imgmath: Move dest image file even with embed enabled

We need the generated image files to be in one common
folder to avoid having to re-generate several times the same image
(the sha1 naming of the file acts as a cache to just read the depth)

The location should be possibly common to all workers,
that's why I choosed to just keep the original destination folder,
ie the file is just moved from tempdir to the build folder before.

This significantly drops the build times from 84min to 26min,
but obviously leaves the unused image files in the output folder.

Another solution would be to use another global temporary folder
shared across all the workers, or to add a delete pass after all workers
have joined, but there does not seem to be an api to do that.

sphinx/ext/imgmath.py

@@ -266,6 +266,10 @@ def render_math(
         self.builder._imgmath_warned_image_translator = True  # type: ignore
         return None, None, None, None
 
+    # Move generated image on tempdir to build dir
+    ensuredir(path.dirname(outfn))
+    shutil.move(imgpath, outfn)
+
     return relfn, depth, imgpath, outfn
 
 
@@ -306,20 +310,17 @@ def html_visit_math(self: HTMLTranslator, node: nodes.math) -> None:
         sm.walkabout(self)
         logger.warning(__('display latex %r: %s'), node.astext(), msg)
         raise nodes.SkipNode from exc
-    if self.builder.config.imgmath_embed:
-        image_format = self.builder.config.imgmath_image_format.lower()
-        img_src = render_maths_to_base64(image_format, imgpath)
-    else:
-        # Move generated image on tempdir to build dir
-        if imgpath is not None:
-            ensuredir(path.dirname(outfn))
-            shutil.move(imgpath, outfn)
-        img_src = fname
-    if img_src is None:
+
+    if fname is None:
         # something failed -- use text-only as a bad substitute
         self.body.append('<span class="math">%s</span>' %
                          self.encode(node.astext()).strip())
     else:
+        if self.builder.config.imgmath_embed:
+            image_format = self.builder.config.imgmath_image_format.lower()
+            img_src = render_maths_to_base64(image_format, outfn)
+        else:
+            img_src = fname
         c = f'<img class="math" src="{img_src}"' + get_tooltip(self, node)
         if depth is not None:
             c += f' style="vertical-align: {-depth:d}px"'
@@ -348,20 +349,17 @@ def html_visit_displaymath(self: HTMLTranslator, node: nodes.math_block) -> None
         self.body.append('<span class="eqno">(%s)' % number)
         self.add_permalink_ref(node, _('Permalink to this equation'))
         self.body.append('</span>')
-    if self.builder.config.imgmath_embed:
-        image_format = self.builder.config.imgmath_image_format.lower()
-        img_src = render_maths_to_base64(image_format, imgpath)
-    else:
-        # Move generated image on tempdir to build dir
-        if imgpath is not None:
-            ensuredir(path.dirname(outfn))
-            shutil.move(imgpath, outfn)
-        img_src = fname
-    if img_src is None:
+
+    if fname is None:
         # something failed -- use text-only as a bad substitute
         self.body.append('<span class="math">%s</span></p>\n</div>' %
                          self.encode(node.astext()).strip())
     else:
+        if self.builder.config.imgmath_embed:
+            image_format = self.builder.config.imgmath_image_format.lower()
+            img_src = render_maths_to_base64(image_format, outfn)
+        else:
+            img_src = fname
         self.body.append(f'<img src="{img_src}"' + get_tooltip(self, node) +
                          '/></p>\n</div>')
     raise nodes.SkipNode