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Here's the newest version. |
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|
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Changes: |
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|
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- added explicit notion of parent directory (missing in previous GLEP |
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but present in implementation), |
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|
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- explicitly named GNU tar format with list of permitted extensions, |
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|
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- changed volume label to 'gpkg-1.txt' file to improve portability; made |
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it explicit version identifier as well, |
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|
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- added info on other package formats to rationale. |
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|
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|
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--- |
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GLEP: 9999 |
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Title: Gentoo binary package container format |
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Author: Michał Górny <mgorny@g.o> |
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Type: Standards Track |
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Status: Draft |
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Version: 1 |
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Created: 2018-11-15 |
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Last-Modified: 2018-11-26 |
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Post-History: 2018-11-17 |
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Content-Type: text/x-rst |
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--- |
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|
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Abstract |
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======== |
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|
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This GLEP proposes a new binary package container format for Gentoo. |
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The current tbz2/XPAK format is shortly described, and its deficiences |
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are explained. Accordingly, the requirements for a new format are set |
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and a gpkg format satisfying them is proposed. The rationale for |
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the design decisions is provided. |
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|
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|
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Motivation |
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========== |
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|
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The current Portage binary package format |
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----------------------------------------- |
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|
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The historical ``.tbz2`` binary package format used by Portage is |
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a concatenation of two distinct formats: header-oriented compressed .tar |
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format (used to hold package files) and trailer-oriented custom XPAK |
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format (used to hold metadata) [#MAN-XPAK]_. The format has already |
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been extended incompatibly twice. |
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|
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The first time, support for storing multiple successive builds of binary |
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package for a single ebuild version has been added. This feature relies |
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on appending additional hyphen, followed by an integer to the package |
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filename. It is disabled by default (preserving backwards |
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compatibility) and controlled by ``binpkg-multi-instance`` feature. |
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|
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The second time, support for additional compression formats has been |
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added. When format other than bzip2 is used, the ``.tbz2`` suffix |
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is replaced by ``.xpak`` and Portage relies on magic bytes to detect |
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compression used. For backwards compatibility, Portage still defaults |
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to using bzip2; compression program can be switched using |
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``BINPKG_COMPRESS`` configuration variable. |
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|
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Additionally, there have been minor changes to the stored metadata |
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and file storage policies. In particular, behavior regarding |
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``INSTALL_MASK``, controllable file compression and stripping has |
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changed over time. |
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|
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|
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The advantages of tbz2/XPAK format |
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---------------------------------- |
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|
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The tbz2/XPAK format used by Portage has three interesting features: |
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|
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1. **Each binary package is fully contained within a single file.** |
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While this might seem unnecessary, it makes it easier for the user |
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to transfer binary packages without having to be concerned about |
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finding all the necessary files to transfer. |
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|
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2. **The binary packages are compatible with regular compressed |
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tarballs, most of the time.** With notable exceptions of historical |
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versions of pbzip2 and the recent zstd compressor, tbz2/XPAK packages |
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can be extracted using regular tar utility with a compressor |
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implementation that discards trailing garbage. |
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|
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3. **The metadata is uncompressed, and can be efficiently accessed |
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without decompressing package contents.** This includes |
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the possibility of rewriting it (e.g. as a result of package moves) |
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without the necessity of repacking the files. |
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|
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|
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Transparency problem with the current binary package format |
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----------------------------------------------------------- |
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|
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Notwithstanding its advantages, the tbz2/XPAK format has a significant |
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design fault that consists of two issues: |
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|
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1. **The XPAK format is a custom binary format with explicit use |
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of binary-encoded file offsets and field lengths.** As such, it is |
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non-trivial to read or edit without specialized tools. Such tools |
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are currently implemented separately from the package manager, |
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as part of the portage-utils toolkit, written in C [#PORTAGE-UTILS]_. |
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|
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2. **The tarball compatibility feature relies on obscure feature of |
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ignoring trailing garbage in compressed files**. While this is |
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implemented consistently in most of the compressors, this feature |
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is not really a part of specification but rather traditional |
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behavior. Given that the original reasons for this no longer apply, |
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new compressor implementations are likely to miss support for this. |
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|
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Both of the issues make the format hard to use without dedicated tools, |
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or when the tools misbehave. This impacts the following scenarios: |
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|
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A. **Using binary packages for system recovery.** In case of serious |
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breakage, it is really preferable that the format depends on as few |
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tools a possible, and especially not on Gentoo-specific tools. |
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|
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B. **Inspecting binary packages in detail exceeding standard package |
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manager facilities.** |
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|
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C. **Modifying binary packages in ways not predicted by the package |
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manager authors.** A real-life example of this is working around |
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broken ``pkg_*`` phases which prevent the package from being |
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installed. |
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|
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|
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OpenPGP extensibility problem |
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----------------------------- |
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|
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There are at least three obvious ways in which the current format could |
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be extended to support OpenPGP signatures, and each of them has its own |
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distinct problem: |
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|
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1. **Adding a detached signature.** This option is non-intrusive but |
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causes the format to no longer be contained in a single file. |
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|
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2. **Wrapping the package in OpenPGP message format.** This would use |
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a standard format and make verification and unpacking relatively |
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easy. However, it would break backwards compatibility and add |
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explicit dependency on OpenPGP implementation in order to unpack |
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the package. |
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|
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3. **Adding OpenPGP signature as extra XPAK member.** This is |
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the clever solution. It implies strengthening the dependency |
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on custom tooling, now additionally necessary to extract |
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the signature and reconstruct the original file to accommodate |
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verification. |
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|
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|
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Goals for a new container format |
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-------------------------------- |
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|
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All of the above considered, the new format should combine |
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the advantages of the existing format and at the same time address its |
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deficiencies whenever possible. Furthermore, since a format replacement |
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is taking place it is worthwhile to consider additional goals that could |
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be satisfied with little change. |
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|
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The following obligatory goals have been set for a replacement format: |
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|
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1. **The packages must remain contained in a single file.** As a matter |
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of user convenience, it should be possible to transfer binary |
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packages without having to use multiple files, and to install them |
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from any location. |
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|
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2. **The file format must be entirely based on common file formats, |
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respecting best practices, with as little customization as necessary |
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to satisfy the requirements.** The format should be transparent |
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enough to let user inspect and manipulate it without special tooling |
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or detailed knowledge. |
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|
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3. **The file format must provide support for OpenPGP signatures.** |
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Preferably, it should use standard OpenPGP message formats. |
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|
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4. **The file format must allow for efficient metadata updates.** |
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In particular, it should be possible to update the metadata without |
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having to recompress package files. |
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|
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Additionally, the following optional goals have been noted: |
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|
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A. **The file format should account for easy recognition both through |
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filename and through contents.** Preferably, it should have distinct |
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features making it possible to detect it via file(1). |
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|
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B. **The file format should provide for partial fetching of binary |
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packages.** It should be possible to easily fetch and read |
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the package metadata without having to download the whole package. |
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|
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C. **The file format should allow for metadata compression.** |
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|
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D. **The file format should make future extensions easily possible |
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without breaking backwards compatibility.** |
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|
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|
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Specification |
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============= |
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|
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The container format |
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-------------------- |
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|
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The gpkg package container is an uncompressed .tar achive whose filename |
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should use ``.gpkg.tar`` suffix. This archive contains the following |
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members, all placed in a single directory whose name matches |
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the basename of the package file, in order: |
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|
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1. The package identifier file ``gpkg-1.txt`` (required). |
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|
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2. A signature for the metadata archive: ``metadata.tar${comp}.sig`` |
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(optional). |
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|
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3. The metadata archive ``metadata.tar${comp}``, optionally compressed |
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(required). |
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|
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4. A signature for the filesystem image archive: |
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``image.tar${comp}.sig`` (optional). |
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|
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5. The filesystem image archive ``image.tar${comp}``, optionally |
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compressed (required). |
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|
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It is recommended that relative order of the archive members is |
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preserved. However, implementations must support archives with members |
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out of order. |
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|
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The container may be extended with additional members in the future. |
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The implementations should ignore unrecognized members and preserve |
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them across package updates. |
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|
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|
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Permitted .tar format features |
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------------------------------ |
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|
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The tar archives should use either the POSIX ustar format or a subset |
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of the GNU format with the following (optional) extensions: |
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|
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- long pathnames and long linknames, |
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|
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- base-256 encoding of large file sizes. |
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|
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Other extensions should be avoided whenever possible. |
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|
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|
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The package identifier file |
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--------------------------- |
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|
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The package identifier file serves the purpose of identifying the binary |
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package format and its version. |
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|
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The implementations must include a package identifier file named |
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``gpkg-1.txt``. The filename includes package format version; |
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implementations should reject packages which do not contain this file |
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as unsupported format. |
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|
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The file can have any contents. Normally, it should be empty. |
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|
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Furthermore, this file should be included in the .tar archive |
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as the first member. This makes it possible to use it as an additional |
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magic at a fixed location that can be used by tools such as file(1) |
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to easily distinguish Gentoo binary packages from regular .tar archives. |
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|
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|
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The metadata archive |
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-------------------- |
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|
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The metadata archive stores the package metadata needed for the package |
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manager to process it. The archive should be included at the beginning |
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of the binary package in order to make it possible to read it out of |
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partially fetched binary package, and to avoid fetching the remaining |
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part of the package if not necessary. |
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|
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The archive contains a single directory called ``metadata``. In this |
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directory, the individual metadata keys are stored as files. The exact |
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keys and metadata format is outside the scope of this specification. |
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|
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The package manager may need to modify the package metadata. In this |
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case, it should replace the metadata archive without having to alter |
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other package members. |
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|
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The metadata archive can optionally be compressed. It can also be |
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supplemented with a detached OpenPGP signature. |
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|
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|
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The image archive |
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----------------- |
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|
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The image archive stores all the files to be installed by the binary |
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package. It should be included as the last of the files in the binary |
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package container. |
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|
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The archive contains a single directory called ``image``. Inside this |
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directory, all package files are stored in filesystem layout, relative |
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to the root directory. |
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|
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The image archive can optionally be compressed. It can also be |
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supplemented with a detached OpenPGP signature. |
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|
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|
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Archive member compression |
298 |
-------------------------- |
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|
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The archive members outlined above support optional compression using |
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one of the compressed file formats supported by the package manager. |
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The exact list of compression types is outside the scope of this |
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specification. |
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|
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The implementations must support archive members being uncompressed, |
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and must support using different compression types for different files. |
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|
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When compressing an archive member, the member filename should be |
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suffixed using the standard suffix for the particular compressed file |
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type (e.g. ``.bz2`` for bzip2 format). |
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|
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|
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OpenPGP member signatures |
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------------------------- |
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|
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The archive members support optional OpenPGP signatures. |
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The implementations must allow the user to specify whether OpenPGP |
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signatures are to be expected in remotely fetched packages. |
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|
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If the signatures are expected and the archive member is unsigned, the |
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package manager must reject processing it. If the signature does not |
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verify, the package manager must reject processing the corresponding |
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archive member. In particular, it must not attempt decompressing |
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compressed members in those circumstances. |
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|
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The signatures are created as binary detached OpenPGP signature files, |
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with filename corresponding to the member filename with ``.sig`` suffix |
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appended. |
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|
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The exact details regarding creating and verifying signatures, as well |
331 |
as maintaining and distributing keys are outside the scope of this |
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specification. |
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|
334 |
|
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Rationale |
336 |
========= |
337 |
|
338 |
Package formats used by other distributions |
339 |
------------------------------------------- |
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|
341 |
The research on the new package format included investigating |
342 |
the possibility of reusing solutions from other operating system |
343 |
distributions. While reusing a foreign package format would be |
344 |
interesting, the differences in Gentoo metadata structure would prevent |
345 |
any real compatibility. Some degree of compatibility might be achieved |
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through adapting the Gentoo metadata, however the costs of such |
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a solution would probably outweigh its usefulness. |
348 |
|
349 |
Debian and its derivates are using the .deb package format. This is |
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a nested archive format, with the outer archive being of ar format, |
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and containing nested tarballs of control information (metadata) |
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and data [#DEB-FORMAT]_. |
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|
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Red Hat, its derivates and some less related distributions are using |
355 |
the RPM format. It is a custom binary format, storing metadata directly |
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and using a trailer cpio archive to store package files. |
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|
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Arch Linux is using xz-compressed tarballs (suffixed ``.pkg.tar.xz``) |
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as its binary package format. The tarballs contain package files |
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on top-level, with specially named dotfiles used for package metadata. |
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OpenPGP signatures are stored as detached ``.sig`` files alongside |
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packages. |
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|
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Exherbo is using the pbins format. In this format, the binary package |
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metadata is stored in repository alike ebuilds, and the binary package |
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files are stored separately and downloaded alike source tarballs. |
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|
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|
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Nested archive format |
370 |
--------------------- |
371 |
|
372 |
The basic problem in designing the new format was how to embed multiple |
373 |
data streams (metadata, image) into a single file. Traditionally, this |
374 |
has been done via using two non-conflicting file formats. However, |
375 |
while such a solution is clever, it suffers in terms of transparency. |
376 |
|
377 |
Therefore, it has been established that the new format should really |
378 |
consist of a single archive format, with all necessary data |
379 |
transparently accessible inside the file. Consequently, it has been |
380 |
debated how different parts of binary package data should be stored |
381 |
inside that archive. |
382 |
|
383 |
The proposal to continue storing image data as top-level data |
384 |
in the package format, and store metadata as special directory in that |
385 |
structure has been discarded as a case of in-band signalling. |
386 |
|
387 |
Finally, the proposal has been shaped to store different kinds of data |
388 |
as nested archives in the outer binary package container. Besides |
389 |
providing a clean way of accessing different kinds of information, it |
390 |
makes it possible to add separate OpenPGP signatures to them. |
391 |
|
392 |
|
393 |
Inner vs. outer compression |
394 |
--------------------------- |
395 |
|
396 |
One of the points in the new format debate was whether the binary |
397 |
package as a whole should be compressed vs. compressing individual |
398 |
members. The first option may seem as an obvious choice, especially |
399 |
given that with a larger data set, the compression may proceed more |
400 |
effectively. However, it has a single strong disadvantage: compression |
401 |
prevents random access and manipulation of the binary package members. |
402 |
|
403 |
While for the purpose of reading binary packages, the problem could be |
404 |
circumvented through convenient member ordering and avoiding disjoint |
405 |
reads of the binary package, metadata updates would either require |
406 |
recompressing the whole package (which could be really time consuming |
407 |
with large packages) or applying complex techniques such as splitting |
408 |
the compressed archive into multiple compressed streams. |
409 |
|
410 |
This considered, the simplest solution is to apply compression to |
411 |
the individual package members, while leaving the container format |
412 |
uncompressed. It provides fast random access to the individual members, |
413 |
as well as capability of updating them without the necessity of |
414 |
recompressing other files in the container. |
415 |
|
416 |
This also makes it possible to easily protect compressed files using |
417 |
standard OpenPGP detached signature format. All this combined, |
418 |
the package manager may perform partial fetch of binary package, verify |
419 |
the signature of its metadata member and process it without having to |
420 |
fetch the potentially-large image part. |
421 |
|
422 |
|
423 |
Container and archive formats |
424 |
----------------------------- |
425 |
|
426 |
During the debate, the actual archive formats to use were considered. |
427 |
The .tar format seemed an obvious choice for the image archive since |
428 |
it is the only widely deployed archive format that stores all kinds |
429 |
of file metadata on POSIX systems. However, multiple options for |
430 |
the outer format has been debated. |
431 |
|
432 |
Firstly, the ZIP format has been proposed as the only commonly supported |
433 |
format supporting adding files from stdin (i.e. making it possible to |
434 |
pipe the inner archives straight into the container without using |
435 |
temporary files). However, this format has been clearly rejected |
436 |
as both not being present in the system set, and being trailer-based |
437 |
and therefore unusable without having to fetch the whole file. |
438 |
|
439 |
Secondly, the ar and cpio formats were considered. The former is used |
440 |
by Debian and its derivative binary packages; the latter is used by Red |
441 |
Hat derivatives. Both formats have the advantage of having less |
442 |
historical baggage than .tar, and having less overhead. However, both |
443 |
are also rather obscure (especially given that ar is actually provided |
444 |
by GNU binutils rather than as a stand-alone archiver), considered |
445 |
obsolete by POSIX and both have file size limitations smaller than .tar. |
446 |
|
447 |
Thirdly, SquashFS was another interesting option. Its main advantage is |
448 |
transparent compression support and ability to mount as a filesystem. |
449 |
However, it has a significant implementation complexity, including mount |
450 |
management and necessity of fallback to unsquashfs. Since the image |
451 |
needs to be writable for the pre-installation manipulations, using it |
452 |
via a mount would additionally require some kind of overlay filesystem. |
453 |
Using it as top-level format has no real gain over a pipeline with tar, |
454 |
and is certainly less portable. Therefore, there does not seem to be |
455 |
a benefit in using SquashFS. |
456 |
|
457 |
All that considered, it has been decided that there is no purpose |
458 |
in using a second archive format in the specification unless it has |
459 |
significant advantage to .tar. Therefore, .tar has also been used |
460 |
as outer package format, even though it has larger overhead than other |
461 |
formats (mostly due to padding). |
462 |
|
463 |
|
464 |
.tar portability issues |
465 |
----------------------- |
466 |
|
467 |
The modern .tar dialects could be considered a dirty extensions |
468 |
of the original .tar format. Three variants may be considered |
469 |
of interest: POSIX ustar, pax (newer POSIX standard) and GNU tar. |
470 |
All three formats are supported by GNU tar, whose presence on systems |
471 |
used to create binary packages could be relied on. Therefore, |
472 |
the portability concerns are related mostly to being able to read |
473 |
and modify binary packages in scenarios of GNU tar being unavailable. |
474 |
|
475 |
For the purpose of this specification, a detailed research |
476 |
on portability of individual tar features has been conducted. |
477 |
The research concluded to: |
478 |
|
479 |
Judging by the test results, the most portability could be |
480 |
achieved by: |
481 |
|
482 |
- using strict POSIX ustar format whenever possible, |
483 |
|
484 |
- using GNU format for long paths (that do not fix in ustar format), |
485 |
|
486 |
- using base-256 (+ pax if already used) encoding for large files, |
487 |
|
488 |
- using pax (+ octal or base-256) for high-range/precision |
489 |
timestamps and user/group identifiers, |
490 |
|
491 |
- using pax attributes for extended metadata and/or volume label. |
492 |
|
493 |
It has been determined that for the purpose of binary package we really |
494 |
only need to be concerned about long paths and huge files. Therefore, |
495 |
the above was limited to the three first points and a guideline was |
496 |
formed from them. |
497 |
|
498 |
Debian has a similar guideline for the inner tar of their package |
499 |
format has been created [#DEB-FORMAT]_. |
500 |
|
501 |
|
502 |
Member ordering |
503 |
--------------- |
504 |
|
505 |
The member ordering is explicitly specified in order to provide for |
506 |
trivially reading metadata from partially fetched archives. |
507 |
By requiring the metadata archive to be stored before the image archive, |
508 |
the package manager may stop fetching after reading it and save |
509 |
bandwidth and/or space. |
510 |
|
511 |
|
512 |
Detached OpenPGP signatures |
513 |
--------------------------- |
514 |
|
515 |
The use of detached OpenPGP signatures is to provide authenticity checks |
516 |
for binary packages. Covering the complete members with signatures |
517 |
provide for trivial verification of all metadata and image contents |
518 |
respectively, without having to invent custom mechanisms for combining |
519 |
them. Covering the compressed archives helps to prevent zipbomb |
520 |
attacks. Covering the individual members rather than the whole package |
521 |
provides for verification of partially fetched binary packages. |
522 |
|
523 |
|
524 |
Format versioning |
525 |
----------------- |
526 |
|
527 |
The format is versioned through an explicit file, with the version |
528 |
stored in the filename. If the format changes incompatible, |
529 |
the filename changes and old implementations do not recognize it |
530 |
as a valid package. |
531 |
|
532 |
Previously, the format tried to avoid an explicit file for this purpose |
533 |
and used volume label instead. However, the use of label has been |
534 |
renounced due to unforeseen portability issues. |
535 |
|
536 |
|
537 |
Backwards Compatibility |
538 |
======================= |
539 |
|
540 |
The format does not preserve backwards compatibility with the tbz2 |
541 |
packages. It has been established that preserving compatibility with |
542 |
the old format was impossible without making the new format even worse |
543 |
than the old one was. |
544 |
|
545 |
For example, adding any visible members to the tarball would cause |
546 |
them to be installed to the filesystem by old Portage versions. Working |
547 |
around this would require some kind of awful hacks that would oppose |
548 |
the goal of using simple and transparent package format. |
549 |
|
550 |
|
551 |
Reference Implementation |
552 |
======================== |
553 |
|
554 |
The proof-of-concept implementation of binary package format converter |
555 |
is available as xpak2gpkg [#XPAK2GPKG]_. It can be used to easily |
556 |
create packages in the new format for early inspection. |
557 |
|
558 |
|
559 |
References |
560 |
========== |
561 |
|
562 |
.. [#MAN-XPAK] xpak - The XPAK Data Format used with Portage binary |
563 |
packages |
564 |
(https://dev.gentoo.org/~zmedico/portage/doc/man/xpak.5.html) |
565 |
|
566 |
.. [#PORTAGE-UTILS] portage-utils: Small and fast Portage helper tools |
567 |
written in C |
568 |
(https://packages.gentoo.org/packages/app-portage/portage-utils) |
569 |
|
570 |
.. [#DEB-FORMAT] deb(5) — Debian binary package format |
571 |
(https://manpages.debian.org/unstable/dpkg-dev/deb.5.en.html) |
572 |
|
573 |
.. [#TAR-PORTABILITY] Michał Górny, Portability of tar features |
574 |
(https://dev.gentoo.org/~mgorny/articles/portability-of-tar-features.html) |
575 |
|
576 |
.. [#XPAK2GPKG] xpak2gpkg: Proof-of-concept converter from tbz2/xpak |
577 |
to gpkg binpkg format |
578 |
(https://github.com/mgorny/xpak2gpkg) |
579 |
|
580 |
|
581 |
Copyright |
582 |
========= |
583 |
This work is licensed under the Creative Commons Attribution-ShareAlike 3.0 |
584 |
Unported License. To view a copy of this license, visit |
585 |
http://creativecommons.org/licenses/by-sa/3.0/. |
586 |
|
587 |
|
588 |
-- |
589 |
Best regards, |
590 |
Michał Górny |