Document how to use Spack to replace Homebrew/Conda (#13083)

* Document how to use Spack to replace Homebrew/Conda
* Initial draft; can iterate more as features become available

lib/spack/docs/workflows.rst

@@ -1095,6 +1095,248 @@ or filesystem views.  However, it has some drawbacks:
    integrate Spack explicitly in their workflow.  Not all users are
    willing to do this.
 
+-------------------------------------
+Using Spack to Replace Homebrew/Conda
+-------------------------------------
+
+Spack is an incredibly powerful package manager, designed for supercomputers
+where users have diverse installation needs. But Spack can also be used to
+handle simple single-user installations on your laptop. Most macOS users are
+already familiar with package managers like Homebrew and Conda, where all
+installed packages are symlinked to a single central location like ``/usr/local``.
+In this section, we will show you how to emulate the behavior of Homebrew/Conda
+using :ref:`environments`!
+
+^^^^^
+Setup
+^^^^^
+
+First, let's create a new environment. We'll assume that Spack is already set up
+correctly, and that you've already sourced the setup script for your shell.
+To create a new environment, simply run:
+
+.. code-block:: console
+
+   $ spack env create myenv
+   ==> Updating view at /Users/me/spack/var/spack/environments/myenv/.spack-env/view
+   ==> Created environment 'myenv' in /Users/me/spack/var/spack/environments/myenv
+   $ spack env activate myenv
+
+Here, *myenv* can be anything you want to name your environment. Next, we can add
+a list of packages we would like to install into our environment. Let's say we
+want a newer version of Bash than the one that comes with macOS, and we want a
+few Python libraries. We can run:
+
+.. code-block:: console
+
+   $ spack add bash
+   ==> Adding bash to environment myenv
+   ==> Updating view at /Users/me/spack/var/spack/environments/myenv/.spack-env/view
+   $ spack add python@3:
+   ==> Adding python@3: to environment myenv
+   ==> Updating view at /Users/me/spack/var/spack/environments/myenv/.spack-env/view
+   $ spack add py-numpy py-scipy py-matplotlib
+   ==> Adding py-numpy to environment myenv
+   ==> Adding py-scipy to environment myenv
+   ==> Adding py-matplotlib to environment myenv
+   ==> Updating view at /Users/me/spack/var/spack/environments/myenv/.spack-env/view
+
+Each package can be listed on a separate line, or combined into a single line.
+Notice that we're explicitly asking for Python 3 here. You can use any spec
+you would normally use on the command line with other Spack commands.
+
+Next, we want to manually configure a couple of things. In the ``myenv``
+directory, we can find the ``spack.yaml`` that actually defines our environment.
+
+.. code-block:: console
+
+   $ vim ~/spack/var/spack/environments/myenv/spack.yaml
+
+.. code-block:: yaml
+
+   # This is a Spack Environment file.
+   #
+   # It describes a set of packages to be installed, along with
+   # configuration settings.
+   spack:
+     # add package specs to the `specs` list
+     specs: [bash, 'python@3:', py-numpy, py-scipy, py-matplotlib]
+     view:
+       default:
+         root: /Users/me/spack/var/spack/environments/myenv/.spack-env/view
+         projections: {}
+     config: {}
+     mirrors: {}
+     modules:
+       enable: []
+     packages: {}
+     repos: []
+     upstreams: {}
+     definitions: []
+     concretization: separately
+
+You can see the packages we added earlier in the ``specs:`` section. If you
+ever want to add more packages, you can either use ``spack add`` or manually
+edit this file.
+
+We also need to change the ``concretization:`` option. By default, Spack
+concretizes each spec *separately*, allowing multiple versions of the same
+package to coexist. Since we want a single consistent environment, we want to
+concretize all of the specs *together*.
+
+Here is what your ``spack.yaml`` looks like with these new settings, and with
+some of the sections we don't plan on using removed:
+
+.. code-block:: diff
+
+   spack:
+   -  specs: [bash, 'python@3:', py-numpy, py-scipy, py-matplotlib]
+   +  specs:
+   +  - bash
+   +  - 'python@3:'
+   +  - py-numpy
+   +  - py-scipy
+   +  - py-matplotlib
+   -  view:
+   -    default:
+   -      root: /Users/me/spack/var/spack/environments/myenv/.spack-env/view
+   -      projections: {}
+   +  view: /Users/me/spack/var/spack/environments/myenv/.spack-env/view
+   -  config: {}
+   -  mirrors: {}
+   -  modules:
+   -    enable: []
+   -  packages: {}
+   -  repos: []
+   -  upstreams: {}
+   -  definitions: []
+   +  concretization: together
+   -  concretization: separately
+
+""""""""""""""""
+Symlink location
+""""""""""""""""
+
+In the ``spack.yaml`` file above, you'll notice that by default, Spack symlinks
+all installations to ``/Users/me/spack/var/spack/environments/myenv/.spack-env/view``.
+You can actually change this to any directory you want. For example, Homebrew
+uses ``/usr/local``, while Conda uses ``/Users/me/anaconda``. In order to access
+files in these locations, you need to update ``PATH`` and other environment variables
+to point to them. Activating the Spack environment does this automatically, but
+you can also manually set them in your ``.bashrc``.
+
+.. warning::
+
+   There are several reasons why you shouldn't use ``/usr/local``:
+
+   1. If you are on macOS 10.11+ (El Capitan and newer), Apple makes it hard
+      for you. You may notice permissions issues on ``/usr/local`` due to their
+      `System Integrity Protection <https://support.apple.com/en-us/HT204899>`_.
+      By default, users don't have permissions to install anything in ``/usr/local``,
+      and you can't even change this using ``sudo chown`` or ``sudo chmod``.
+   2. Other package managers like Homebrew will try to install things to the
+      same directory. If you plan on using Homebrew in conjunction with Spack,
+      don't symlink things to ``/usr/local``.
+   3. If you are on a shared workstation, or don't have sudo priveleges, you
+      can't do this.
+
+   If you still want to do this anyway, there are several ways around SIP.
+   You could disable SIP by booting into recovery mode and running
+   ``csrutil disable``, but this is not recommended, as it can open up your OS
+   to security vulnerabilities. Another technique is to run ``spack concretize``
+   and ``spack install`` using ``sudo``. This is also not recommended.
+
+   The safest way I've found is to create your installation directories using
+   sudo, then change ownership back to the user like so:
+
+   .. code-block:: bash
+
+      for directory in .spack bin contrib include lib man share
+      do
+          sudo mkdir -p /usr/local/$directory
+          sudo chown $(id -un):$(id -gn) /usr/local/$directory
+      done
+
+   Depending on the packages you install in your environment, the exact list of
+   directories you need to create may vary. You may also find some packages
+   like Java libraries that install a single file to the installation prefix
+   instead of in a subdirectory. In this case, the action is the same, just replace
+   ``mkdir -p`` with ``touch`` in the for-loop above.
+
+   But again, it's safer just to use the default symlink location.
+
+
+^^^^^^^^^^^^
+Installation
+^^^^^^^^^^^^
+
+To actually concretize the environment, run:
+
+.. code-block:: console
+
+   $ spack concretize
+
+This will tell you which if any packages are already installed, and alert you
+to any conflicting specs.
+
+To actually install these packages and symlink them to your ``view:``
+directory, simply run:
+
+.. code-block:: console
+
+   $ spack install
+
+Now, when you type ``which python3``, it should find the one you just installed.
+
+In order to change the default shell to our newer Bash installation, we first
+need to add it to this list of acceptable shells. Run:
+
+.. code-block:: console
+
+   $ sudo vim /etc/shells
+
+and add the absolute path to your bash executable. Then run:
+
+.. code-block:: console
+
+   $ chsh -s /path/to/bash
+
+Now, when you log out and log back in, ``echo $SHELL`` should point to the
+newer version of Bash.
+
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Updating Installed Packages
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Let's say you upgraded to a new version of macOS, or a new version of Python
+was released, and you want to rebuild your entire software stack. To do this,
+simply run the following commands:
+
+.. code-block:: console
+
+   $ spack env activate myenv
+   $ spack concretize --force
+   $ spack install
+
+The ``--force`` flag tells Spack to overwrite its previous concretization
+decisions, allowing you to choose a new version of Python. If any of the new
+packages like Bash are already installed, ``spack install`` won't re-install
+them, it will keep the symlinks in place.
+
+^^^^^^^^^^^^^^
+Uninstallation
+^^^^^^^^^^^^^^
+
+If you decide that Spack isn't right for you, uninstallation is simple.
+Just run:
+
+.. code-block:: console
+
+   $ spack env activate myenv
+   $ spack uninstall --all
+
+This will uninstall all packages in your environment and remove the symlinks.
+
 ------------------------
 Using Spack on Travis-CI
 ------------------------