Adding upstream version 2.7.12.upstream/2.7.12

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+# 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 file formats
+  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!
+* `flatMetaKey`, `flatDescription`: 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.