view mercurial/help/internals/bundles.txt @ 36858:01f6bba64424

hgweb: remove support for POST form data (BC)

Previously, we called out to cgi.parse(), which for POST requests
parsed multipart/form-data and application/x-www-form-urlencoded
Content-Type requests for form data, combined it with query string
parameters, returned a union of the values.

As far as I know, nothing in Mercurial actually uses this mechanism
to submit data to the HTTP server. The wire protocol has its own
mechanism for passing parameters. And the web interface only does
GET requests. Removing support for parsing POST data doesn't break
any tests.

Another reason to not like this feature is that cgi.parse() may
modify the QUERY_STRING environment variable as a side-effect.
In addition, it merges both POST data and the query string into
one data structure. This prevents consumers from knowing whether
a variable came from the query string or POST data. That can matter
for some operations.

I suspect we use cgi.parse() because back when this code was
initially implemented, it was the function that was readily
available. In other words, I don't think there was conscious
choice to support POST data: we just got it because cgi.parse()
supported it.

Since nothing uses the feature and it is untested, let's remove
support for parsing POST form data. We can add it back in easily
enough if we need it in the future.

.. bc::

   Hgweb no longer reads form data in POST requests from
   multipart/form-data and application/x-www-form-urlencoded
   requests. Arguments should be specified as URL path components
   or in the query string in the URL instead.

Differential Revision: https://phab.mercurial-scm.org/D2774

A bundle is a container for repository data.

Bundles are used as standalone files as well as the interchange format
over the wire protocol used when two Mercurial peers communicate with
each other.

Headers
=======

Bundles produced since Mercurial 0.7 (September 2005) have a 4 byte
header identifying the major bundle type. The header always begins with
``HG`` and the follow 2 bytes indicate the bundle type/version. Some
bundle types have additional data after this 4 byte header.

The following sections describe each bundle header/type.

HG10
----

``HG10`` headers indicate a *changegroup bundle*. This is the original
bundle format, so it is sometimes referred to as *bundle1*. It has been
present since version 0.7 (released September 2005).

This header is followed by 2 bytes indicating the compression algorithm
used for data that follows. All subsequent data following this
compression identifier is compressed according to the algorithm/method
specified.

Supported algorithms include the following.

``BZ``
   *bzip2* compression.

   Bzip2 compressors emit a leading ``BZ`` header. Mercurial uses this
   leading ``BZ`` as part of the bundle header. Therefore consumers
   of bzip2 bundles need to *seed* the bzip2 decompressor with ``BZ`` or
   seek the input stream back to the beginning of the algorithm component
   of the bundle header so that decompressor input is valid. This behavior
   is unique among supported compression algorithms.

   Supported since version 0.7 (released December 2006).

``GZ``
  *zlib* compression.

   Supported since version 0.9.2 (released December 2006).

``UN``
  *Uncompressed* or no compression. Unmodified changegroup data follows.

  Supported since version 0.9.2 (released December 2006).

3rd party extensions may implement their own compression. However, no
authority reserves values for their compression algorithm identifiers.

HG2X
----

``HG2X`` headers (where ``X`` is any value) denote a *bundle2* bundle.
Bundle2 bundles are a container format for various kinds of repository
data and capabilities, beyond changegroup data (which was the only data
supported by ``HG10`` bundles.

``HG20`` is currently the only defined bundle2 version.

The ``HG20`` format is documented at :hg:`help internals.bundle2`.

Initial ``HG20`` support was added in Mercurial 3.0 (released May
2014). However, bundle2 bundles were hidden behind an experimental flag
until version 3.5 (released August 2015), when they were enabled in the
wire protocol. Various commands (including ``hg bundle``) did not
support generating bundle2 files until Mercurial 3.6 (released November
2015).

HGS1
----

*Experimental*

A ``HGS1`` header indicates a *streaming clone bundle*. This is a bundle
that contains raw revlog data from a repository store. (Typically revlog
data is exchanged in the form of changegroups.)

The purpose of *streaming clone bundles* are to *clone* repository data
very efficiently.

The ``HGS1`` header is always followed by 2 bytes indicating a
compression algorithm of the data that follows. Only ``UN``
(uncompressed data) is currently allowed.

``HGS1UN`` support was added as an experimental feature in version 3.6
(released November 2015) as part of the initial offering of the *clone
bundles* feature.