Support VISUAL environment variable for editing. (#10898)

If the user has set the environment variable VISUAL, it will be used
in preference to EDITOR for all Spack editing activities. If VISUAL
is not set or fails (perhaps due to a lack of graphical editing
capabilities),EDITOR will be used instead. We fall back to one of
several common editors if neither bears fruit.

This feature has been tailored to:

* Provide identical behavior to the previous implementation in the
  case that VISUAL is not set.
* Not require any change to code utilizing the editor feature.
* Follow usual UNIX behavior concerning VISUAL and EDITOR.

lib/spack/spack/util/editor.py

@@ -5,31 +5,37 @@
 
 """Module for finding the user's preferred text editor.
 
-Defines one variable: ``editor``, which is a
-``spack.util.executable.Executable`` object that can be called to invoke
-the editor.
-
-If no ``editor`` is found, an ``EnvironmentError`` is raised when
-``editor`` is invoked.
+Defines one function, editor(), which invokes the editor defined by the
+user's VISUAL environment variable if set. We fall back to the editor
+defined by the EDITOR environment variable if VISUAL is not set or the
+specified editor fails (e.g. no DISPLAY for a graphical editor). If
+neither variable is set, we fall back to one of several common editors,
+raising an EnvironmentError if we are unable to find one.
 """
+import copy
 import os
 
 from spack.util.executable import Executable, which
 
-# Set up the user's editor
-# $EDITOR environment variable has the highest precedence
-editor = os.environ.get('EDITOR')
+_visual_exe\
+    = Executable(os.environ['VISUAL']) if 'VISUAL' in os.environ else None
+_editor_exe\
+    = Executable(os.environ['EDITOR']) \
+    if 'EDITOR' in os.environ else which('vim', 'vi', 'emacs', 'nano')
+
 
-# if editor is not set, use some sensible defaults
-if editor is not None:
-    editor = Executable(editor)
-else:
-    editor = which('vim', 'vi', 'emacs', 'nano')
+# Invoke the user's editor.
+def editor(*args, **kwargs):
+    if _visual_exe:
+        visual_kwargs = copy.copy(kwargs)
+        visual_kwargs['fail_on_error'] = False
+        _visual_exe(*args, **visual_kwargs)
+        if _visual_exe.returncode == 0:
+            return  # Otherwise, fall back to EDITOR.
 
-# If there is no editor, only raise an error if we actually try to use it.
-if not editor:
-    def editor_not_found(*args, **kwargs):
+    if _editor_exe:
+        _editor_exe(*args, **kwargs)
+    else:
         raise EnvironmentError(
-            'No text editor found! Please set the EDITOR environment variable '
-            'to your preferred text editor.')
-    editor = editor_not_found
+            'No text editor found! Please set the VISUAL and/or EDITOR '
+            'environment variable(s) to your preferred text editor.')