cause the removal of a package from the system

An unpack stanza starts with an Unpack field and supports the following fields:

  • Unpack: (mandatory). The value is a package identifier, referencing one of the package stanzas of the package universe via its APT-ID field.

  • All fields supported by package stanzas.

Configure and Remove stanzas require and support the same fields with the exception of the Unpack field which is replaced in these instances with the Configure or Remove field respectively.

The order of the stanzas is significant (unlike in the EDSP protocol), with the first stanza being the first performed action. If multiple stanzas of the same type appear in direct succession the order in such a set isn’t significant through.

The solution needs to be valid (it is not allowed to configure a package before it was unpacked, dependency relations must be satisfied, …), but they don’t need to be complete: A planner can and should expect that any package which wasn’t explicitly configured will be configured at the end automatically. That also means through that a planner is not allowed to produce a solution in which a package remains unconfigured. Also, packages which are requested to be removed will be automatically removed at the end if not marked for removal explicitly earlier.

In terms of expressivity, all stanzas can carry one single field each, as APT-IDs are enough to pinpoint packages to be installed/removed. Nonetheless, for protocol readability, it is recommended that planners either add unconditionally the fields Package, Version, and Architecture to all install/remove stanzas or, alternatively, that they support a --verbose command line flag that explicitly enables the output of those fields in solutions.

Error

An error is a single Deb 822 stanza, starting the field Error. The following fields are supported in error stanzas:

  • Error: (mandatory). The value of this field is ignored, although it should be a unique error identifier, such as a UUID.

  • Message: (mandatory). The value of this field is a text string, meant to be read by humans, that explains the cause of the planner error. Message fields might be multi-line, like the Description field in the dpkg database. The first line conveys a short message, which can be explained in more details using subsequent lines.

Progress

During dependency solving, an external planner may send progress information to APT using progress stanzas. A progress stanza starts with the Progress field and might contain the following fields:

  • Progress: (mandatory). The value of this field is a date and time timestamp from the UTC timezone, in RFC 2822 format (see ‘date -uR’ as an example). The timestamp provides a time annotation for the progress report.

  • Percentage: (optional). An integer from 0 to 100, representing the completion of the installation planning process, as declared by the planner.

  • Message: (optional). A textual message, meant to be read by the APT user, telling what is going on within the installation planner (e.g. the current phase of planning, as declared by the planner).

Future extensions

Potential future extensions to this protocol are to be discussed on deity@lists.debian.org.