Add PEP 438: Transitioning to release-file hosting on PyPI, submitted by Holger Krekel.
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PEP: 438
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Title: Transitioning to release-file hosting on PyPI
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Version: $Revision$
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Last-Modified: $Date$
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Author: Holger Krekel <holger@merlinux.eu>, Carl Meyer <carl@oddbird.net>
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Discussions-To: catalog-sig@python.org
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Status: Draft
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Type: Process
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Content-Type: text/x-rst
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Created: 15-Mar-2013
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Post-History:
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Abstract
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========
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This PEP proposes a backward-compatible two-phase transition process
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to speed up, simplify and robustify installing from the
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pypi.python.org (PyPI) package index. To ease the transition and
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minimize client-side friction, **no changes to distutils or existing
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installation tools are required in order to benefit from the first
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transition phase, which will result in faster, more reliable installs
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for most existing packages**.
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The first transition phase implements an easy and explicit means for a
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package maintainer to control which release file links are served to
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present-day installation tools. The first phase also includes the
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implementation of analysis tools for present-day packages, to support
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communication with package maintainers and the automated setting of
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default modes for controlling release file links. The first phase
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also will default newly-registered projects on PyPI to only serve
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links to release files which were uploaded to PyPI.
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The second transition phase concerns end-user installation tools,
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which shall default to only install release files that are hosted on
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PyPI and tell the user if external release files exist, offering a
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choice to automatically use those external files.
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Rationale
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=========
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.. _history:
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History and motivations for external hosting
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--------------------------------------------
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When PyPI went online, it offered release registration but had no
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facility to host release files itself. When hosting was added, no
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automated downloading tool existed yet. When Philip Eby implemented
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automated downloading (through setuptools), he made the choice to
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allow people to use download hosts of their choice. The finding of
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externally-hosted packages was implemented as follows:
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#. The PyPI ``simple/`` index for a package contains all links found
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by scraping them from that package's long_description metadata for
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any release. Links in the "Download-URL" and "Home-page" metadata
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fields are given ``rel=download`` and ``rel=homepage`` attributes,
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respectively.
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#. Any of these links whose target is a file whose name appears to be
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in the form of an installable source or binary distribution, with
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name in the form "packagename-version.ARCHIVEEXT", is considered a
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potential installation candidate by installation tools.
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#. Similarly, any links suffixed with an "#egg=packagename-version"
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fragment are considered an installation candidate.
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#. Additionally, the ``rel=homepage`` and ``rel=download`` links are
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crawled by installation tools and, if HTML, are themselves scraped
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for release-file links in the above formats.
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Today, most packages released on PyPI host their release files on
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PyPI, but a small percentage (XXX need updated data) rely on external
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hosting.
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There are many reasons [2]_ why people have chosen external
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hosting. To cite just a few:
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- release processes and scripts have been developed already and upload
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to external sites
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- it takes too long to upload large files from some places in the
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world
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- export restrictions e.g. for crypto-related software
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- company policies which require offering open source packages through
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own sites
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- problems with integrating uploading to PyPI into one's release
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process (because of release policies)
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- desiring download statistics different from those maintained by PyPI
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- perceived bad reliability of PyPI
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- not aware that PyPI offers file-hosting
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Irrespective of the present-day validity of these reasons, there
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clearly is a history why people choose to host files externally and it
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even was for some time the only way you could do things. This PEP
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takes the position that there are at least some valid reasons for
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external hosting.
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Problem
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-------
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**Today, python package installers (pip, easy_install, buildout, and
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others) often need to query many non-PyPI URLs even if there are no
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externally hosted files**. Apart from querying pypi.python.org's
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simple index pages, also all homepages and download pages ever
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specified with any release of a package are crawled by an installer.
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The need for installers to crawl external sites slows down
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installation and makes for a brittle and unreliable installation
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process. Those sites and packages also don't take part in the
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:pep:`381` mirroring infrastructure, further decreasing reliability
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and speed of automated installation processes around the world.
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Most packages are hosted directly on pypi.python.org [1]_. Even for
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these packages, installers still crawl their homepage and
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download-url, if specified. Many package uploaders are not aware that
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specifying the "homepage" or "download-url" in their package metadata
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will needlessly slow down the installation process for all users.
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Relying on third party sites also opens up more attack vectors for
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injecting malicious packages into sites using automated installs. A
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simple attack might just involve getting hold of an old now-unused
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homepage domain and placing malicious packages there. Moreover,
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performing a Man-in-The-Middle (MITM) attack between an installation
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site and any of the download sites can inject malicious packages on
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the installation site. As many homepages and download locations are
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using HTTP and not HTTPS, such attacks are not hard to launch. Such
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MITM attacks can easily happen even for packages which never intended
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to host files externally as their homepages are contacted by
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installers anyway.
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There is currently no way for package maintainers to avoid
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external-link crawling, other than removing all homepage/download url
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metadata for all historic releases. While a script [3]_ has been
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written to perform this action, it is not a good general solution
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because it removes useful metadata from PyPI releases.
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Even if the sites referenced by "Homepage" and "Download-URL" links
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were not scraped for further links, there is no obvious way under the
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current system for a package owner to link to an installable file from
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a long_description metadata field (which is shown as package
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documentation on ``/pypi/PKG``) without installation tools
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automatically considering that file a candidate for installation.
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Conversely, there is no way to explicitly register multiple external
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release files without putting them in metadata fields.
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Goals
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-----
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These are the goals to be achieved by implementation of this PEP:
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* Package owners should be able to explicitly control which files are
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presented by PyPI to installer tools as installation
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candidates. Installation should not be slowed and made less reliable
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by extensive and unnecessary crawling of links that package owners
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did not explicitly nominate as installation files.
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* It should remain possible for package owners to choose to host their
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release files on their own hosting, external to PyPI. It should be
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easy for a user to request the installation of such releases using
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automated installer tools.
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* Automated installer tools should not install externally-hosted
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packages **by default**, but only when explicitly authorized to do
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so by the user. When tools refuse to install such a package by
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default, they should tell the user exactly which external link(s)
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they would need to follow, and what option(s) the user can provide
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to authorize the tool to follow those links. PyPI should provide all
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necessary metadata for installer tools to implement this easily and
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within a single request/reply interaction.
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* Migration from the status quo to the above points should be gradual
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and minimize breakage. This includes tooling that makes it easy for
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package owners with an existing release process that uploads to
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non-PyPI hosting to also upload those release files to PyPI.
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Solution / two transition phases
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================================
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The first transition phase introduces a "hosting-mode" field for each
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project on PyPI, allowing package owners explicit control of which
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release file links are served to present-day installation tools in the
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machine-readable ``simple/`` index. The first transition will, after
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successful hosting-mode manipulations by individual early-adopters,
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set a default hosting mode for existing packages, based on automated
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analysis. **Maintainers will be notified one month ahead of any such
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automated change**. At completion of the first transition phase,
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**all present-day existing release and installation processes and
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tools are expected to continue working**. Any remaining errors or
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problems are expected to only relate to installation of individual
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packages and can be easily corrected by package maintainers or PyPI
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admins if maintainers are not reachable.
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Also in the first phase, each link served in the ``simple/`` index
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will be explicitly marked as ``rel="internal"`` (hosted by the index
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itself) or ``rel="external"`` (linking to an external site that is not
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part of the index).
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In the second transition phase, PyPI client installation tools shall
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be updated to default to only install ``rel="internal"`` packages
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unless a user specifies option(s) to permit installing from external
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links.
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Maintainers of packages which currently host release files on non-PyPI
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sites shall receive instructions and tools to ease "re-hosting" of
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their historic and future package release files. This re-hosting tool
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MUST be available before automated hosting-mode changes are announced
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to package maintainers.
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Implementation
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==============
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Hosting modes
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-------------
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The foundation of the first transition phase is the introduction of
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three "modes" of PyPI hosting for a package, affecting which links are
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generated for the ``simple/`` index. These modes are implemented
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without requiring changes to installation tools via changes to the
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algorithm for generating the machine-readable ``simple/`` index.
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The modes are:
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- ``pypi-scrape-crawl``: no change from the current situation of
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generating machine-readable links for installation tools, as
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outlined in the history_.
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- ``pypi-scrape``: for a package in this mode, links to be added to
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the ``simple/`` index are still scraped from package
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metadata. However, the "Home-page" and "Download-url" links are
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given ``rel=ext-homepage`` and ``rel=ext-download`` attributes
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instead of ``rel=homepage`` and ``rel=download``. The effect of this
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(with no change in installation tools necessary) is that these links
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will not be followed and scraped for further candidate links by
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present-day installation tools: only installable files directly
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hosted from PyPI or linked directly from PyPI metadata will be
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considered for installation. Installation tools MAY evolve to offer
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an option to use the new rel-attribution to crawl external pages but
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MUST NOT default to it.
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- ``pypi-explicit``: for a package in this mode, only links to release
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files uploaded to PyPI, and external links to release files
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explicitly nominated by the package owner (via a new interface
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exposed by PyPI) will be added to the ``simple/`` index.
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Thus the hope is that eventually all projects on PyPI can be migrated
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to the ``pypi-explicit`` mode, while preserving the ability to install
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release files hosted externally via installer tools. Deprecation of
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hosting modes to eventually only allow the ``pypi-explicit`` mode is
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NOT REGULATED by this PEP but is expected to become feasible some time
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after successful implementation of the transition phases described in
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this PEP. It is expected that deprecation requires **a new process to
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deal with abandoned packages** because of unreachable maintainers for
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still popular packages.
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First transition phase (PyPI)
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-----------------------------
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The proposed solution consists of multiple implementation and
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communication steps:
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#. Implement in PyPI the three modes described above, with an
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interface for package owners to select the mode for each package
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and register explicit external file URLs.
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#. For packages in all modes, label all links in the ``simple/`` index
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with ``rel="internal"`` or ``rel="external"``, to make it easier
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for client tools to distinguish the types of links in the second
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transition phase.
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#. Default all newly-registered packages to ``pypi-explicit`` mode
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(package owners can still switch to the other modes as desired).
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#. Determine (via an automated analysis tool) which packages have all
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installable files available on PyPI itself (group A), which have
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all installable files linked directly from PyPI metadata (group B),
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and which have installable versions available that are linked only
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from external homepage/download HTML pages (group C).
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#. Send mail to maintainers of projects in group A that their project
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will be automatically configured to ``pypi-explicit`` mode in one
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month, and similarly to maintainers of projects in group B that
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their project will be automatically configured to ``pypi-scrape``
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mode. Inform them that this change is not expected to affect
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installability of their project at all, but will result in faster
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and safer installs for their users. Encourage them to set this
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mode themselves sooner to benefit their users.
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#. Send mail to maintainers of packages in group C that their package
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hosting mode is ``pypi-scrape-crawl``, list the URLs which
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currently are crawled, and suggest that they either re-host their
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packages directly on PyPI and switch to ``pypi-explicit``, or at
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least provide direct links to release files in PyPI metadata and
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switch to ``pypi-scrape``. Provide instructions and tools to help
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with these transitions.
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Second transition phase (installer tools)
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-----------------------------------------
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For the second transition phase, maintainers of installation tools are
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asked to release two updates.
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The first update shall provide clear warnings if externally-hosted
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release files (that is, files whose link is ``rel="external"``) are
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selected for download, for which projects and URLs exactly this
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happens, and warn that in future versions externally-hosted downloads
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will be disabled by default.
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The second update should change the default mode to allow only
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installation of ``rel="internal"`` package files, and allow
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installation of externally-hosted packages only when the user supplies
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an option (ideally an option specifying exactly which external domains
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are to be trusted as download sources). When download of an
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externally-hosted package is disallowed, the user should be notified,
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with instructions for how to make the install succeed and warnings
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about the implication (that a file will be downloaded from a site that
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is not part of the package index).
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Open Questions / tasks
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======================
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- Should we introduce some form of PyPI API versioning in this PEP?
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(it might complicate matters and delay the implementation but is
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often seen as good practise).
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- Do another round of discussions with installation tool authors and
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see about incorporating their feedback. There is one known issue in
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particular from Philip J. Eby who considers a host-based pattern
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matching algorithm preferable to interpreting "rel" attributes.
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References
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==========
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.. [1] Donald Stufft, ratio of externally hosted versus pypi-hosted,
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http://mail.python.org/pipermail/catalog-sig/2013-March/005549.html
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(XXX need to update this data for all easy_install-supported formats)
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.. [2] Marc-Andre Lemburg, reasons for external hosting,
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http://mail.python.org/pipermail/catalog-sig/2013-March/005626.html
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.. [3] Holger Krekel, script to remove homepage/download metadata for
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all releases
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http://mail.python.org/pipermail/catalog-sig/2013-February/005423.html
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Acknowledgments
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===============
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Philip Eby for precise information and the basic ideas to implement
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the transition via server-side changes only.
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Donald Stufft for pushing away from external hosting and offering to
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implement both a Pull Request for the necessary PyPI changes and the
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analysis tool to drive the transition phase 1.
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Marc-Andre Lemburg, Nick Coghlan and catalog-sig in general for
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thinking through issues regarding getting rid of "external hosting".
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Copyright
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=========
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This document has been placed in the public domain.
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..
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Local Variables:
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||||||
|
mode: indented-text
|
||||||
|
indent-tabs-mode: nil
|
||||||
|
sentence-end-double-space: t
|
||||||
|
fill-column: 70
|
||||||
|
coding: utf-8
|
||||||
|
End:
|
||||||
Loading…
Reference in New Issue