hgext/clonebundles.py
author Augie Fackler <augie@google.com>
Mon, 12 Feb 2018 10:21:44 -0500
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parent 32773 d25802b0eef5
child 37498 aacfca6f9767
permissions -rw-r--r--
bdiff: add to clang-format oversight This file didn't require any complex fixes, so we may as well enable clang-format while I'm editing it. Differential Revision: https://phab.mercurial-scm.org/D2177
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# This software may be used and distributed according to the terms of the
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# GNU General Public License version 2 or any later version.
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"""advertise pre-generated bundles to seed clones
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"clonebundles" is a server-side extension used to advertise the existence
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of pre-generated, externally hosted bundle files to clients that are
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cloning so that cloning can be faster, more reliable, and require less
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resources on the server.
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Cloning can be a CPU and I/O intensive operation on servers. Traditionally,
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the server, in response to a client's request to clone, dynamically generates
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a bundle containing the entire repository content and sends it to the client.
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There is no caching on the server and the server will have to redundantly
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generate the same outgoing bundle in response to each clone request. For
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servers with large repositories or with high clone volume, the load from
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clones can make scaling the server challenging and costly.
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This extension provides server operators the ability to offload potentially
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expensive clone load to an external service. Here's how it works.
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1. A server operator establishes a mechanism for making bundle files available
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   on a hosting service where Mercurial clients can fetch them.
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2. A manifest file listing available bundle URLs and some optional metadata
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   is added to the Mercurial repository on the server.
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3. A client initiates a clone against a clone bundles aware server.
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4. The client sees the server is advertising clone bundles and fetches the
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   manifest listing available bundles.
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5. The client filters and sorts the available bundles based on what it
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   supports and prefers.
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6. The client downloads and applies an available bundle from the
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   server-specified URL.
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7. The client reconnects to the original server and performs the equivalent
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   of :hg:`pull` to retrieve all repository data not in the bundle. (The
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   repository could have been updated between when the bundle was created
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   and when the client started the clone.)
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Instead of the server generating full repository bundles for every clone
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request, it generates full bundles once and they are subsequently reused to
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bootstrap new clones. The server may still transfer data at clone time.
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However, this is only data that has been added/changed since the bundle was
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created. For large, established repositories, this can reduce server load for
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clones to less than 1% of original.
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To work, this extension requires the following of server operators:
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* Generating bundle files of repository content (typically periodically,
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  such as once per day).
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* A file server that clients have network access to and that Python knows
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  how to talk to through its normal URL handling facility (typically an
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  HTTP server).
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* A process for keeping the bundles manifest in sync with available bundle
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  files.
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Strictly speaking, using a static file hosting server isn't required: a server
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operator could use a dynamic service for retrieving bundle data. However,
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static file hosting services are simple and scalable and should be sufficient
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for most needs.
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Bundle files can be generated with the :hg:`bundle` command. Typically
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:hg:`bundle --all` is used to produce a bundle of the entire repository.
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:hg:`debugcreatestreamclonebundle` can be used to produce a special
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*streaming clone bundle*. These are bundle files that are extremely efficient
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to produce and consume (read: fast). However, they are larger than
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traditional bundle formats and require that clients support the exact set
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of repository data store formats in use by the repository that created them.
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Typically, a newer server can serve data that is compatible with older clients.
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However, *streaming clone bundles* don't have this guarantee. **Server
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operators need to be aware that newer versions of Mercurial may produce
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streaming clone bundles incompatible with older Mercurial versions.**
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A server operator is responsible for creating a ``.hg/clonebundles.manifest``
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file containing the list of available bundle files suitable for seeding
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clones. If this file does not exist, the repository will not advertise the
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existence of clone bundles when clients connect.
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The manifest file contains a newline (\\n) delimited list of entries.
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Each line in this file defines an available bundle. Lines have the format:
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    <URL> [<key>=<value>[ <key>=<value>]]
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That is, a URL followed by an optional, space-delimited list of key=value
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pairs describing additional properties of this bundle. Both keys and values
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are URI encoded.
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Keys in UPPERCASE are reserved for use by Mercurial and are defined below.
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All non-uppercase keys can be used by site installations. An example use
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for custom properties is to use the *datacenter* attribute to define which
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data center a file is hosted in. Clients could then prefer a server in the
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data center closest to them.
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The following reserved keys are currently defined:
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BUNDLESPEC
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   A "bundle specification" string that describes the type of the bundle.
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   These are string values that are accepted by the "--type" argument of
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   :hg:`bundle`.
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   The values are parsed in strict mode, which means they must be of the
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   "<compression>-<type>" form. See
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   mercurial.exchange.parsebundlespec() for more details.
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   :hg:`debugbundle --spec` can be used to print the bundle specification
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   string for a bundle file. The output of this command can be used verbatim
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   for the value of ``BUNDLESPEC`` (it is already escaped).
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   Clients will automatically filter out specifications that are unknown or
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   unsupported so they won't attempt to download something that likely won't
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   apply.
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   The actual value doesn't impact client behavior beyond filtering:
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   clients will still sniff the bundle type from the header of downloaded
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   files.
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   **Use of this key is highly recommended**, as it allows clients to
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   easily skip unsupported bundles. If this key is not defined, an old
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   client may attempt to apply a bundle that it is incapable of reading.
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REQUIRESNI
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   Whether Server Name Indication (SNI) is required to connect to the URL.
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   SNI allows servers to use multiple certificates on the same IP. It is
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   somewhat common in CDNs and other hosting providers. Older Python
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   versions do not support SNI. Defining this attribute enables clients
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   with older Python versions to filter this entry without experiencing
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   an opaque SSL failure at connection time.
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   If this is defined, it is important to advertise a non-SNI fallback
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   URL or clients running old Python releases may not be able to clone
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   with the clonebundles facility.
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   Value should be "true".
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Manifests can contain multiple entries. Assuming metadata is defined, clients
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will filter entries from the manifest that they don't support. The remaining
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entries are optionally sorted by client preferences
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(``ui.clonebundleprefers`` config option). The client then attempts
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to fetch the bundle at the first URL in the remaining list.
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**Errors when downloading a bundle will fail the entire clone operation:
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clients do not automatically fall back to a traditional clone.** The reason
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for this is that if a server is using clone bundles, it is probably doing so
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because the feature is necessary to help it scale. In other words, there
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is an assumption that clone load will be offloaded to another service and
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that the Mercurial server isn't responsible for serving this clone load.
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If that other service experiences issues and clients start mass falling back to
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the original Mercurial server, the added clone load could overwhelm the server
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due to unexpected load and effectively take it offline. Not having clients
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automatically fall back to cloning from the original server mitigates this
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scenario.
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Because there is no automatic Mercurial server fallback on failure of the
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bundle hosting service, it is important for server operators to view the bundle
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hosting service as an extension of the Mercurial server in terms of
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availability and service level agreements: if the bundle hosting service goes
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down, so does the ability for clients to clone. Note: clients will see a
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message informing them how to bypass the clone bundles facility when a failure
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occurs. So server operators should prepare for some people to follow these
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instructions when a failure occurs, thus driving more load to the original
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Mercurial server when the bundle hosting service fails.
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"""
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from __future__ import absolute_import
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from mercurial import (
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    extensions,
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    wireproto,
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)
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testedwith = 'ships-with-hg-core'
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def capabilities(orig, repo, proto):
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    caps = orig(repo, proto)
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    # Only advertise if a manifest exists. This does add some I/O to requests.
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    # But this should be cheaper than a wasted network round trip due to
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    # missing file.
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    if repo.vfs.exists('clonebundles.manifest'):
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        caps.append('clonebundles')
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    return caps
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def extsetup(ui):
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    extensions.wrapfunction(wireproto, '_capabilities', capabilities)