Trac #33608: Building documentation is broken in Sage 9.6.beta6

Building the documentation is currently broken and fails with the
following error messages:

$ make doc-clean; make doc-uninstall; make
{{{
...
[sagemath_doc_html-none] [semirings] building [html]: targets for 3
source files that are out of date
[sagemath_doc_html-none] [semirings] dumping search index in English
(code: en)... done
[sagemath_doc_html-none] [semirings] The HTML pages are in
../../home/gitpod/sage-local/share/doc/sage/html/en/reference/semirings.
[sagemath_doc_html-none] [semirings] Extension error
(matplotlib.sphinxext.plot_directive):
[sagemath_doc_html-none] [semirings] Handler <function _copy_css_file at
0x7f0673973e50> for event 'build-finished' threw an exception
(exception: [Errno 17] File exists: '/home/gitpod/sage-
local/share/doc/sage/html/en/reference/semirings/_static')
[sagemath_doc_html-none] Build finished. The built documents can be
found in /home/gitpod/sage-
local/share/doc/sage/html/en/reference/semirings
...
}}}

URL: https://trac.sagemath.org/33608
Reported by: gh-tobiasdiez
Ticket author(s): Kwankyu Lee
Reviewer(s): Tobias Diez

src/sage_docbuild/ext/multidocs.py

@@ -226,7 +226,7 @@ def citation_dir(app: Sphinx) -> Path:
         if dirs[0] == '/':
             dirs.pop(0)
         tail = dirs[1:]
-        citedir = (sage_doc / "inventory").joinpath(*tail) 
+        citedir = (sage_doc / "inventory").joinpath(*tail)
     else:
         citedir = outdir / "inventory"
     os.makedirs(citedir, exist_ok=True)
@@ -303,6 +303,10 @@ def copy_static_files(self):
                     shutil.rmtree(static_dir)
                 except OSError:
                     os.unlink(static_dir)
+            # This ensures that the symlink we are creating points to an
+            # existing directory. See trac #33608.
+            os.makedirs(os.path.join(app.builder.outdir, master_static_dir),
+                        exist_ok=True)
             os.symlink(master_static_dir, static_dir)
 
         app.builder.copy_static_files = link_static_files
@@ -313,10 +317,9 @@ def copy_static_files(self):
         app.emit('env-check-consistency', app.env)
 
 
-
 def setup(app: Sphinx):
     app.add_config_value('multidocs_is_master', True, True)
     app.add_config_value('multidocs_subdoc_list', [], True)
     app.add_config_value('multidoc_first_pass', 0, False)   # 1 = deactivate the loading of the inventory
     app.connect('builder-inited', init_subdoc)
-    return {'parallel_read_safe': True} 
+    return {'parallel_read_safe': True}