Declare additional dependency to sphinx-build in an extension

TL,DR: From a Sphinx extension, how do I tell sphinx-build to treat an additional file as a dependency? In my immediate use case, this is the extension's source code, but the question could equally apply to some auxiliary file used by the extension.

I'm generating documentation with Sphinx using a custom extension. I'm using sphinx-build to build the documentation. For example, I use this command to generate the HTML (this is the command in the makefile generated by sphinx-quickstart):

sphinx-build -b html -d _build/doctrees   . _build/html

Since my custom extension is maintained together with the source of the documentation, I want sphinx-build to treat it as a dependency of the generated HTML (and LaTeX, etc.). So whenever I change my extension's source code, I want sphinx-build to regenerate the output.

How do I tell sphinx-build to treat an additional file as a dependency? That is not mentioned in the toctree, since it isn't part of the source. Logically, this should be something I do from my extension's setup function.

---

Sample extension (my_extension.py):

from docutils import nodes
from docutils.parsers.rst import Directive

class Foo(Directive):
    def run(self):
        node = nodes.paragraph(text='Hello world\n')
        return [node]

def setup(app):
    app.add_directive('foo', Foo)

Sample source (index.rst):

.. toctree::
   :maxdepth: 2

.. foo::

Sample conf.py (basically the output of sphinx-quickstart plus my extension):

import sys
import os
sys.path.insert(0, os.path.abspath('.'))
extensions = ['my_extension']
templates_path = ['_templates']
source_suffix = '.rst'
master_doc = 'index'
project = 'Hello directive'
copyright = '2019, Gilles'
author = 'Gilles'
version = '1'
release = '1'
language = None
exclude_patterns = ['_build']
pygments_style = 'sphinx'
todo_include_todos = False
html_theme = 'alabaster'
html_static_path = ['_static']
htmlhelp_basename = 'Hellodirectivedoc'
latex_elements = {
}
latex_documents = [
    (master_doc, 'Hellodirective.tex', 'Hello directive Documentation',
     'Gilles', 'manual'),
]
man_pages = [
    (master_doc, 'hellodirective', 'Hello directive Documentation',
     [author], 1)
]
texinfo_documents = [
    (master_doc, 'Hellodirective', 'Hello directive Documentation',
     author, 'Hellodirective', 'One line description of project.',
     'Miscellaneous'),
]

Validation of a solution:

1. Run make html (or sphinx-build as above).
2. Modify my_extension.py to replace Hello world by Hello again.
3. Run make html again.
4. The generated HTML (_build/html/index.html) must now contain Hello again instead of Hello world.

It looks like the note_dependency method in the build environment API should do what I want. But when should I call it? I tried various events but none seemed to hit the environment object in the right state. What did work was to call it from a directive.

import os
from docutils import nodes
from docutils.parsers.rst import Directive
import sphinx.application

class Foo(Directive):
    def run(self):
        self.state.document.settings.env.note_dependency(__file__)
        node = nodes.paragraph(text='Hello done\n')
        return [node]

def setup(app):
    app.add_directive('foo', Foo)

If a document contains at least one foo directive, it'll get marked as stale when the extension that introduces this directive changes. This makes sense, although it could get tedious if an extension adds many directives or makes different changes. I don't know if there's a better way.

Inspired by Luc Van Oostenryck's autodoc-C.