editing: add higher-precedence SPACK_EDITOR environment variable

Other tools like git support `GIT_EDITOR` which takes higher precedence than the
standard `VISUAL` or `EDITOR` variables. This adds similar support for Spack, in the
`SPACK_EDITOR` env var.

- [x] consolidate editor code from hooks into `spack.util.editor`
- [x] add more editor tests
- [x] add support for `SPACK_EDITOR`
- [x] add a documentation section for controlling the editor and reference it

lib/spack/docs/getting_started.rst

@@ -368,7 +368,8 @@ Manual compiler configuration
 
 If auto-detection fails, you can manually configure a compiler by
 editing your ``~/.spack/<platform>/compilers.yaml`` file.  You can do this by running
-``spack config edit compilers``, which will open the file in your ``$EDITOR``.
+``spack config edit compilers``, which will open the file in
+:ref:`your favorite editor <controlling-the-editor>`.
 
 Each compiler configuration in the file looks like this:
 

lib/spack/docs/packaging_guide.rst

@@ -229,7 +229,8 @@ always choose to download just one tarball initially, and run
 
 Spack automatically creates a directory in the appropriate repository,
 generates a boilerplate template for your package, and opens up the new
-``package.py`` in your favorite ``$EDITOR``:
+``package.py`` in your favorite ``$EDITOR`` (see :ref:`controlling-the-editor`
+for details):
 
 .. code-block:: python
    :linenos:
@@ -335,6 +336,31 @@ The rest of the tasks you need to do are as follows:
    :ref:`installation_process` is
    covered in detail later.
 
+
+.. _controlling-the-editor:
+
+^^^^^^^^^^^^^^^^^^^^^^^^^
+Controlling the editor
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When Spack needs to open an editor for you (e.g., for commands like
+:ref:`cmd-spack-create` or :ref:`spack edit`, it looks at several environment variables
+to figure out what to use. The order of precedence is:
+
+* ``SPACK_EDITOR``: highest precedence, in case you want something specific for Spack;
+* ``VISUAL``: standard environment variable for full-screen editors like ``vim`` or ``emacs``;
+* ``EDITOR``: older environment variable for your editor.
+
+You can set any of these to the command you want to run, e.g., in ``bash`` you might run
+one of these::
+
+  export VISUAL=vim
+  export EDITOR="emacs -nw"
+  export SPACK_EDITOR=nano
+
+If Spack finds none of these variables set, it will look for ``vim``, ``vi``, ``emacs``,
+``nano``, and ``notepad``, in that order.
+
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 Non-downloadable software
 ^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -1787,7 +1813,7 @@ for instructions on setting up a mirror.
 After running ``spack install pgi``, the first thing that will happen is
 Spack will create a global license file located at
 ``$SPACK_ROOT/etc/spack/licenses/pgi/license.dat``. It will then open up the
-file using the editor set in ``$EDITOR``, or vi if unset. It will look like
+file using :ref:`your favorite editor <controlling-the-editor>`. It will look like
 this:
 
 .. code-block:: sh

lib/spack/spack/test/util/editor.py

@@ -19,7 +19,7 @@
 
 
 # env vars that control the editor
-EDITOR_VARS = ["VISUAL", "EDITOR"]
+EDITOR_VARS = ["SPACK_EDITOR", "VISUAL", "EDITOR"]
 
 
 @pytest.fixture(scope="module", autouse=True)
@@ -104,6 +104,30 @@ def assert_exec(exe, args):
     assert ed.editor("/path/to/file", exec_fn=assert_exec)
 
 
+def test_editor_precedence(good_exe, gvim_exe, vim_exe, bad_exe):
+    """Ensure we prefer editor variables in order of precedence."""
+    os.environ["SPACK_EDITOR"] = good_exe
+    os.environ["VISUAL"] = gvim_exe
+    os.environ["EDITOR"] = vim_exe
+    correct_exe = good_exe
+
+    def assert_callback(exe, args):
+        result = ed.executable(exe, args)
+        if result == 0:
+            assert exe == correct_exe
+        return result
+
+    ed.editor(exec_fn=assert_callback)
+
+    os.environ["SPACK_EDITOR"] = bad_exe
+    correct_exe = gvim_exe
+    ed.editor(exec_fn=assert_callback)
+
+    os.environ["VISUAL"] = bad_exe
+    correct_exe = vim_exe
+    ed.editor(exec_fn=assert_callback)
+
+
 def test_find_exe_from_env_var_no_editor():
     if "FOO" in os.environ:
         os.environ.unset("FOO")

lib/spack/spack/util/editor.py

@@ -127,6 +127,8 @@ def try_env_var(var):
         return try_exec(exe, full_args, var)
 
     # try standard environment variables
+    if try_env_var("SPACK_EDITOR"):
+        return True
     if try_env_var("VISUAL"):
         return True
     if try_env_var("EDITOR"):