Fix relative references in SVGs generated by `sphinx.ext.graphviz` (#11078)

Co-authored-by: Ralf Grubenmann <ralf.grubenmann@gmail.com>
Co-authored-by: Adam Turner <9087854+aa-turner@users.noreply.github.com>

tests/roots/test-ext-graphviz/_static/images/test.svg

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg version="1.1"
+     height="128" width="128"
+     xmlns="http://www.w3.org/2000/svg">
+
+  <rect width="100%" height="100%" fill="red" />
+
+</svg>

tests/roots/test-ext-graphviz/conf.py

@@ -1,2 +1,3 @@
 extensions = ['sphinx.ext.graphviz']
 exclude_patterns = ['_build']
+html_static_path = ["_static"]

tests/roots/test-ext-graphviz/index.rst

@@ -31,3 +31,13 @@ Hello |graph| graphviz world
    :align: center
 
    centered
+
+.. graphviz::
+   :align: center
+
+   digraph test {
+     foo [label="foo", URL="#graphviz", target="_parent"]
+     bar [label="bar", image="./_static/images/test.svg"]
+     baz [label="baz", URL="./_static/images/test.svg"]
+     foo -> bar -> baz
+   }

tests/test_ext_graphviz.py

@@ -82,6 +82,17 @@ def test_graphviz_svg_html(app, status, warning):
             r'</div>')
     assert re.search(html, content, re.S)
 
+    image_re = r'.*data="([^"]+)".*?digraph test'
+    image_path_match = re.search(image_re, content, re.S)
+    assert image_path_match
+
+    image_path = image_path_match.group(1)
+    image_content = (app.outdir / image_path).read_text(encoding='utf8')
+    assert '"./_static/' not in image_content
+    assert '<ns0:image ns1:href="../_static/images/test.svg"' in image_content
+    assert '<ns0:a ns1:href="../_static/images/test.svg"' in image_content
+    assert '<ns0:a ns1:href="..#graphviz"' in image_content
+
 
 @pytest.mark.sphinx('latex', testroot='ext-graphviz')
 @pytest.mark.usefixtures('if_graphviz_found')