docs: automatically track manual dependencies

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/sphinx/depfile.py

@@ -0,0 +1,51 @@
+# coding=utf-8
+#
+# QEMU depfile generation extension
+#
+# Copyright (c) 2020 Red Hat, Inc.
+#
+# This work is licensed under the terms of the GNU GPLv2 or later.
+# See the COPYING file in the top-level directory.
+
+"""depfile is a Sphinx extension that writes a dependency file for
+   an external build system"""
+
+import os
+import sphinx
+
+__version__ = '1.0'
+
+def get_infiles(env):
+    for x in env.found_docs:
+        yield env.doc2path(x)
+        yield from ((os.path.join(env.srcdir, dep)
+                    for dep in env.dependencies[x]))
+
+def write_depfile(app, env):
+    if not env.config.depfile:
+        return
+
+    # Using a directory as the output file does not work great because
+    # its timestamp does not necessarily change when the contents change.
+    # So create a timestamp file.
+    if env.config.depfile_stamp:
+        with open(env.config.depfile_stamp, 'w') as f:
+            pass
+
+    with open(env.config.depfile, 'w') as f:
+        print((env.config.depfile_stamp or app.outdir) + ": \\", file=f)
+        print(*get_infiles(env), file=f)
+        for x in get_infiles(env):
+            print(x + ":", file=f)
+
+
+def setup(app):
+    app.add_config_value('depfile', None, 'env')
+    app.add_config_value('depfile_stamp', None, 'env')
+    app.connect('env-updated', write_depfile)
+
+    return dict(
+        version = __version__,
+        parallel_read_safe = True,
+        parallel_write_safe = True
+    )