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diff --git a/doc/external-dependency-solver-protocol.md b/doc/external-dependency-solver-protocol.md new file mode 100644 index 0000000..6890281 --- /dev/null +++ b/doc/external-dependency-solver-protocol.md @@ -0,0 +1,377 @@ +# APT External Dependency Solver Protocol (EDSP) - version 0.5 + +This document describes the communication protocol between APT and +external dependency solvers. The protocol is called APT EDSP, for "APT +External Dependency Solver 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 solver** for dependencies, + which is identified by the string `internal`. +- **External solver**: an *external* software component able to resolve + dependencies on behalf of APT. + +At each interaction with APT, a single solver is in use. When there is +a total of 2 or more solvers, internals or externals, the user can +choose which one to use. + +Each solver is identified by an unique string, the **solver +name**. Solver names must be formed using only alphanumeric ASCII +characters, dashes, and underscores; solver names must start with a +lowercase ASCII letter. The special name `internal` denotes APT's +internal solver, is reserved, and cannot be used by external solvers. + + +## Installation + +Each external solver is installed as a file under Dir::Bin::Solvers (see +below), which defaults to `/usr/lib/apt/solvers`. We will assume in the +remainder of this section that such a default value is in effect. + +The naming scheme is `/usr/lib/apt/solvers/NAME`, where `NAME` is the +name of the external solver. + +Each file under `/usr/lib/apt/solvers` corresponding to an external +solver must be executable. + +No non-solver files must be installed under `/usr/lib/apt/solvers`, so +that an index of available external solvers can be obtained by listing +the content of that directory. + + +## Configuration + +Several APT options can be used to affect dependency solving 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::Solver**: the name of the solver to be used for + dependency solving. Defaults to `internal` + +- **Dir::Bin::Solvers**: absolute path of the directory where to look for + external solvers. Defaults to `/usr/lib/apt/solvers`. + +- **APT::Solver::Strict-Pinning**: whether pinning must be strictly + respected (as the internal solver does) or can be slightly deviated + from. Defaults to `yes`. + +- **APT::Solver::Preferences**: user preference string used during + dependency solving by the requested solver. Check the documentation + of the solver you are using if and what is supported as a value here. + Defaults to the empty string. + +- **APT::Solver::RunAsUser**: if APT itself is run as root it will + change to this user before executing the solver. Defaults to the value + of APT::Sandbox::User, which itself defaults to `_apt`. Can be + disabled by set this option to `root`. + +The options **Strict-Pinning** and **Preferences** can also be set for +a specific solver only via **APT::Solver::NAME::Strict-Pinning** and +**APT::Solver::NAME::Preferences** respectively where `NAME` is the name +of the external solver this option should apply to. These options if set +override the generic options; for simplicity the documentation will +refer only to the generic options. + + +## Protocol + +When configured to use an external solver, APT will resort to it to +decide which packages should be installed or removed. + +The interaction happens **in batch**: APT will invoke the external +solver passing the current status of installed and available packages, +as well as the user request to alter the set of installed packages. The +external solver will compute a new complete set of installed packages +and gives APT a "diff" listing of which *additional* packages should be +installed and of which currently installed packages should be +*removed*. (Note: the order in which those actions have to be performed +will be up to APT to decide.) + +External solvers are invoked by executing them. Communications happens +via the file descriptors: **stdin** (standard input) and **stdout** +(standard output). stderr is not used by the EDSP protocol. Solvers 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 solver +2. APT send to the solver a dependency solving **scenario** +3. The solver solves dependencies. During this phase the solver may + send, repeatedly, **progress** information to APT. +4. The solver sends back to APT an **answer**, i.e. either a *solution* + or an *error* report. +5. The external solver 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 a dependency solving scenario, a request represents the action on +installed packages requested by the user. + +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 EDSP +protocol which will be used to communicate. At present, the string must +be `EDSP 0.5`. Request fields are mainly used to identify the beginning +of a request stanza; their actual values are otherwise not used by the +EDSP 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. + +- **Upgrade-All:** (optional, defaults to `no`). Allowed values `yes`, + `no`. When set to `yes`, an upgrade of all installed packages has been + requested, usually via an upgrade command like 'apt full-upgrade'. + +- **Autoremove:** (optional, defaults to `no`). Allowed values: `yes`, + `no`. When set to `yes`, a clean up of unused automatically installed + packages has been requested, usually via an APT `autoremove` request. + +- **Upgrade:** (deprecated, optional, defaults to `no`). Allowed values: + `yes`, `no`. When set to `yes`, an upgrade of all installed packages + has been requested, usually via an APT `upgrade` request. A value of + `yes` is equivalent to the fields `Upgrade-All`, + `Forbid-New-Install`and `Forbid-Remove` all set to `yes`. + +- **Dist-Upgrade:** (deprecated, optional, defaults to `no`). Allowed + values: `yes`, `no`. Same as Upgrade, but for APT `dist-upgrade` + requests. A value of `yes` is equivalent to the field `Upgrade-All` + set to `yes` and the fields `Forbid-New-Install`and `Forbid-Remove` + set to `no`. + +The following **preference fields** are supported in request stanzas: + +- **Strict-Pinning:** (optional, defaults to `yes`). Allowed values: + `yes`, `no`. When set to `yes`, APT pinning is strict, in the sense + that the solver must not propose to install packages which are not APT + candidates (see the `APT-Pin` and `APT-Candidate` fields in the + package universe). When set to `no`, the solver does only a best + effort attempt to install APT candidates. Usually, the value of this + field comes from the `APT::Solver::Strict-Pinning` configuration + option. + +- **Forbid-New-Install:* (optional, defaults to `no`). Allowed values: + `yes`, `no`. When set to `yes` the resolver is forbidden to install + new packages in its returned solution. + +- **Forbid-Remove:* (optional, defaults to `no`). Allowed values: `yes`, + `no`. When set to `yes` the resolver is forbidden to remove currently + installed packages in its returned solution. + +- **Solver:** (optional, defaults to the empty string) a purely + informational string specifying to which solver this request was send + initially. + +- **Preferences:** (optional, defaults to the empty string) + a solver-specific optimization string, usually coming from the + `APT::Solver::Preferences` configuration option. + + +#### 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: + +- All fields contained in the dpkg database, with the exception of + fields marked as "internal" (see the manpage `dpkg-query (1)`). Among + those fields, the following are mandatory for all package stanzas: + Package, Version, Architecture. + + It is recommended not to pass the Description field to external + solvers or, alternatively, to trim it to the short description only. + +- **Installed:** (optional, defaults to `no`). Allowed values: `yes`, + `no`. When set to `yes`, the corresponding package is currently + installed. + + Note: the Status field present in the dpkg database must not be passed + to the external solver, as it's an internal dpkg field. Installed and + other fields permit one to encode the most relevant aspects of Status + in communications with solvers. + +- **Hold:** (optional, defaults to `no`). Allowed values: `yes`, + `no`. When set to `yes`, the corresponding package is marked as "on + hold" by dpkg. + +- **APT-ID:** (mandatory). Unique package identifier, according to APT. + +- **APT-Pin:** (mandatory). Must be an integer. Package pin value, + according to APT policy. + +- **APT-Candidate:** (optional, defaults to `no`). Allowed values: + `yes`, `no`. When set to `yes`, the corresponding package is the APT + candidate for installation among all available packages with the same + name and architecture. + +- **APT-Automatic:** (optional, defaults to `no`). Allowed values: + `yes`, `no`. When set to `yes`, the corresponding package is marked by + APT as automatic installed. Note that automatic installed packages + should be removed by the solver only when the Autoremove action is + requested (see Request section). + +- **APT-Release:** (optional) The releases the package belongs to, according to + APT. The format of this field is multiline with one value per line and the + first line (the one containing the field name) empty. Each subsequent line + corresponds to one of the releases the package belongs to and looks like + this: `o=Debian,a=unstable,n=sid,l=Debian,c=main`. That is, each release line + is a comma-separated list of "key=value" pairs, each of which denotes a + Release file entry (Origin, Label, Codename, etc.) in the format of + APT_PREFERENCES(5). + +- **Source:** (optional) The name of the source package the binary + package this record is for was built from. + This field does NOT include the version of the source package unlike + the Source field in the dpkg database. The version is optionally + available in the **Source-Version:** field. + + +### Answer + +An answer from the external solver to APT is either a *solution* or an +*error*. + +The following invariant on **exit codes** must hold true. When the +external solver 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 solver 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 solver +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 install +stanza (telling APT to install a specific new package or to upgrade or +downgrade a package to a specific version), a remove stanza (telling APT to +remove one), or an autoremove stanza (telling APT about the *future* +possibility of removing a package using the Autoremove action). + +An **install stanza** starts with an Install field and supports the +following fields: + +- **Install:** (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. + +**Remove stanzas** are similar to install stanzas, but have **Remove** +fields instead of Install fields. + +**Autoremove stanzas** are similar to install stanzas, but have +**Autoremove** fields instead of Install fields. Autoremove stanzas +should be output so that APT can inform the user of which packages they +can now autoremove, as a consequence of the executed action. However, +this protocol makes no assumption on the fact that a subsequent +invocation of an Autoremove action will actually remove the very same +packages indicated by Autoremove stanzas in the former solution. + +A package can't be installed in multiple versions at the same time, so +for each package there can at most one version be selected either for +installation or removal. This especially means that a solver is neither +allowed to represent package upgrades as a remove of the installed +version and the installation of another (the remove is implicit and must +be omitted from the solution) nor is it supported to revert previous +actions in the solution with later actions. APT is allowed to show +warnings and might even misbehave in earlier versions if a solver is +violating this assumption. + +In terms of expressivity, install and remove 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 solvers 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 solver + 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 solver 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 dependency solving process, as declared by the + solver. + +- **Message:** (optional). A textual message, meant to be read by the + APT user, telling what is going on within the dependency solving + (e.g. the current phase of dependency solving, as declared by the + solver). + + +# Future extensions + +Potential future extensions to this protocol, listed in no specific +order, include: + +- fixed error types to identify common failures across solvers and + enable APT to translate error messages +- structured error data to explain failures in terms of packages and + dependencies |