# This software may be used and distributed according to the terms of the

# GNU General Public License version 2 or any later version.

"clonebundles" is a server-side extension used to advertise the existence

of pre-generated, externally hosted bundle files to clients that are

cloning so that cloning can be faster, more reliable, and require less

resources on the server.

Cloning can be a CPU and I/O intensive operation on servers. Traditionally,

the server, in response to a client's request to clone, dynamically generates

a bundle containing the entire repository content and sends it to the client.

There is no caching on the server and the server will have to redundantly

generate the same outgoing bundle in response to each clone request. For

servers with large repositories or with high clone volume, the load from

clones can make scaling the server challenging and costly.

This extension provides server operators the ability to offload potentially

expensive clone load to an external service. Here's how it works.

1. A server operator establishes a mechanism for making bundle files available

   on a hosting service where Mercurial clients can fetch them.

2. A manifest file listing available bundle URLs and some optional metadata

   is added to the Mercurial repository on the server.

3. A client initiates a clone against a clone bundles aware server.

4. The client sees the server is advertising clone bundles and fetches the

   manifest listing available bundles.

5. The client filters and sorts the available bundles based on what it

   supports and prefers.

6. The client downloads and applies an available bundle from the

   server-specified URL.

7. The client reconnects to the original server and performs the equivalent

   of :hg:`pull` to retrieve all repository data not in the bundle. (The

   repository could have been updated between when the bundle was created

   and when the client started the clone.)

Instead of the server generating full repository bundles for every clone

request, it generates full bundles once and they are subsequently reused to

bootstrap new clones. The server may still transfer data at clone time.

However, this is only data that has been added/changed since the bundle was

created. For large, established repositories, this can reduce server load for

clones to less than 1% of original.

To work, this extension requires the following of server operators:

* Generating bundle files of repository content (typically periodically,

  such as once per day).

* A file server that clients have network access to and that Python knows

  how to talk to through its normal URL handling facility (typically an

  HTTP server).

* A process for keeping the bundles manifest in sync with available bundle

  files.

Strictly speaking, using a static file hosting server isn't required: a server

operator could use a dynamic service for retrieving bundle data. However,

static file hosting services are simple and scalable and should be sufficient

for most needs.

Bundle files can be generated with the :hg:`bundle` command. Typically

:hg:`bundle --all` is used to produce a bundle of the entire repository.

:hg:`debugcreatestreamclonebundle` can be used to produce a special

*streaming clone bundle*. These are bundle files that are extremely efficient

to produce and consume (read: fast). However, they are larger than

traditional bundle formats and require that clients support the exact set

of repository data store formats in use by the repository that created them.

Typically, a newer server can serve data that is compatible with older clients.

However, *streaming clone bundles* don't have this guarantee. **Server

operators need to be aware that newer versions of Mercurial may produce

streaming clone bundles incompatible with older Mercurial versions.**

The list of requirements printed by :hg:`debugcreatestreamclonebundle` should

be specified in the ``requirements`` parameter of the *bundle specification

string* for the ``BUNDLESPEC`` manifest property described below. e.g.

``BUNDLESPEC=none-packed1;requirements%3Drevlogv1``.

A server operator is responsible for creating a ``.hg/clonebundles.manifest``

file containing the list of available bundle files suitable for seeding

clones. If this file does not exist, the repository will not advertise the

existence of clone bundles when clients connect.

The manifest file contains a newline (\n) delimited list of entries.

Each line in this file defines an available bundle. Lines have the format:

    <URL> [<key>=<value>[ <key>=<value>]]

That is, a URL followed by an optional, space-delimited list of key=value

pairs describing additional properties of this bundle. Both keys and values

are URI encoded.

Keys in UPPERCASE are reserved for use by Mercurial and are defined below.

All non-uppercase keys can be used by site installations. An example use

for custom properties is to use the *datacenter* attribute to define which

data center a file is hosted in. Clients could then prefer a server in the

data center closest to them.

The following reserved keys are currently defined:

BUNDLESPEC

   A "bundle specification" string that describes the type of the bundle.

   These are string values that are accepted by the "--type" argument of

   :hg:`bundle`.

   The values are parsed in strict mode, which means they must be of the

   "<compression>-<type>" form. See

   mercurial.exchange.parsebundlespec() for more details.

   Clients will automatically filter out specifications that are unknown or

   unsupported so they won't attempt to download something that likely won't

   apply.

   The actual value doesn't impact client behavior beyond filtering:

   clients will still sniff the bundle type from the header of downloaded

   files.

   **Use of this key is highly recommended**, as it allows clients to

   easily skip unsupported bundles.

REQUIRESNI

   Whether Server Name Indication (SNI) is required to connect to the URL.

   SNI allows servers to use multiple certificates on the same IP. It is

   somewhat common in CDNs and other hosting providers. Older Python

   versions do not support SNI. Defining this attribute enables clients

   with older Python versions to filter this entry without experiencing

   an opaque SSL failure at connection time.

   If this is defined, it is important to advertise a non-SNI fallback

   URL or clients running old Python releases may not be able to clone

   with the clonebundles facility.

   Value should be "true".

Manifests can contain multiple entries. Assuming metadata is defined, clients

will filter entries from the manifest that they don't support. The remaining

entries are optionally sorted by client preferences

(``experimental.clonebundleprefers`` config option). The client then attempts

to fetch the bundle at the first URL in the remaining list.

**Errors when downloading a bundle will fail the entire clone operation:

clients do not automatically fall back to a traditional clone.** The reason

for this is that if a server is using clone bundles, it is probably doing so

because the feature is necessary to help it scale. In other words, there

is an assumption that clone load will be offloaded to another service and

that the Mercurial server isn't responsible for serving this clone load.

If that other service experiences issues and clients start mass falling back to

the original Mercurial server, the added clone load could overwhelm the server

due to unexpected load and effectively take it offline. Not having clients

automatically fall back to cloning from the original server mitigates this

scenario.

Because there is no automatic Mercurial server fallback on failure of the

bundle hosting service, it is important for server operators to view the bundle

hosting service as an extension of the Mercurial server in terms of

availability and service level agreements: if the bundle hosting service goes

down, so does the ability for clients to clone. Note: clients will see a

message informing them how to bypass the clone bundles facility when a failure

occurs. So server operators should prepare for some people to follow these

instructions when a failure occurs, thus driving more load to the original

Mercurial server when the bundle hosting service fails.

"""

from mercurial import (

    extensions,

    wireproto,

)

testedwith = 'internal'

def capabilities(orig, repo, proto):

    caps = orig(repo, proto)

    # Only advertise if a manifest exists. This does add some I/O to requests.

    # But this should be cheaper than a wasted network round trip due to

    # missing file.

    if repo.opener.exists('clonebundles.manifest'):

        caps.append('clonebundles')

    return caps

def extsetup(ui):

    extensions.wrapfunction(wireproto, '_capabilities', capabilities)