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diff --git a/doc/acquire-additional-files.txt b/doc/acquire-additional-files.txt new file mode 100644 index 000000000..5a07c2bec --- /dev/null +++ b/doc/acquire-additional-files.txt @@ -0,0 +1,142 @@ +# 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 frontends 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 frontend +might want to use and would therefore need to be downloaded as well +(e.g. apt-file). + +This file describes the configuration scheme such a frontend can use to +instruct the Acquire system to download those additional files. + +Note that you can't download files from other sources. It must be files +in the same repository and below the Release file. The Release file must +also contain hashes for the file itself as well as for the compressed +file if wanted, otherwise a download isn't even tried! + + +# 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): + + APT::Acquire::Targets::deb::Packages { + URI "$(COMPONENT)/binary-$(ARCHITECTURE)/Packages"; + ShortDescription "Packages"; + Description "$(SITE) $(RELEASE)/$(COMPONENT) $(ARCHITECTURE) Packages"; + + flatURI "Packages"; + flatDescription "$(SITE) $(RELEASE) Packages"; + + Optional "false"; + }; + +All files which should be downloaded (nicknamed 'Targets') are mentioned +below the APT::Acquire::Targets 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 URI for 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 never shown or used. + +All targets have three main properties you can define: +* URI: 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 should you try to 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. + +Additional optional properties: +* flat{URI,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 repositories + 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. +* Optional: The default value is 'true' 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 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 examples + +The stanzas for Translation-* files as well as for Sources files would +look like this: + +APT::Acquire::Targets { + deb::Translations { + URI "$(COMPONENT)/i18n/Translation-$(LANGUAGE)"; + ShortDescription "Translation-$(LANGUAGE)"; + Description "$(SITE) $(RELEASE)/$(COMPONENT) Translation-$(LANGUAGE)"; + + flatURI "$(LANGUAGE)"; + flatDescription "$(SITE) $(RELEASE) Translation-$(LANGUAGE)"; + }; + + deb-src::Sources { + URI "$(COMPONENT)/source/Sources"; + ShortDescription "Sources"; + Description "$(SITE) $(RELEASE)/$(COMPONENT) Sources"; + + flatURI "Sources"; + flatDescription "$(SITE) $(RELEASE) Sources"; + + Optional "false"; + }; +}; + +# 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 literally. + +* $(SITE): An identifier of the site we access, e.g. + "http://example.org/". +* $(RELEASE): This is usually an archive- or codename, e.g. "stable" or + "stretch". Note that flat-style repositories do not have a 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". + +These values are defined both for 'deb' as well as 'deb-src' targets. +'deb' targets additional have the variable: + +* $(ARCHITECTURE): Values are all entries of configuration option + APT::Architectures (potentially modified by sources.list options), + e.g. "amd64", "i386" or "armel". |