Support independent includes with conditional, optional, and remote entries (#48784)

Supersedes #46792.
Closes #40018.
Closes #31026.
Closes #2700.

There were a number of feature requests for os-specific config. This enables os-specific
config without adding a lot of special sub-scopes.

Support `include:` as an independent configuration schema, allowing users to include
configuration scopes from files or directories. Includes can be:
* conditional (similar to definitions in environments), and/or
* optional (i.e., the include will be skipped if it does not exist).

Includes can be paths or URLs (`ftp`, `https`, `http` or `file`). Paths can be absolute or
relative . Environments can include configuration files using the same schema. Remote includes 
must be checked by `sha256`.

Includes can also be recursive, and this modifies the config system accordingly so that
we push included configuration scopes on the stack *before* their including scopes, and
we remove configuration scopes from the stack when their including scopes are removed.

For example, you could have an `include.yaml` file (e.g., under `$HOME/.spack`) to specify
global includes:

```
include:
- ./enable_debug.yaml
- path: https://github.com/spack/spack-configs/blob/main/NREL/configs/mac/config.yaml
  sha256: 37f982915b03de18cc4e722c42c5267bf04e46b6a6d6e0ef3a67871fcb1d258b
```

Or an environment `spack.yaml`:

```
spack:
  include:
  - path: "/path/to/a/config-dir-or-file"
    when: os == "ventura"
  - ./path/relative/to/containing/file/that/is/required
  - path: "/path/with/spack/variables/$os/$target"
    optional: true
  - path: https://raw.githubusercontent.com/spack/spack-configs/refs/heads/main/path/to/required/raw/config.yaml
    sha256: 26e871804a92cd07bb3d611b31b4156ae93d35b6a6d6e0ef3a67871fcb1d258b
```

Updated TODO:
- [x] Get existing unit tests to pass with Todd's changes
- [x] Resolve new (or old) circular imports
- [x] Ensure remote includes (global) work
- [x] Ensure remote includes for environments work (note: caches remote
      files under user cache root)
- [x] add sha256 field to include paths, validate, and require for remote includes
- [x] add sha256 remote file unit tests
- [x] revisit how diamond includes should work
- [x] support recursive includes
- [x] add recursive include unit tests
- [x] update docs and unit test to indicate ordering of recursive includes with
      conflicting options is deferred to follow-on work

---------

Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>
Co-authored-by: Peter Scheibel <scheibel1@llnl.gov>
Co-authored-by: Todd Gamblin <tgamblin@llnl.gov>

lib/spack/docs/configuration.rst

@@ -14,6 +14,7 @@ case you want to skip directly to specific docs:
 * :ref:`compilers.yaml <compiler-config>`
 * :ref:`concretizer.yaml <concretizer-options>`
 * :ref:`config.yaml <config-yaml>`
+* :ref:`include.yaml <include-yaml>`
 * :ref:`mirrors.yaml <mirrors>`
 * :ref:`modules.yaml <modules>`
 * :ref:`packages.yaml <packages-config>`

lib/spack/docs/environments.rst

@@ -670,24 +670,45 @@ This configuration sets the default compiler for all packages to
 Included configurations
 ^^^^^^^^^^^^^^^^^^^^^^^
 
-Spack environments allow an ``include`` heading in their yaml
-schema. This heading pulls in external configuration files and applies
-them to the environment.
+Spack environments allow an ``include`` heading in their yaml schema.
+This heading pulls in external configuration files and applies them to
+the environment.
 
 .. code-block:: yaml
 
    spack:
      include:
-     - relative/path/to/config.yaml
+     - environment/relative/path/to/config.yaml
      - https://github.com/path/to/raw/config/compilers.yaml
      - /absolute/path/to/packages.yaml
+     - path: /path/to/$os/$target/environment
+       optional: true
+     - path: /path/to/os-specific/config-dir
+       when: os == "ventura"
+
+Included configuration files are required *unless* they are explicitly optional
+or the entry's condition evaluates to ``false``. Optional includes are specified
+with the ``optional`` clause and conditional with the ``when`` clause. (See
+:ref:`include-yaml` for more information on optional and conditional entries.)
+
+Files are listed using paths to individual files or directories containing them.
+Path entries may be absolute or relative to the environment or specified as 
+URLs. URLs to individual files need link to the **raw** form of the file's 
+contents (e.g., `GitHub
+<https://docs.github.com/en/repositories/working-with-files/using-files/viewing-and-understanding-files#viewing-or-copying-the-raw-file-content>`_ 
+or `GitLab
+<https://docs.gitlab.com/ee/api/repository_files.html#get-raw-file-from-repository>`_).
+Only the ``file``, ``ftp``, ``http`` and ``https`` protocols (or schemes) are
+supported. Spack-specific, environment and user path variables can be used.
+(See :ref:`config-file-variables` for more information.)
+
+.. warning::
+
+   Recursive includes are not currently processed in a breadth-first manner
+   so the value of a configuration option that is altered by multiple included
+   files may not be what you expect. This will be addressed in a future
+   update.
 
-Environments can include files or URLs. File paths can be relative or
-absolute. URLs include the path to the text for individual files or
-can be the path to a directory containing configuration files.
-Spack supports ``file``, ``http``, ``https`` and ``ftp`` protocols (or
-schemes). Spack-specific, environment and user path variables may be
-used in these paths. See :ref:`config-file-variables` for more information.
 
 ^^^^^^^^^^^^^^^^^^^^^^^^
 Configuration precedence

lib/spack/docs/include_yaml.rst

@@ -0,0 +1,51 @@
+.. Copyright Spack Project Developers. See COPYRIGHT file for details.
+
+   SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+.. _include-yaml:
+
+===============================
+Include Settings (include.yaml)
+===============================
+
+Spack allows you to include configuration files through ``include.yaml``.
+Using the ``include:`` heading results in pulling in external configuration
+information to be used by any Spack command.
+
+Included configuration files are required *unless* they are explicitly optional
+or the entry's condition evaluates to ``false``. Optional includes are specified
+with the ``optional`` clause and conditional with the ``when`` clause. For
+example,
+
+.. code-block:: yaml
+
+   include:
+   - /path/to/a/required/config.yaml
+   - path: /path/to/$os/$target/config
+     optional: true
+   - path: /path/to/os-specific/config-dir
+     when: os == "ventura"
+
+shows all three. The first entry, ``/path/to/a/required/config.yaml``,
+indicates that included ``config.yaml`` file is required (so must exist).
+Use of ``optional: true`` for ``/path/to/$os/$target/config`` means
+the path is only included if it exists. The condition ``os == "ventura"``
+in the ``when`` clause for ``/path/to/os-specific/config-dir`` means the
+path is only included when the operating system (``os``) is ``ventura``.
+
+The same conditions and variables in `Spec List References 
+<https://spack.readthedocs.io/en/latest/environments.html#spec-list-references>`_
+can be used for conditional activation in the ``when`` clauses.
+
+Included files can be specified by path or by their parent directory.
+Paths may be absolute, relative (to the configuration file including the path), 
+or specified as URLs. Only the ``file``, ``ftp``, ``http`` and ``https`` protocols (or
+schemes) are supported. Spack-specific, environment and user path variables
+can be used. (See :ref:`config-file-variables` for more information.)
+
+.. warning::
+
+   Recursive includes are not currently processed in a breadth-first manner
+   so the value of a configuration option that is altered by multiple included
+   files may not be what you expect. This will be addressed in a future
+   update.

lib/spack/docs/index.rst

@@ -71,6 +71,7 @@ or refer to the full manual below.
 
    configuration
    config_yaml
+   include_yaml
    packages_yaml
    build_settings
    environments