Bug 1213959 - Set SHELL in Read the Docs environment; r=ted

The Read the Docs environment doesn't have SHELL defined. This causes
mach.mixin.process's import to fail, as it insists on finding an active
shell. While we could fix mach.mixin.process to not raise if the
variable is not set, this is a bit more work and has wider build system
and mach implications. So we employ a quick hack instead.

DONTBUILD (NPOTB)

tools/docs/conf.py

@@ -57,10 +57,19 @@ version = re.sub(r'[ab]\d+$', '', release)
 exclude_patterns = ['_build', '_staging', '_venv']
 pygments_style = 'sphinx'
 
-# Read The Docs can't import sphinx_rtd_theme, so don't import it there.
+# We need to perform some adjustment of the settings and environment
+# when running on Read The Docs.
 on_rtd = os.environ.get('READTHEDOCS', None) == 'True'
 
-if not on_rtd:
+if on_rtd:
+    # SHELL isn't set on RTD and mach.mixin.process's import raises if a
+    # shell-related environment variable can't be found. Set the variable here
+    # to hack us into working on RTD.
+    assert 'SHELL' not in os.environ
+    os.environ['SHELL'] = '/bin/bash'
+else:
+    # We only need to set the RTD theme when not on RTD because the RTD
+    # environment handles this otherwise.
     import sphinx_rtd_theme
     html_theme = 'sphinx_rtd_theme'
     html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]