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diff --git a/doc/acquire-additional-files.txt b/doc/acquire-additional-files.txt new file mode 100644 index 000000000..c55d5fe19 --- /dev/null +++ b/doc/acquire-additional-files.txt @@ -0,0 +1,319 @@ +# Acquire additional files in 'update' operations + +The download and verification of data from multiple sources in different +compression formats, with partial downloads and patches is an involved +process which is hard to implement correctly and securely. + +APT front-ends share the code and binaries to make this happen in libapt +with the Acquire system, supported by helpers shipped in the apt package +itself and additional transports in individual packages like +apt-transport-https. + +For its own operation libapt needs or can make use of Packages, Sources +and Translation-* files, which it will acquire by default, but +a repository might contain more data files (e.g. Contents) a front-end +(e.g. apt-file) might want to use and would therefore need to be +downloaded as well. + +This file describes the configuration scheme such a front-end can use to +instruct the Acquire system to download those additional files. + +# The Configuration Stanza + +The Acquire system uses the same configuration settings to implement the +files it downloads by default. These settings are the default, but if +they would be written in a configuration file the configuration +instructing the Acquire system to download the Packages files would look +like this (see also apt.conf(5) manpage for configuration file syntax): + + Acquire::IndexTargets::deb::Packages { + MetaKey "$(COMPONENT)/binary-$(ARCHITECTURE)/Packages"; + ShortDescription "Packages"; + Description "$(RELEASE)/$(COMPONENT) $(ARCHITECTURE) Packages"; + + flatMetaKey "Packages"; + flatDescription "$(RELEASE) Packages"; + + Optional "no"; + }; + +All files which should be downloaded (nicknamed 'Targets') are mentioned +below the Acquire::IndexTargets scope. 'deb' is here the type of the +sources.list entry the file should be acquired for. The only other +supported value is hence 'deb-src'. Beware: You can't specify multiple +types here and you can't download the same (evaluated) MetaKey from +multiple types! + +After the type you can pick any valid and unique string which preferable +refers to the file it downloads (In the example we picked 'Packages'). +This string is used as identifier (if not explicitly set otherwise) for +the target class and accessible as 'Identifier' and 'Created-By' e.g. +in the "apt-get indextargets" output as detailed below. The identifier +is also used to allow user to enable/disable targets per sources.list +entry. + +All targets have three main properties you can define: +* MetaKey: The identifier of the file to be downloaded as used in the + Release file. It is also the relative location of the file from the + Release file. You can neither download from a different server + entirely (absolute URI) nor access directories above the Release file + (e.g. "../../"). +* ShortDescription: Very short string intended to be displayed to the + user e.g. while reporting progress. apt will e.g. use this string in + the last line to indicate progress of e.g. the download of a specific + item. +* Description: A preferable human understandable and readable identifier + of which file is acquired exactly. Mainly used for progress reporting + and error messages. apt will e.g. use this string in the Get/Hit/Err + progress lines. + An identifier of the site accessed as seen in the sources.list (e.g. + "http://example.org/debian" or "file:/path/to/a/repository") is + automatically prefixed for this property. + + +Additional optional properties: +* Identifier: The default value is the unique string identifying this + file (in the example above it was 'Packages') also accessible as + Created-By. The difference is that using this property multiple files + can be subsumed under one identifier e.g. if you configure multiple + possible locations for the files (with Fallback-Of), but the front-end + doesn't need to handle files from the different locations differently. +* DefaultEnabled: The default value is 'yes' which means that apt will + try to acquire this target from all sources. If set to 'no' the user + has to explicitly enable this target in the sources.list file with the + Targets option(s) – or override this value in a config file. +* Optional: The default value is 'yes' and should be kept at this value. + If enabled the acquire system will skip the download if the file isn't + mentioned in the Release file. Otherwise this is treated as a hard + error and the update process fails. Note that failures while + downloading (e.g. 404 or hash verification errors) are failures, + regardless of this setting. +* KeepCompressed: The default is the value of Acquire::GzipIndexes, + which defaults to false. If true, the acquire system will keep the + file compressed on disk rather than extract it. If your front-end can't + deal with compressed files transparently you have to explicitly set + this option to false to avoid problems with users setting the option + globally. On the other hand, if you set it to true or don't set it you + have to ensure your front-end can deal with all compressed fileformats + supported by apt (libapt users can e.g. use FileFd, others can use + the cat-file command of /usr/lib/apt/apt-helper). +* Fallback-Of: Is by default not set. If it is set and specifies another + target name (see Created-By) which was found in the Release file the + download of this target will be skipped. This can be used to implement + fallback(chain)s to allow transitions like the rename of target files. + The behavior if cycles are formed with Fallback-Of is undefined! +* flat{MetaKey,Description}: APT supports two types of repositories: + dists-style repositories which are the default and by far the most + common which are named after the fact that the files are in an + elaborated directory structure. In contrast a flat-style repository + lumps all files together in one directory. Support for these flat + repositories exists mainly for legacy purposes only. It is therefore + recommend to not set these values. + + +The acquire system will automatically choose to download a compressed +file if it is available and uncompress it for you, just as it will also +use PDiff patching if provided by the repository and enabled by the +user. You only have to ensure that the Release file contains the +information about the compressed files/PDiffs to make this happen. +*NO* properties have to be set to enable this! + + +More properties exist, but these should *NOT* be set by front-ends +requesting files. They exist for internal and end-user usage only. +Some of these are – which are documented here only to ensure that they +aren't accidentally used by front-ends: +* PDiffs: controls if apt will try to use PDiffs for this target. + Defaults to the value of Acquire::PDiffs which is true by default. + Can be overridden per-source by the sources.list option of the same + name. See the documentation for both of these for details. +* By-Hash: controls if apt will try to use an URI constructed from + a hashsum of the file to download. See the documentation for config + option Acquire::By-Hash and sources.list option By-Hash for details. +* CompressionTypes: The default value is a space separated list of + compression types supported by apt (see Acquire::CompressionTypes). + You can set this option to prevent apt from downloading a compression + type a front-end can't open transparently. This should always be + a temporary workaround through and a bug should be reported against + the front-end in question. +* KeepCompressedAs: The default value is a space separated list of + compression types supported by apt (see previous option) which is + sorted by the cost-value of the compression in ascending order, + except that cost=0 "compressions" (like uncompressed) are listed last. + + +# More examples + +The stanzas for Translation-* files as well as for Sources files would +look like this: + +Acquire::IndexTargets { + deb::Translations { + MetaKey "$(COMPONENT)/i18n/Translation-$(LANGUAGE)"; + ShortDescription "Translation-$(LANGUAGE)"; + Description "$(RELEASE)/$(COMPONENT) Translation-$(LANGUAGE)"; + + flatMetaKey "$(LANGUAGE)"; + flatDescription "$(RELEASE) Translation-$(LANGUAGE)"; + }; + + deb-src::Sources { + MetaKey "$(COMPONENT)/source/Sources"; + ShortDescription "Sources"; + Description "$(RELEASE)/$(COMPONENT) Sources"; + + flatMetaKey "Sources"; + flatDescription "$(RELEASE) Sources"; + + Optional "no"; + }; +}; + +# Substitution variables + +As seen in the examples, properties can contain placeholders filled in +by the acquire system. The following variables are known; note that +unknown variables have no default value nor are they touched: They are +printed as-is. + +* $(RELEASE): This is usually an archive- or codename, e.g. "stable" or + "stretch". Note that flat-style repositories do not have an archive- + or codename per-se, so the value might very well be just "/" or so. +* $(COMPONENT): as given in the sources.list, e.g. "main", "non-free" or + "universe". Note that flat-style repositories again do not really + have a meaningful value here. +* $(LANGUAGE): Values are all entries (expect "none") of configuration + option Acquire::Languages, e.g. "en", "de" or "de_AT". +* $(ARCHITECTURE): Values are all entries of configuration option + APT::Architectures (potentially modified by sources.list options), + e.g. "amd64", "i386" or "armel" for the 'deb' type. In type 'deb-src' + this variable has the value "source". +* $(NATIVE_ARCHITECTURE): The architecture apt treats as the native + architecture for this system configured as APT::Architecture + defaulting to the architecture apt itself was built for. + +Note that while more variables might exist in the implementation, these +are to be considered undefined and their usage strongly discouraged. If +you have a need for other variables contact us. + +# Accessing files + +Do NOT hardcode specific file locations, names or compression types in +your application! You will notice that the configuration options give +you no choice over where the downloaded files will be stored. This is by +design so multiple applications can download and use the same file +rather than each and every one of them potentially downloads and uses +its own copy somewhere on disk. + +"apt-get indextargets" can be used to get the location as well as other +information about all files downloaded (aka: you will see Packages, +Sources and Translation-* files here as well). Provide a line of the +default output format as parameter to filter out all entries which do +not have such a line. With --format, you can further more define your +own output style. The variables are what you see in the output, just all +uppercase and wrapped in $(), as in the configuration file. + +To get all the filenames of all Translation-en files you can e.g. call: + apt-get indextargets --format '$(FILENAME)' "Identifier: Translations" "Language: en" + +The line-based filtering and the formatting is rather crude and feature- +less by design: The default format is Debian's standard format deb822 +(in particular: Field names are case-insensitive and the order of fields +in the stanza is undefined), so instead of apt reimplementing powerful +filters and formatting for this command, it is recommend to use piping +and dedicated tools like 'grep-dctrl' if you need more than the basics +provided. + +Accessing this information via libapt is done by reading the +sources.lists (pkgSourceList), iterating over the metaIndex objects this +creates and calling GetIndexTargets() on them. See the source code of +"apt-get indextargets" for a complete example. + +Note that by default targets are not listed if they weren't downloaded. +If you want to see all targets, you can use the --no-release-info, which +also removes the Codename, Suite, Version, Origin, Label and Trusted +fields from the output as these also display data which needs to be +downloaded first and could hence be inaccurate [on the pro-side: This +mode is faster as it doesn't require a valid binary cache to operate]. +The most notable difference perhaps is in the Filename field through: By +default it indicates an existing file, potentially compressed (Hint: +libapt users can use FileFd to open compressed files transparently). In +the --no-release-info mode the indicated file doesn't need to exist and +it will always refer to an uncompressed file, even if the index would be +(or is) stored compressed. + +Remarks on fields only available in (default) --release-info mode: +* Trusted: Denotes with a 'yes' or 'no' if the data in this file is + authenticated by a trust chain rooted in a trusted gpg key. You should + be careful with untrusted data and warn the user if you use it. +* Codename, Suite, Version, Origin and Label are fields from the Release + file, are only present if they are present in the Release file and + contain the same data. + +Remarks on other available fields: +* MetaKey, ShortDesc, Description, Site, Release: as defined + by the configuration and described further above. +* Identifier: Defaults to the value of Created-By, but can be set + explicitly in the configuration (see above). Prefer this field over + Created-By to subsume multiple file(location)s (see Fallback-Of). +* Created-By: configuration entity responsible for this target +* Target-Of: type of the sources.list entry +* URI, Repo-URI: avoid using. Contains potentially username/password. + Prefer 'Site', especially for display. +* Optional, DefaultEnabled, KeepCompressed: Decode the options of the + same name from the configuration. +* Language, Architecture, Component: as defined further above, but with + the catch that they might be missing if they don't effect the target + (aka: They weren't used while evaluating the MetaKey template). + +Again, additional fields might be visible in certain implementations, +but you should avoid using them and instead talk to us about a portable +implementation. + +# Multiple applications requiring the same files + +It is highly encouraged that applications talk to each other and to us +about which files they require. It is usually best to have a common +package ship the configuration needed to get the files, but specific +needs might require specific solutions. Again: talk to us. + +Bad things will happen if multiple front-ends request the same file(s) +via different targets, which is another reason why coordination is very +important! + +# Acquiring files not mentioned in the Release file + +You can't. This is by design as these files couldn't be verified to not +be modified in transit, corrupted by the download process or simple if +they are present at all on the server, which would require apt to probe +for them. APT did this in the past for legacy reasons, we do not intend +to go back to these dark times. + +This is also why you can't request files from a different server. It +would have the additional problem that this server might not even be +accessible (e.g. proxy settings) or that local sources (file:/, cdrom:/) +start requesting online files… + +In other words: We would be opening Pandora's box. + +# Acquiring files to a specific location on disk + +You can't by design to avoid multiple front-ends requesting the same file +to be downloaded to multiple different places on (different) disks +(among other reasons). See the next point for a solution if you really +have to force a specific location by creating symlinks. + +# Post processing the acquired files + +You can't modify the files apt has downloaded as apt keeps state with +e.g. the modification times of the files and advanced features like +PDiffs break. + +You can however install an APT::Update::Post-Invoke{-Success,} hook +script and use them to copy (modified) files to a different location. +Use 'apt-get indextargets' (or similar) to get the filenames – do not +look into /var/lib/apt/lists directly! + +Please avoid time consuming calculations in the scripts and instead just +trigger a background task as there is little to no feedback for the user +while hook scripts run. |