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| -rw-r--r-- | doc/apt-solver-protocol.mdwn | 153 |
1 files changed, 104 insertions, 49 deletions
diff --git a/doc/apt-solver-protocol.mdwn b/doc/apt-solver-protocol.mdwn index b1435102e..85cd98d62 100644 --- a/doc/apt-solver-protocol.mdwn +++ b/doc/apt-solver-protocol.mdwn @@ -1,6 +1,6 @@ ** TENTATIVE PROPOSAL, VERY VERY VERY DRAFT ** -# APT External Dependency Solver Protocol (EDSP) - version 0.1 +# APT External Dependency Solver Protocol (EDSP) - version 0.2 This document describes the communication protocol between APT and external dependency solvers. The protocol is called APT EDSP, for "APT @@ -13,13 +13,18 @@ External Dependency Solver Protocol". - 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. Each external solver is identified by - an unique string (other than `internal`) called the solver **name**. - + 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 @@ -32,8 +37,8 @@ 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 simply -looking at the content of that directory. +that an index of available external solvers can be obtained by listing +the content of that directory. ## Configuration @@ -49,9 +54,10 @@ configuration documentation for more, and more up to date, information. respected (as the internal solver does) or can be slightly deviated from. Defaults to `yes`. -- **APT::Solver::Preferences**: solver-specific user preferences used - during dependency solving. Check your solver documentation for what is - supported here. Default to empty. +- **APT::Solver::NAME::Preferences** (where NAME is a solver name): + solver-specific user preference string used during dependency solving, + when the solver NAME is in use. Check solver-specific documentation + for what is supported here. Defaults to the empty string. ## Protocol @@ -74,13 +80,15 @@ via the file descriptors: **stdin** (standard input) and **stdout** therefore use stderr to dump debugging information that could be inspected separately. -After invocation, the protocol passes through 3 separate phases: +After invocation, the protocol passes through a sequence of phases: -1. APT send to the solver a dependency solving **scenario** -2. The solver solves dependencies. No communication with APT happens - during this phase. -3. The solver sends back to APT an **answer**, i.e. either a *solution* +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 @@ -104,7 +112,7 @@ and followed by a mixture of action and preference 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 0.1`. +be `EDSP 0.2`. a unique request identifier, such as an UUID. Request fields are mainly used to identify the beginning of a @@ -154,32 +162,42 @@ 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 supported by Debian Packages file (see one of the - `/var/lib/apt/lists/*Packages` file for an example), *with the - exception of the Description field* that is not allowed. +- 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. - Among those fields, the following are mandatory: 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, default value `no`). Allowed values: `yes`, +- **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 to encode the most relevant aspects of Status in + communications with solvers. - ##TODO## changed with respect to current prototype, which uses Status +- **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 a non-negative integer. Package pin - value, according to current APT policy. +- **APT-Pin:** (mandatory). Must be an integer. Package pin value, + according to APT policy. -- **APT-Candidate:** (optional, default value `no`). Allowed values: - `yes`, `no`. When set to `yes`, the corresponding package is granted - to have the highest pinning value among all the packages having the - same name. - - ##TODO## what about multi-arch? is the pin value granted to be the - higest also across different architectures? - +- **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. + +- **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). ### Answer @@ -189,7 +207,7 @@ An answer from the external solver to APT is either a *solution* or an 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 aware of that), it +external solver is *unable to find a solution* (and s 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 @@ -198,21 +216,29 @@ user. #### Solution -A solution is a single Deb 822 stanza, starting with the field -Solution. The following fields are supported in solution stanzas: +A solution is a list of Deb 822 stanzas. Each of them is either an +install stanza, telling APT to install a specific package, or a remove +stanza, telling APT to remove one. + +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. -- **Solution:** (mandatory). The value of this field is ignored, - although it should be a unique solution identifier, such as a UUID. +- All fields supported by package stanzas. -- **Install:** (optional, defaults to the empty string). A space - separated list of strings of the form `PACKAGE=VERSION` where - `PACKAGE` is a package name and `VERSION` is an available version of - that package. The list denotes a set of packages that must be - installed to satisfy user request. +**Remove stanzas** are similar to install stanzas, but have **Remove** +fields instead of Install fields. -- **Remove:** (optional, defaults to the empty string). Same as Install, - but denoting a set of packages that must be removed to satisfy user - request. +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 @@ -225,11 +251,40 @@ following fields are supported in error stanzas: - **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. + 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, in RFC 2822 format. 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: - ##TODO## can we support line continuations throughout this format? If - yes, they might come handy both for error stanzas and for solution - stanzas (which might have very long install/remove lines) +- 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 ** TENTATIVE PROPOSAL, VERY VERY VERY DRAFT ** |