view mercurial/help/scripting.txt @ 29561:1a782fabf80d

sslutil: print a warning when using TLS 1.0 on legacy Python

Mercurial now requires TLS 1.1+ when TLS 1.1+ is supported by the
client. Since we made the decision to require TLS 1.1+ when running
with modern Python versions, it makes sense to do something for
legacy Python versions that only support TLS 1.0.

Feature parity would be to prevent TLS 1.0 connections out of the
box and require a config option to enable them. However, this is
extremely user hostile since Mercurial wouldn't talk to https://
by default in these installations! I can easily see how someone
would do something foolish like use "--insecure" instead - and
that would be worse than allowing TLS 1.0!

This patch takes the compromise position of printing a warning when
performing TLS 1.0 connections when running on old Python
versions. While this warning is no more annoying than the
CA certificate / fingerprint warnings in Mercurial 3.8, we provide
a config option to disable the warning because to many people
upgrading Python to make the warning go away is not an available
recourse (unlike pinning fingerprints is for the CA warning).

The warning appears as optional output in a lot of tests.

It is common for machines (as opposed to humans) to consume Mercurial.
This help topic describes some of the considerations for interfacing
machines with Mercurial.

Choosing an Interface
=====================

Machines have a choice of several methods to interface with Mercurial.
These include:

- Executing the ``hg`` process
- Querying a HTTP server
- Calling out to a command server

Executing ``hg`` processes is very similar to how humans interact with
Mercurial in the shell. It should already be familiar to you.

:hg:`serve` can be used to start a server. By default, this will start
a "hgweb" HTTP server. This HTTP server has support for machine-readable
output, such as JSON. For more, see :hg:`help hgweb`.

:hg:`serve` can also start a "command server." Clients can connect
to this server and issue Mercurial commands over a special protocol.
For more details on the command server, including links to client
libraries, see https://mercurial.selenic.com/wiki/CommandServer.

:hg:`serve` based interfaces (the hgweb and command servers) have the
advantage over simple ``hg`` process invocations in that they are
likely more efficient. This is because there is significant overhead
to spawn new Python processes.

.. tip::

   If you need to invoke several ``hg`` processes in short order and/or
   performance is important to you, use of a server-based interface
   is highly recommended.

Environment Variables
=====================

As documented in :hg:`help environment`, various environment variables
influence the operation of Mercurial. The following are particularly
relevant for machines consuming Mercurial:

HGPLAIN
    If not set, Mercurial's output could be influenced by configuration
    settings that impact its encoding, verbose mode, localization, etc.

    It is highly recommended for machines to set this variable when
    invoking ``hg`` processes.

HGENCODING
    If not set, the locale used by Mercurial will be detected from the
    environment. If the determined locale does not support display of
    certain characters, Mercurial may render these character sequences
    incorrectly (often by using "?" as a placeholder for invalid
    characters in the current locale).

    Explicitly setting this environment variable is a good practice to
    guarantee consistent results. "utf-8" is a good choice on UNIX-like
    environments.

HGRCPATH
    If not set, Mercurial will inherit config options from config files
    using the process described in :hg:`help config`. This includes
    inheriting user or system-wide config files.

    When utmost control over the Mercurial configuration is desired, the
    value of ``HGRCPATH`` can be set to an explicit file with known good
    configs. In rare cases, the value can be set to an empty file or the
    null device (often ``/dev/null``) to bypass loading of any user or
    system config files. Note that these approaches can have unintended
    consequences, as the user and system config files often define things
    like the username and extensions that may be required to interface
    with a repository.

Consuming Command Output
========================

It is common for machines to need to parse the output of Mercurial
commands for relevant data. This section describes the various
techniques for doing so.

Parsing Raw Command Output
--------------------------

Likely the simplest and most effective solution for consuming command
output is to simply invoke ``hg`` commands as you would as a user and
parse their output.

The output of many commands can easily be parsed with tools like
``grep``, ``sed``, and ``awk``.

A potential downside with parsing command output is that the output
of commands can change when Mercurial is upgraded. While Mercurial
does generally strive for strong backwards compatibility, command
output does occasionally change. Having tests for your automated
interactions with ``hg`` commands is generally recommended, but is
even more important when raw command output parsing is involved.

Using Templates to Control Output
---------------------------------

Many ``hg`` commands support templatized output via the
``-T/--template`` argument. For more, see :hg:`help templates`.

Templates are useful for explicitly controlling output so that
you get exactly the data you want formatted how you want it. For
example, ``log -T {node}\n`` can be used to print a newline
delimited list of changeset nodes instead of a human-tailored
output containing authors, dates, descriptions, etc.

.. tip::

   If parsing raw command output is too complicated, consider
   using templates to make your life easier.

The ``-T/--template`` argument allows specifying pre-defined styles.
Mercurial ships with the machine-readable styles ``json`` and ``xml``,
which provide JSON and XML output, respectively. These are useful for
producing output that is machine readable as-is.

.. important::

   The ``json`` and ``xml`` styles are considered experimental. While
   they may be attractive to use for easily obtaining machine-readable
   output, their behavior may change in subsequent versions.

   These styles may also exhibit unexpected results when dealing with
   certain encodings. Mercurial treats things like filenames as a
   series of bytes and normalizing certain byte sequences to JSON
   or XML with certain encoding settings can lead to surprises.

Command Server Output
---------------------

If using the command server to interact with Mercurial, you are likely
using an existing library/API that abstracts implementation details of
the command server. If so, this interface layer may perform parsing for
you, saving you the work of implementing it yourself.

Output Verbosity
----------------

Commands often have varying output verbosity, even when machine
readable styles are being used (e.g. ``-T json``). Adding
``-v/--verbose`` and ``--debug`` to the command's arguments can
increase the amount of data exposed by Mercurial.

An alternate way to get the data you need is by explicitly specifying
a template.

Other Topics
============

revsets
   Revisions sets is a functional query language for selecting a set
   of revisions. Think of it as SQL for Mercurial repositories. Revsets
   are useful for querying repositories for specific data.

   See :hg:`help revsets` for more.

share extension
   The ``share`` extension provides functionality for sharing
   repository data across several working copies. It can even
   automatically "pool" storage for logically related repositories when
   cloning.

   Configuring the ``share`` extension can lead to significant resource
   utilization reduction, particularly around disk space and the
   network. This is especially true for continuous integration (CI)
   environments.

   See :hg:`help -e share` for more.