config: overrides for caches and system and user scopes (#26735)

Spack's `system` and `user` scopes provide ways for administrators and
users to set global defaults for all Spack instances, but for use cases
where one wants a clean Spack installation, these scopes can be undesirable.
For example, users may want to opt out of global system configuration, or
they may want to ignore their own home directory settings when running in
a continuous integration environment.

Spack also, by default, keeps various caches and user data in `~/.spack`,
but users may want to override these locations.

Spack provides three environment variables that allow you to override or
opt out of configuration locations:

 * `SPACK_USER_CONFIG_PATH`: Override the path to use for the
   `user` (`~/.spack`) scope.

 * `SPACK_SYSTEM_CONFIG_PATH`: Override the path to use for the
   `system` (`/etc/spack`) scope.

 * `SPACK_DISABLE_LOCAL_CONFIG`: set this environment variable to completely
   disable *both* the system and user configuration directories. Spack will
   only consider its own defaults and `site` configuration locations.

And one that allows you to move the default cache location:

 * `SPACK_USER_CACHE_PATH`: Override the default path to use for user data
   (misc_cache, tests, reports, etc.)

With these settings, if you want to isolate Spack in a CI environment, you can do this:

   export SPACK_DISABLE_LOCAL_CONFIG=true
   export SPACK_USER_CACHE_PATH=/tmp/spack

This is a stop-gap approach until we have figured out how to deal with
the system and user config scopes more generally, as there are plans to
potentially / eventually get rid of them.

**User config**

Spack is a bit of a pain when you have:

- a shared $HOME folder across different systems.
- multiple Spack versions on the same system.

**System config**

- On shared systems with a versioned programming environment / toolkit,
  system administrators want to provide config for each version (e.g.
  21.09, 21.10) of the programming environment, and the user Spack
  instance should be able to pick this up without a steep learning
  curve.
- On shared systems the user should be able to opt out of the
  hard-coded config scope in /etc/spack, since it may be incompatible
  with their particular instance. Currently Spack can only opt out of all
  config scopes through overrides with `"config:":`, `"packages:":`, but that
  also drops the defaults config, which would have to be repeated, which
  is undesirable, especially the lengthy packages.yaml.

An example use case is: having config in this folder:

```
/path/to/programming/environment/{version}/{compilers,packages}.yaml
```

and have `module load spack-system-config` set the variable

```
SPACK_SYSTEM_CONFIG_PATH=/path/to/programming/environment/{version}
```

where the user no longer has to worry about what `{version}` they are
on.

**Continuous integration**

Finally, there is the use case of continuous integration, which may
clone an arbitrary Spack version, which optimally should not pick up
system or user config from the previous run (like may happen in
classical bare metal non-containerized filesystem side effect ridden
jenkins pipelines). In fact this is very similar to how spack itself
tries to avoid picking up system dependencies during builds...

**But environments solve this?**

- You could do `include`s in environment files to get similar behavior
  to the spack_system_config_path example, but environments require you
  to:
  1) require paths to individual config files, not directories.
  2) fail if the listed config file does not exist
- They allow you to override config scopes, but this is generally too
  rigurous, as it requires you to repeat the default config, in
  particular packages.yaml, and just defies the point of layered config.

Co-authored-by: Tom Scogland <tscogland@llnl.gov>
Co-authored-by: Tim Fuller <tjfulle@sandia.gov>
Co-authored-by: Steve Leak <sleak@lbl.gov>
Co-authored-by: Todd Gamblin <tgamblin@llnl.gov>

7 files changed, 228 insertions, 47 deletions

diff --git a/lib/spack/docs/configuration.rst b/lib/spack/docs/configuration.rst
index 10b442ab74..5d41f86997 100644
--- a/lib/spack/docs/configuration.rst
+++ b/lib/spack/docs/configuration.rst
@@ -402,12 +402,17 @@ Spack-specific variables
 
 Spack understands several special variables. These are:
 
+* ``$env``: name of the currently active :ref:`environment <environments>`
 * ``$spack``: path to the prefix of this Spack installation
 * ``$tempdir``: default system temporary directory (as specified in
   Python's `tempfile.tempdir
   <https://docs.python.org/2/library/tempfile.html#tempfile.tempdir>`_
   variable.
 * ``$user``: name of the current user
+* ``$user_config_path``: user configuration directory (``~/.spack`` unless
+  :ref:`overridden <local-config-overrides>`)
+* ``$user_cache_path``: user cache directory (``~/.spack`` unless
+  :ref:`overridden <local-config-overrides>`)
 
 Note that, as with shell variables, you can write these as ``$varname``
 or with braces to distinguish the variable from surrounding characters:
@@ -562,3 +567,39 @@ built in and are not overridden by a configuration file. The
 command line. ``dirty`` and ``install_tree`` come from the custom
 scopes ``./my-scope`` and ``./my-scope-2``, and all other configuration
 options come from the default configuration files that ship with Spack.
+
+.. _local-config-overrides:
+
+------------------------------
+Overriding Local Configuration
+------------------------------
+
+Spack's ``system`` and ``user`` scopes provide ways for administrators and users to set
+global defaults for all Spack instances, but for use cases where one wants a clean Spack
+installation, these scopes can be undesirable. For example, users may want to opt out of
+global system configuration, or they may want to ignore their own home directory
+settings when running in a continuous integration environment.
+
+Spack also, by default, keeps various caches and user data in ``~/.spack``, but
+users may want to override these locations.
+
+Spack provides three environment variables that allow you to override or opt out of
+configuration locations:
+
+* ``SPACK_USER_CONFIG_PATH``: Override the path to use for the
+  ``user`` (``~/.spack``) scope.
+* ``SPACK_SYSTEM_CONFIG_PATH``: Override the path to use for the ``system``
+  (``/etc/spack``) scope.
+* ``SPACK_DISABLE_LOCAL_CONFIG``: set this environment variable to completely disable
+  **both** the system and user configuration directories. Spack will only consider its
+  own defaults and ``site`` configuration locations.
+
+And one that allows you to move the default cache location:
+
+* ``SPACK_USER_CACHE_PATH``: Override the default path to use for user data
+  (misc_cache, tests, reports, etc.)
+
+With these settings, if you want to isolate Spack in a CI environment, you can do this::
+
+  export SPACK_DISABLE_LOCAL_CONFIG=true
+  export SPACK_USER_CACHE_PATH=/tmp/spack
diff --git a/lib/spack/spack/bootstrap.py b/lib/spack/spack/bootstrap.py
index 0bdf49c5e8..78ed54429f 100644
--- a/lib/spack/spack/bootstrap.py
+++ b/lib/spack/spack/bootstrap.py
@@ -582,7 +582,7 @@ def store_path():
 def _root_path():
     """Root of all the bootstrap related folders"""
     return spack.config.get(
-        'bootstrap:root', spack.paths.user_bootstrap_path
+        'bootstrap:root', spack.paths.default_user_bootstrap_path
     )
 
 
diff --git a/lib/spack/spack/caches.py b/lib/spack/spack/caches.py
index 67c93cb306..71695aaf5c 100644
--- a/lib/spack/spack/caches.py
+++ b/lib/spack/spack/caches.py
@@ -23,11 +23,8 @@ def misc_cache_location():
     Currently the ``misc_cache`` stores indexes for virtual dependency
     providers and for which packages provide which tags.
     """
-    path = spack.config.get('config:misc_cache')
-    if not path:
-        path = os.path.join(spack.paths.user_config_path, 'cache')
-    path = spack.util.path.canonicalize_path(path)
-    return path
+    path = spack.config.get('config:misc_cache', spack.paths.default_misc_cache_path)
+    return spack.util.path.canonicalize_path(path)
 
 
 def _misc_cache():
@@ -47,7 +44,7 @@ def fetch_cache_location():
     """
     path = spack.config.get('config:source_cache')
     if not path:
-        path = os.path.join(spack.paths.var_path, "cache")
+        path = spack.paths.default_fetch_cache_path
     path = spack.util.path.canonicalize_path(path)
     return path
 
diff --git a/lib/spack/spack/config.py b/lib/spack/spack/config.py
index 470a2b958c..c82729b007 100644
--- a/lib/spack/spack/config.py
+++ b/lib/spack/spack/config.py
@@ -89,24 +89,6 @@ configuration_defaults_path = (
     'defaults', os.path.join(spack.paths.etc_path, 'spack', 'defaults')
 )
 
-#: Builtin paths to configuration files in Spack
-configuration_paths = (
-    # Default configuration scope is the lowest-level scope. These are
-    # versioned with Spack and can be overridden by systems, sites or users
-    configuration_defaults_path,
-
-    # System configuration is per machine.
-    # No system-level configs should be checked into spack by default
-    ('system', os.path.join(spack.paths.system_etc_path, 'spack')),
-
-    # Site configuration is per spack instance, for sites or projects
-    # No site-level configs should be checked into spack by default.
-    ('site', os.path.join(spack.paths.etc_path, 'spack')),
-
-    # User configuration can override both spack defaults and site config
-    ('user', spack.paths.user_config_path)
-)
-
 #: Hard-coded default values for some key configuration options.
 #: This ensures that Spack will still work even if config.yaml in
 #: the defaults scope is removed.
@@ -808,8 +790,37 @@ def _config():
     cfg = Configuration()
 
     # first do the builtin, hardcoded defaults
-    defaults = InternalConfigScope('_builtin', config_defaults)
-    cfg.push_scope(defaults)
+    builtin = InternalConfigScope('_builtin', config_defaults)
+    cfg.push_scope(builtin)
+
+    # Builtin paths to configuration files in Spack
+    configuration_paths = [
+        # Default configuration scope is the lowest-level scope. These are
+        # versioned with Spack and can be overridden by systems, sites or users
+        configuration_defaults_path,
+    ]
+
+    disable_local_config = "SPACK_DISABLE_LOCAL_CONFIG" in os.environ
+
+    # System configuration is per machine.
+    # This is disabled if user asks for no local configuration.
+    if not disable_local_config:
+        configuration_paths.append(
+            ('system', spack.paths.system_config_path),
+        )
+
+    # Site configuration is per spack instance, for sites or projects
+    # No site-level configs should be checked into spack by default.
+    configuration_paths.append(
+        ('site', os.path.join(spack.paths.etc_path, 'spack')),
+    )
+
+    # User configuration can override both spack defaults and site config
+    # This is disabled if user asks for no local configuration.
+    if not disable_local_config:
+        configuration_paths.append(
+            ('user', spack.paths.user_config_path)
+        )
 
     # add each scope and its platform-specific directory
     for name, path in configuration_paths:
diff --git a/lib/spack/spack/paths.py b/lib/spack/spack/paths.py
index 627be54bd7..70d138a932 100644
--- a/lib/spack/spack/paths.py
+++ b/lib/spack/spack/paths.py
@@ -41,30 +41,88 @@ build_systems_path    = os.path.join(module_path, 'build_systems')
 operating_system_path = os.path.join(module_path, 'operating_systems')
 test_path             = os.path.join(module_path, "test")
 hooks_path            = os.path.join(module_path, "hooks")
-var_path              = os.path.join(prefix, "var", "spack")
-repos_path            = os.path.join(var_path, "repos")
-tests_path            = os.path.join(var_path, "tests")
+opt_path              = os.path.join(prefix, "opt")
 share_path            = os.path.join(prefix, "share", "spack")
+etc_path              = os.path.join(prefix, "etc")
 
-# Paths to built-in Spack repositories.
-packages_path      = os.path.join(repos_path, "builtin")
-mock_packages_path = os.path.join(repos_path, "builtin.mock")
 
-#: User configuration location
-user_config_path = os.path.expanduser('~/.spack')
-user_bootstrap_path = os.path.join(user_config_path, 'bootstrap')
-reports_path = os.path.join(user_config_path, "reports")
-monitor_path = os.path.join(reports_path, "monitor")
+#
+# Things in $spack/var/spack
+#
+var_path              = os.path.join(prefix, "var", "spack")
 
-# We cache repositories (git) in first, extracted metadata in second
-user_repos_cache_path = os.path.join(user_config_path, 'git_repos')
+# read-only things in $spack/var/spack
+repos_path            = os.path.join(var_path, "repos")
+packages_path         = os.path.join(repos_path, "builtin")
+mock_packages_path    = os.path.join(repos_path, "builtin.mock")
 
-opt_path        = os.path.join(prefix, "opt")
-etc_path        = os.path.join(prefix, "etc")
-system_etc_path = '/etc'
+#
+# Writable things in $spack/var/spack
+# TODO: Deprecate these, as we want a read-only spack prefix by default.
+# TODO: These should probably move to user cache, or some other location.
+#
+# fetch cache for downloaded files
+default_fetch_cache_path = os.path.join(var_path, "cache")
 
 # GPG paths.
 gpg_keys_path      = os.path.join(var_path, "gpg")
 mock_gpg_data_path = os.path.join(var_path, "gpg.mock", "data")
 mock_gpg_keys_path = os.path.join(var_path, "gpg.mock", "keys")
 gpg_path           = os.path.join(opt_path, "spack", "gpg")
+
+
+# Below paths are where Spack can write information for the user.
+# Some are caches, some are not exactly caches.
+#
+# The options that start with `default_` below are overridable in
+# `config.yaml`, but they default to use `user_cache_path/<location>`.
+#
+# You can override the top-level directory (the user cache path) by
+# setting `SPACK_USER_CACHE_PATH`. Otherwise it defaults to ~/.spack.
+#
+def _get_user_cache_path():
+    return os.path.expanduser(os.getenv('SPACK_USER_CACHE_PATH') or "~/.spack")
+
+
+user_cache_path = _get_user_cache_path()
+
+#: junit, cdash, etc. reports about builds
+reports_path = os.path.join(user_cache_path, "reports")
+
+#: spack monitor analysis directories
+monitor_path = os.path.join(reports_path, "monitor")
+
+#: git repositories fetched to compare commits to versions
+user_repos_cache_path = os.path.join(user_cache_path, 'git_repos')
+
+#: bootstrap store for bootstrapping clingo and other tools
+default_user_bootstrap_path = os.path.join(user_cache_path, 'bootstrap')
+
+#: transient caches for Spack data (virtual cache, patch sha256 lookup, etc.)
+default_misc_cache_path = os.path.join(user_cache_path, 'cache')
+
+
+# Below paths pull configuration from the host environment.
+#
+# There are three environment variables you can use to isolate spack from
+# the host environment:
+# - `SPACK_USER_CONFIG_PATH`: override `~/.spack` location (for config and caches)
+# - `SPACK_SYSTEM_CONFIG_PATH`: override `/etc/spack` configuration scope.
+# - `SPACK_DISABLE_LOCAL_CONFIG`: disable both of these locations.
+
+
+# User configuration and caches in $HOME/.spack
+def _get_user_config_path():
+    return os.path.expanduser(os.getenv('SPACK_USER_CONFIG_PATH') or "~/.spack")
+
+
+# Configuration in /etc/spack on the system
+def _get_system_config_path():
+    return os.path.expanduser(os.getenv('SPACK_SYSTEM_CONFIG_PATH') or "/etc/spack")
+
+
+#: User configuration location
+user_config_path = _get_user_config_path()
+
+#: System configuration location
+system_config_path = _get_system_config_path()
diff --git a/lib/spack/spack/test/config.py b/lib/spack/spack/test/config.py
index 704e9994ed..f2f8626b4f 100644
--- a/lib/spack/spack/test/config.py
+++ b/lib/spack/spack/test/config.py
@@ -432,6 +432,20 @@ def test_substitute_user(mock_low_high_config):
     )
 
 
+def test_substitute_user_config(mock_low_high_config):
+    user_config_path = spack.paths.user_config_path
+    assert user_config_path + '/baz' == spack_path.canonicalize_path(
+        '$user_cache_path/baz'
+    )
+
+
+def test_substitute_user_cache(mock_low_high_config):
+    user_cache_path = spack.paths.user_cache_path
+    assert user_cache_path + '/baz' == spack_path.canonicalize_path(
+        '$user_cache_path/baz'
+    )
+
+
 def test_substitute_tempdir(mock_low_high_config):
     tempdir = tempfile.gettempdir()
     assert tempdir == spack_path.canonicalize_path('$tempdir')
@@ -1167,3 +1181,59 @@ def test_internal_config_scope_cache_clearing():
     internal_scope.clear()
     # Check that this didn't affect the scope object
     assert internal_scope.sections['config'] == data
+
+
+def test_system_config_path_is_overridable(working_env):
+    p = "/some/path"
+    os.environ['SPACK_SYSTEM_CONFIG_PATH'] = p
+    assert spack.paths._get_system_config_path() == p
+
+
+def test_system_config_path_is_default_when_env_var_is_empty(working_env):
+    os.environ['SPACK_SYSTEM_CONFIG_PATH'] = ''
+    assert "/etc/spack" == spack.paths._get_system_config_path()
+
+
+def test_user_config_path_is_overridable(working_env):
+    p = "/some/path"
+    os.environ['SPACK_USER_CONFIG_PATH'] = p
+    assert p == spack.paths._get_user_config_path()
+
+
+def test_user_config_path_is_default_when_env_var_is_empty(working_env):
+    os.environ['SPACK_USER_CONFIG_PATH'] = ''
+    assert os.path.expanduser("~/.spack") == spack.paths._get_user_config_path()
+
+
+def test_local_config_can_be_disabled(working_env):
+    os.environ['SPACK_DISABLE_LOCAL_CONFIG'] = 'true'
+    cfg = spack.config._config()
+    assert "defaults" in cfg.scopes
+    assert "system" not in cfg.scopes
+    assert "site" in cfg.scopes
+    assert "user" not in cfg.scopes
+
+    os.environ['SPACK_DISABLE_LOCAL_CONFIG'] = ''
+    cfg = spack.config._config()
+    assert "defaults" in cfg.scopes
+    assert "system" not in cfg.scopes
+    assert "site" in cfg.scopes
+    assert "user" not in cfg.scopes
+
+    del os.environ['SPACK_DISABLE_LOCAL_CONFIG']
+    cfg = spack.config._config()
+    assert "defaults" in cfg.scopes
+    assert "system" in cfg.scopes
+    assert "site" in cfg.scopes
+    assert "user" in cfg.scopes
+
+
+def test_user_cache_path_is_overridable(working_env):
+    p = "/some/path"
+    os.environ['SPACK_USER_CACHE_PATH'] = p
+    assert spack.paths._get_user_cache_path() == p
+
+
+def test_user_cache_path_is_default_when_env_var_is_empty(working_env):
+    os.environ['SPACK_USER_CACHE_PATH'] = ''
+    assert os.path.expanduser("~/.spack") == spack.paths._get_user_cache_path()
diff --git a/lib/spack/spack/util/path.py b/lib/spack/spack/util/path.py
index 49bb65f7d3..8f76db93bb 100644
--- a/lib/spack/spack/util/path.py
+++ b/lib/spack/spack/util/path.py
@@ -30,6 +30,8 @@ replacements = {
     'spack': spack.paths.prefix,
     'user': getpass.getuser(),
     'tempdir': tempfile.gettempdir(),
+    'user_config_path': spack.paths.user_config_path,
+    'user_cache_path': spack.paths.user_cache_path,
 }
 
 # This is intended to be longer than the part of the install path
@@ -74,10 +76,12 @@ def substitute_config_variables(path):
 
     Spack allows paths in configs to have some placeholders, as follows:
 
-    - $spack     The Spack instance's prefix
-    - $user      The current user's username
-    - $tempdir   Default temporary directory returned by tempfile.gettempdir()
-    - $env       The active Spack environment.
+    - $env               The active Spack environment.
+    - $spack             The Spack instance's prefix
+    - $tempdir           Default temporary directory returned by tempfile.gettempdir()
+    - $user              The current user's username
+    - $user_config_path  The user configuration directory (~/.spack, unless overridden)
+    - $user_cache_path   The user cache directory (~/.spack, unless overridden)
 
     These are substituted case-insensitively into the path, and users can
     use either ``$var`` or ``${var}`` syntax for the variables. $env is only