sage: fix docbuild

The docbuild (or rather its tests) failed because sphinx built some
English parts of the documentation in Russian. The added debian patch
fixes that by isolating docbuilding threads in sphinx using
proper subprocesses.

pkgs/applications/science/math/sage/sage-src.nix

@@ -50,6 +50,15 @@ stdenv.mkDerivation rec {
 
     # Fixes a potential race condition which can lead to transient doctest failures.
     ./patches/fix-ecl-race.patch
+
+    # Parallelize docubuild using subprocesses, fixing an isolation issue. See
+    # https://groups.google.com/forum/#!topic/sage-packaging/YGOm8tkADrE
+    (fetchpatch {
+      name = "sphinx-docbuild-subprocesses.patch";
+      url = "https://salsa.debian.org/science-team/sagemath/raw/8a215b17e6f791ddfae6df8ce6d01dfb89acb434/debian/patches/df-subprocess-sphinx.patch";
+      sha256 = "07p9i0fwjgapmfvmi436yn6v60p8pvmxqjc93wsssqgh5kd8qw3n";
+      stripLen = 1;
+    })
   ];
 
   # Patches needed because of package updates. We could just pin the versions of