doc/source/format_project.rst

.. _projectconf:


Project configuration
=====================
The project configuration file should be named ``project.conf`` and
be located at the project root. It holds information such as Source
aliases relevant for the sources used in the given project as well as
overrides for the configuration of element types used in the project.

Values specified in the project configuration override any of the
default BuildStream project configuration, which is included
:ref:`here <project_defaults>` for reference.


.. _project_essentials:

Essentials
----------


.. _project_format_name:

Project name
~~~~~~~~~~~~
The project name is a unique symbol for your project and will
be used to distinguish your project from others in user preferences,
namespacing of your project's artifacts in shared artifact caches,
and in any case where BuildStream needs to distinguish between multiple
projects.

The first thing to setup in your ``project.conf`` should be the name
of your project.

.. code:: yaml

   name: my-project-name

.. note::

   The project name may contain alphanumeric characters, dashes and
   underscores, and may not start with a leading digit.


.. _project_min_version:

Minimum version
~~~~~~~~~~~~~~~
The BuildStream format is guaranteed to be backwards compatible
with any earlier minor point releases, which is to say that
BuildStream 1.4 can read projects written for BuildStream 1.0,
and that BuildStream 2.2 can read projects written for BuildStream 2.0.

Projects are required to specify the minimum version of BuildStream
which it requires, this allows project authors to convey a useful
error message to their users and peers, in the case that a user needs
to get a newer version of BuildStream in order to work with a given
project.

The project's minimum required BuildStream version must be specified
in ``project.conf`` using the ``min-version`` field, e.g.:

.. code:: yaml

  # This project uses features which were added in 2.2
  min-version: 2.2

It is recommended that when using new features, always consult this
documentation and observe which BuildStream version a feature you are
using was added in. If a feature in the BuildStream YAML format is
not documented with a specific *Since* version, you can assume that
it has been there from the beginning.


.. note::

   External :mod:`Element <buildstream.element>` and :mod:`Source <buildstream.source>`
   plugins also implement their own YAML configuration fragments and as
   such are revisioned separately from the core format.


.. _project_element_path:

Element path
~~~~~~~~~~~~
To allow the user to structure their project nicely, BuildStream
allows the user to specify a project subdirectory where element
``.bst`` files are stored.

.. code:: yaml

   element-path: elements

Note that elements are referred to by their relative paths, whenever
elements are referred to in a ``.bst`` file or on the command line.


.. _project_format_ref_storage:

Ref storage
~~~~~~~~~~~
By default, BuildStream expects to read and write source references
directly in the :ref:`source declaration <format_sources>`, but this
can be inconvenient and prohibitive in some workflows.

Alternatively, BuildStream allows source references to be stored
centrally in a :ref:`project.refs file <projectrefs>` in the toplevel
:ref:`project directory <format_structure>`.

This can be controlled with the ``ref-storage`` option, which is
allowed to be configured with the following values:

* ``inline``

  Source references are stored directly in the
  :ref:`source declaration <format_sources>`

* ``project.refs``

  Source references are stored in the ``project.refs`` file, and
  junction source references are stored in the ``junction.refs`` file.

To enable storing of source references in ``project.refs``, add the
following to your ``project.conf``:

.. code:: yaml

  ref-storage: project.refs


.. _configurable_warnings:

Configurable Warnings
~~~~~~~~~~~~~~~~~~~~~
Warnings can be configured as fatal using the ``fatal-warnings`` configuration item.
When a warning is configured as fatal, where a warning would usually be thrown instead an error will be thrown
causing the build to fail.

Individual warnings can be configured as fatal by setting ``fatal-warnings`` to a list of warnings.

.. code:: yaml

  fatal-warnings:
  - overlaps
  - ref-not-in-track
  - <plugin>:<warning>

BuildStream provides a collection of :class:`Core Warnings <buildstream.types.CoreWarnings>` which may be raised
by a variety of plugins. Other configurable warnings are plugin specific and should be noted within their individual documentation.


.. _project_source_aliases:

Source aliases
~~~~~~~~~~~~~~
In order to abstract the download location of source code and
any assets which need to be downloaded, and also as a matter of
convenience, BuildStream allows one to create named aliases for
URLs which are to be used in the individual ``.bst`` files.

.. code:: yaml

   aliases:
     foo: git://git.foo.org/
     bar: http://bar.com/downloads/


Sandbox options
~~~~~~~~~~~~~~~
Sandbox options for the whole project can be supplied in
``project.conf`` in the same way as in an element. See :ref:`element configuration <format_sandbox>`
for more detail.

.. code:: yaml

   # Specify a user id and group id to use in the build sandbox.
   sandbox:
     build-uid: 1003
     build-gid: 1001


.. _project_essentials_artifacts:

Artifact server
~~~~~~~~~~~~~~~
If you have setup an :ref:`artifact server <cache_servers>` for your
project then it is convenient to configure the following in your ``project.conf``
so that users need not have any additional configuration to communicate
with an artifact share.

.. code:: yaml

  #
  # Artifacts
  #
  artifacts:
    # A remote cache from which to download prebuilt artifacts
    - url: https://foo.com:11001
      server-cert: server.crt
    # A remote cache from which to upload/download built/prebuilt artifacts
    - url: https://foo.com:11002
      push: true
      server-cert: server.crt
      client-cert: client.crt
      client-key: client.key

.. note::

    You can also specify a list of different caches here; earlier entries in the
    list will have higher priority than later ones.

The use of ports are required to distinguish between pull only access and
push/pull access. For information regarding the server/client certificates
and keys, please see: :ref:`Key pair for the server <server_authentication>`.

.. note::

   Buildstream artifact servers have changed since 1.2 to use protocol buffers
   to store artifact information rather than a directory structure, as well as a
   new server API. As a result newer buildstream clients won't work with older
   servers.

.. _project_essentials_split_artifacts:

Split cache servers
~~~~~~~~~~~~~~~~~~~

Should you need to configure an artifact cache to work with a CAS
server that does not support BuildStream's artifact format, you can
"split" that cache and run an artifacts-only server separately. The
format for that is as such:

.. code:: yaml

  #
  # Artifacts
  #
  artifacts:
    # A remote cache from which to download prebuilt artifacts
    - url: https://storage.foo.com:11001
      server-cert: server.crt
      # "storage" remotes store the artifact contents only - this can
      # be a normal CAS implementation such as those provided by
      # Buildbarn or Bazel Buildfarm
      type: storage
    - url: https://index.foo.com:11001
      server-cert: server.crt
      # "index" remotes store only artifact metadata. This is
      # currently only provided by the bst-artifact-server and BuildGrid
      type: index
    # A remote cache from which to upload/download built/prebuilt artifacts
    - url: https://foo.com:11002
      push: true
      server-cert: server.crt
      client-cert: client.crt
      client-key: client.key
      # Caches that support both can omit the type, or set it to "both" -
      # currently, also only supported by bst-artifact-server and BuildGrid
      type: both

.. _project_source_cache:

Source cache server
~~~~~~~~~~~~~~~~~~~
Exactly the same as artifact servers, source cache servers can be specified.

.. code:: yaml

  #
  # Source caches
  #
  source-caches:
    # A remote cache from which to download prestaged sources
    - url: https://foo.com:11001
      server.cert: server.crt
    # A remote cache from which to upload/download prestaged sources
    - url: https://foo.com:11002
      push: true
      server-cert: server.crt
      client-cert: client.crt
      client-key: client.key

.. note::

   Source caches also support "splitting" like :ref:`artifact servers
   <project_essentials_split_artifacts>`.

.. _project_remote_execution:

Remote execution
~~~~~~~~~~~~~~~~
BuildStream supports remote execution using the Google Remote Execution API
(REAPI). A description of how remote execution works is beyond the scope
of this document, but you can specify a remote server complying with the REAPI
using the `remote-execution` option:

.. code:: yaml

  remote-execution:

    # A url defining a remote execution server
    execution-service:
      url: http://buildserver.example.com:50051
      instance-name: development-emea-1
    storage-service:
      url: https://foo.com:11002/
      server-cert: server.crt
      client-cert: client.crt
      client-key: client.key
      instance-name: development-emea-1
    action-cache-service:
      url: http://bar.action.com:50052
      instance-name: development-emea-1

storage-service specifies a remote CAS store and the parameters are the
same as those used to specify an :ref:`artifact server <cache_servers>`.

The action-cache-service specifies where built actions are cached, allowing
buildstream to check whether an action has already been executed and download it
if so. This is similar to the artifact cache but REAPI specified, and is
optional for remote execution to work.

The storage service may be the same endpoint used for artifact
caching. Remote execution cannot work without push access to the
storage endpoint though.

Instance name is optional. Instance names separate different shards on
the same endpoint (url).  You can supply a different instance name for
`execution-service` and `storage-service`, if needed.  The instance
name should be given to you by the service provider of each
service. Not all remote execution and storage services support
instance names.

The Remote Execution API can be found via https://github.com/bazelbuild/remote-apis.

Remote execution configuration can be also provided in the `user
configuration <user_config_remote_execution>`.

.. _project_essentials_mirrors:

Mirrors
~~~~~~~
A list of mirrors can be defined that couple a location to a mapping of aliases to a
list of URIs, e.g.

.. code:: yaml

  mirrors:
  - name: middle-earth
    aliases:
      foo:
      - http://www.middle-earth.com/foo/1
      - http://www.middle-earth.com/foo/2
      bar:
      - http://www.middle-earth.com/bar/1
      - http://www.middle-earth.com/bar/2
  - name: oz
    aliases:
      foo:
      - http://www.oz.com/foo
      bar:
      - http://www.oz.com/bar

The order that the mirrors (and the URIs therein) are consulted is in the order
they are defined when fetching, and in reverse-order when tracking.

A default mirror to consult first can be defined via
:ref:`user config <config_default_mirror>`, or the command-line argument
:ref:`--default-mirror <invoking_bst>`.


.. _project_plugins:

Loading plugins
---------------
If your project makes use of any custom :mod:`Element <buildstream.element>` or
:mod:`Source <buildstream.source>` plugins, then the project must inform BuildStream
of the plugins it means to make use of and the origin from which they can be loaded.

Note that plugins with the same name from different origins are not permitted.


Local plugins
~~~~~~~~~~~~~
Local plugins are expected to be found in a subdirectory of the actual
BuildStream project. :mod:`Element <buildstream.element>` and
:mod:`Source <buildstream.source>` plugins should be stored in separate
directories to avoid namespace collisions, you can achieve this by
specifying a separate *origin* for sources and elements.

.. code:: yaml

   plugins:

   - origin: local
     path: plugins/sources

     # We want to use the `mysource` source plugin located in our
     # project's `plugins/sources` subdirectory.
     sources:
     - mysource

There is no strict versioning policy for plugins loaded from the local
origin because the plugin is provided with the project data and as such,
it is considered to be completely deterministic.

Usually your project will be managed by a VCS like git, and any changes
to your local plugins may have an impact on your project, such as changes
to the artifact cache keys produced by elements which use these plugins.
Changes to plugins might provide new YAML configuration options, changes
in the semantics of existing configurations or even removal of existing
YAML configurations.


Pip plugins
~~~~~~~~~~~
Plugins loaded from the ``pip`` origin are expected to be installed
separately on the host operating system using python's package management
system.

.. code:: yaml

   plugins:

   - origin: pip

     # Specify the name of the python package containing
     # the plugins we want to load. The name one would use
     # on the `pip install` command line.
     #
     package-name: potato

     # We again must specify specifically which plugins we
     # want loaded from this origin.
     #
     elements:
     - starch

Unlike local plugins, plugins loaded from the ``pip`` origin are
loaded from the active *python environment*, and as such you do not
usually have full control over the plugins your project uses unless
one uses strict :ref:`version constraints <project_plugins_pip_version_constraints>`.

The official plugin packages maintained by the BuildStream community are
guaranteed to be fully API stable. If one chooses to load these plugins
from the ``pip`` origin, then it is recommended to use *minimal bound dependency*
:ref:`constraints <project_plugins_pip_version_constraints>` when using
official plugin packages so as to be sure that you have access to all the
features you intend to use in your project.


.. _project_plugins_pip_version_constraints:

Versioning constraints
''''''''''''''''''''''
When loading plugins from the ``pip`` plugin origin, it is possible to
specify constraints on the versions of packages you want to load
your plugins from.

The syntax for specifying constraints are `explained here <https://python-poetry.org/docs/versions/>`_,
and they are the same format supported by the ``pip`` package manager.

.. note::

   In order to be certain that versioning constraints work properly, plugin
   packages should be careful to adhere to `PEP 440, Version Identification and Dependency
   Specification <https://www.python.org/dev/peps/pep-0440/>`_.

Here are a couple of examples:

**Specifying minimal bound dependencies**:

.. code:: yaml

   plugins:

   - origin: pip

     # This project uses the API stable potato project and
     # requires features from at least version 1.2
     #
     package-name: potato>=1.2

**Specifying exact versions**:

.. code:: yaml

   plugins:

   - origin: pip

     # This project requires plugins from the potato
     # project at exactly version 1.2.3
     #
     package-name: potato==1.2.3

**Specifying version constraints**:

.. code:: yaml

   plugins:

   - origin: pip

     # This project requires plugins from the potato
     # project from version 1.2.3 onward until 1.3.
     #
     package-name: potato>=1.2.3,<1.3

.. important::

   **Unstable plugin packages**

   When using unstable plugins loaded from the ``pip`` origin, the installed
   plugins can sometimes be incompatible with your project.

   **Use virtual environments**

   Your operating system's default python environment can only have one
   version of a given package installed at a time, if you work on multiple
   BuildStream projects on the same host, they may not agree on which versions
   of plugins to use.

   In order to guarantee that you can use a specific version of a plugin,
   you may need to install BuildStream into a `virtual environment
   <https://docs.python.org/3/tutorial/venv.html>`_ in order to control which
   python package versions are available when using your project.

   Follow `these instructions
   <https://buildstream.build/source_install.html#installing-in-virtual-environments>`_
   to install BuildStream in a virtual environment.

   **Possible junction conflicts**

   If you have multiple projects which are connected through
   :mod:`junction <elements.junction>` elements, these projects can disagree
   on which version of a plugin is needed from the ``pip`` origin.

   Since only one version of a given plugin *package* can be installed
   at a time in a given *python environment*, you must ensure that all
   projects connected through :mod:`junction <elements.junction>` elements
   agree on which versions of API unstable plugin packages to use.


.. _project_plugins_deprecation:

Suppressing deprecation warnings
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Plugins can be deprecated over time, and using deprecated plugins will
trigger a warning when loading elements and sources which use
deprecated plugin kinds.

These deprecation warnings can be suppressed for the entire plugin
origin or on a per plugin kind basis.

To suppress all deprecation warnings from the origin, set the
``allow-deprecated`` flag for the origin as follows:

.. code:: yaml

   plugins:

   - origin: local
     path: plugins/sources

     # Suppress deprecation warnings for any plugins loaded here
     allow-deprecated: True

     sources:
     - mysource


In order to suppress deprecation warnings for a single element or
source kind within an origin, you will have to use a dictionary
to declare the specific plugin kind and set the ``allow-deprecated`` flag
on that dictionary as follows:

.. code:: yaml

   plugins:

   - origin: pip
     package-name: potato

     # Here we use a dictionary to declare the "starch"
     # element kind, and specify that it is allowed to
     # be deprecated.
     #
     elements:
     - kind: starch
       allow-deprecated: True


.. _project_options:

Options
-------
Options are how BuildStream projects can define parameters which
can be configured by users invoking BuildStream to build your project.

Options are declared in the ``project.conf`` in the main ``options``
dictionary.

.. code:: yaml

   options:
     debug:
       type: bool
       description: Whether to enable debugging
       default: False

Users can configure those options when invoking BuildStream with the
``--option`` argument::

    $ bst --option debug True ...

.. note::

   The name of the option may contain alphanumeric characters
   underscores, and may not start with a leading digit.


Common properties
~~~~~~~~~~~~~~~~~
All option types accept the following common attributes

* ``type``

  Indicates the type of option to declare

* ``description``

  A description of the meaning of the option

* ``variable``

  Optionally indicate a :ref:`variable <format_variables>` name to
  export the option to. A string form of the selected option will
  be used to set the exported value.

  If used, this value will override any existing value for the
  variable declared in ``project.conf``, and will be overridden in
  the regular :ref:`composition order <format_composition>`.

  .. note::

     The name of the variable to export may contain alphanumeric
     characters, dashes, underscores, and may not start with a leading
     digit.


.. _project_options_bool:

Boolean
~~~~~~~
The ``bool`` option type allows specifying boolean values which
can be cased in conditional expressions.


**Declaring**

.. code:: yaml

   options:
     debug:
       type: bool
       description: Whether to enable debugging
       default: False


**Evaluating**

Boolean options can be tested in expressions with equality tests:

.. code:: yaml

   variables:
     enable-debug: False
     (?):
     - debug == True:
         enable-debug: True

Or simply treated as truthy values:

.. code:: yaml

   variables:
     enable-debug: False
     (?):
     - debug:
         enable-debug: True


**Exporting**

When exporting boolean options as variables, a ``True`` option value
will be exported as ``1`` and a ``False`` option as ``0``


.. _project_options_enum:

Enumeration
~~~~~~~~~~~
The ``enum`` option type allows specifying a string value
with a restricted set of possible values.


**Declaring**

.. code:: yaml

   options:
     loglevel:
       type: enum
       description: The logging level
       values:
       - debug
       - info
       - warning
       default: info


**Evaluating**

Enumeration options must be tested as strings in conditional
expressions:

.. code:: yaml

   variables:
     enable-debug: False
     (?):
     - loglevel == "debug":
         enable-debug: True


**Exporting**

When exporting enumeration options as variables, the value is
exported as a variable directly, as it is a simple string.


.. _project_options_flags:

Flags
~~~~~
The ``flags`` option type allows specifying a list of string
values with a restricted set of possible values.

In contrast with the ``enum`` option type, the *default* value
need not be specified and will default to an empty set.


**Declaring**

.. code:: yaml

   options:
     logmask:
       type: flags
       description: The logging mask
       values:
       - debug
       - info
       - warning
       default:
       - info


**Evaluating**

Options of type ``flags`` can be tested in conditional expressions using
a pythonic *in* syntax to test if an element is present in a set:

.. code:: yaml

   variables:
     enable-debug: False
     (?):
     - ("debug" in logmask):
         enable-debug: True


**Exporting**

When exporting flags options as variables, the value is
exported as a comma separated list of selected value strings.


.. _project_options_arch:

Architecture
~~~~~~~~~~~~
The ``arch`` option type is a special enumeration option which defaults via
`uname -m` results to the following list.

* aarch32
* aarch64
* aarch64-be
* power-isa-be
* power-isa-le
* sparc-v9
* x86-32
* x86-64

The reason for this, opposed to using just `uname -m`, is that we want an
OS-independent list, as well as several results mapping to the same architecture
(e.g. i386, i486 etc. are all x86-32). It does not support assigning any default
in the project configuration.

.. code:: yaml

   options:
     machine_arch:
       type: arch
       description: The machine architecture
       values:
       - aarch32
       - aarch64
       - x86-32
       - x86-64


Architecture options can be tested with the same expressions
as other Enumeration options.


.. _project_options_os:

OS
~~

The ``os`` option type is a special enumeration option, which defaults to the
results of `uname -s`. It does not support assigning any default in the project
configuration.

.. code:: yaml

    options:
      machine_os:
        type: os
        description: The machine OS
        values:
        - Linux
        - SunOS
        - Darwin
        - FreeBSD

Os options can be tested with the same expressions as other Enumeration options.


.. _project_options_element_mask:

Element mask
~~~~~~~~~~~~
The ``element-mask`` option type is a special Flags option
which automatically allows only element names as values.

.. code:: yaml

   options:
     debug_elements:
       type: element-mask
       description: The elements to build in debug mode

This can be convenient for automatically declaring an option
which might apply to any element, and can be tested with the
same syntax as other Flag options.


.. code:: yaml

   variables:
     enable-debug: False
     (?):
     - ("element.bst" in debug_elements):
         enable-debug: True



.. _project_defaults:

Element default configuration
-----------------------------
The ``project.conf`` plays a role in defining elements by
providing default values and also by overriding values declared
by plugins on a plugin wide basis.

See the :ref:`composition <format_composition>` documentation for
more detail on how elements are composed.


.. _project_defaults_variables:

Variables
~~~~~~~~~
The defaults for :ref:`Variables <format_variables>` used in your
project is defined here.

.. code:: yaml

   variables:
     prefix: "/usr"


.. _project_defaults_environment:

Environment
~~~~~~~~~~~
The defaults environment for the build sandbox is defined here.

.. code:: yaml

   environment:
     PATH: /usr/bin:/bin:/usr/sbin:/sbin

Additionally, the special ``environment-nocache`` list which specifies
which environment variables do not affect build output, and are thus
not considered in the calculation of artifact keys can be defined here.

.. code:: yaml

   environment-nocache:
   - MAXJOBS

Note that the ``environment-nocache`` list only exists so that we can
control parameters such as ``make -j ${MAXJOBS}``, allowing us to control
the number of jobs for a given build without affecting the resulting
cache key.


.. _project_split_rules:

Split rules
~~~~~~~~~~~
The project wide :ref:`split rules <public_split_rules>` defaults can
be specified here.

.. code:: yaml

   split-rules:
     devel:
     - |
       %{includedir}
     - |
       %{includedir}/**
     - |
       %{libdir}/lib*.a
     - |
       %{libdir}/lib*.la


.. _project_overrides:

Overriding plugin defaults
--------------------------
Base attributes declared by element and source plugins can be overridden
on a project wide basis. This section explains how to make project wide
statements which augment the configuration of an element or source plugin.


.. _project_element_overrides:

Element overrides
~~~~~~~~~~~~~~~~~
The elements dictionary can be used to override variables, environments
or plugin specific configuration data as shown below.


.. code:: yaml

   elements:

     # Override default values for all autotools elements
     autotools:

       variables:
         bindir: "%{prefix}/bin"

       config:
         configure-commands: ...

       environment:
         PKG_CONFIG_PATH=%{libdir}/pkgconfig


.. _project_source_overrides:

Source overrides
~~~~~~~~~~~~~~~~
The sources dictionary can be used to override source plugin
specific configuration data as shown below.


.. code:: yaml

   sources:

     # Override default values for all git sources
     git:

       config:
         checkout-submodules: False


.. _project_shell:

Customizing the shell
---------------------
Since BuildStream cannot know intimate details about your host or about
the nature of the runtime and software that you are building, the shell
environment for debugging and testing applications may need some help.

The ``shell`` section allows some customization of the shell environment.


Interactive shell command
~~~~~~~~~~~~~~~~~~~~~~~~~
By default, BuildStream will use ``sh -i`` when running an interactive
shell, unless a specific command is given to the ``bst shell`` command.

BuildStream will automatically set a convenient prompt via the ``PS1``
environment variable for interactive shells; which might be overwritten
depending on the shell you use in your runtime.

If you are using ``bash``, we recommend the following configuration to
ensure that the customized prompt is not overwritten:

.. code:: yaml

   shell:

     # Specify the command to run by default for interactive shells
     command: [ 'bash', '--noprofile', '--norc', '-i' ]


Environment assignments
~~~~~~~~~~~~~~~~~~~~~~~
In order to cooperate with your host environment, a debugging shell
sometimes needs to be configured with some extra knowledge inheriting
from your host environment.

This can be achieved by setting up the shell ``environment`` configuration,
which is expressed as a dictionary very similar to the
:ref:`default environment <project_defaults_environment>`, except that it
supports host side environment variable expansion in values.

For example, to share your host ``DISPLAY`` and ``DBUS_SESSION_BUS_ADDRESS``
environments with debugging shells for your project, specify the following:

.. code:: yaml

   shell:

     # Share some environment variables from the host environment
     environment:
       DISPLAY: '$DISPLAY'
       DBUS_SESSION_BUS_ADDRESS: '$DBUS_SESSION_BUS_ADDRESS'

Or, a more complex example is how one might share the host pulseaudio
server with a ``bst shell`` environment:

.. code:: yaml

   shell:

     # Set some environment variables explicitly
     environment:
       PULSE_SERVER: 'unix:${XDG_RUNTIME_DIR}/pulse/native'


Host files
~~~~~~~~~~
It can be useful to share some files on the host with a shell so that
it can integrate better with the host environment.

The ``host-files`` configuration allows one to specify files and
directories on the host to be bind mounted into the sandbox.

.. warning::

   One should never mount directories where one expects to
   find data and files which belong to the user, such as ``/home``
   on POSIX platforms.

   This is because the unsuspecting user may corrupt their own
   files accidentally as a result. Instead users can use the
   ``--mount`` option of ``bst shell`` to mount data into the shell.


The ``host-files`` configuration is an ordered list of *mount specifications*.

Members of the list can be *fully specified* as a dictionary, or a simple
string can be used if only the defaults are required.

The fully specified dictionary has the following members:

* ``path``

  The path inside the sandbox. This is the only mandatory
  member of the mount specification.

* ``host_path``

  The host path to mount at ``path`` in the sandbox. This
  will default to ``path`` if left unspecified.

* ``optional``

  Whether the mount should be considered optional. This
  is ``False`` by default.


Here is an example of a *fully specified mount specification*:

.. code:: yaml

   shell:

     # Mount an arbitrary resolv.conf from the host to
     # /etc/resolv.conf in the sandbox, and avoid any
     # warnings if the host resolv.conf doesnt exist.
     host-files:
     - host_path: '/usr/local/work/etc/resolv.conf'
       path: '/etc/resolv.conf'
       optional: True

Here is an example of using *shorthand mount specifications*:

.. code:: yaml

   shell:

     # Specify a list of files to mount in the sandbox
     # directory from the host.
     #
     # If these do not exist on the host, a warning will
     # be issued but the shell will still be launched.
     host-files:
     - '/etc/passwd'
     - '/etc/group'
     - '/etc/resolv.conf'

Host side environment variable expansion is also supported:

.. code:: yaml

   shell:

     # Mount a host side pulseaudio server socket into
     # the shell environment at the same location.
     host-files:
     - '${XDG_RUNTIME_DIR}/pulse/native'


.. _project_default_targets:

Default targets
---------------
When running BuildStream commands from a project directory or subdirectory
without specifying any target elements on the command line, the default targets
of the project will be used.  The default targets can be configured in the
``defaults`` section as follows:

.. code:: yaml

   defaults:

     # List of default target elements
     targets:
     - app.bst

If no default targets are configured in ``project.conf``, BuildStream commands
will default to all ``.bst`` files in the configured element path.

Commands that cannot support junctions as target elements (``bst build``,
``bst artifact push``, and ``bst artifact pull``) ignore junctions in the list
of default targets.

When running BuildStream commands from a workspace directory (that is not a
BuildStream project directory), project default targets are not used and the
workspace element will be used as the default target instead.

``bst artifact checkout``, ``bst source checkout``, and ``bst shell`` are
currently limited to a single target element and due to this, they currently
do not use project default targets.  However, they still use the workspace
element as default target when run from a workspace directory.


.. _project_builtin_defaults:

Builtin defaults
----------------
BuildStream defines some default values for convenience, the default
values overridden by your project's ``project.conf`` are presented here:

  .. literalinclude:: ../../src/buildstream/data/projectconfig.yaml
     :language: yaml