sage: fix docbuild

This is only a hotfix without really knowing the issue. See
https://groups.google.com/forum/#!msg/sage-packaging/VU4h8IWGFLA/mrmCMocYBwAJ

pkgs/applications/science/math/sage/patches/revert-sphinx-always-fork.patch

@@ -0,0 +1,71 @@
+commit f1c59929c3c180ac283334c2b3c901ac8c82f6b1
+Author: Timo Kaufmann <timokau@zoho.com>
+Date:   Sat Oct 20 20:07:41 2018 +0200
+
+    Revert "Something related to the sphinxbuild seems to be leaking memory"
+    
+    This reverts commit 7d85dc796c58c3de57401bc22d3587b94e205091.
+
+diff --git a/src/sage_setup/docbuild/__init__.py b/src/sage_setup/docbuild/__init__.py
+index 0b24b1a60b..084c3f89d7 100644
+--- a/src/sage_setup/docbuild/__init__.py
++++ b/src/sage_setup/docbuild/__init__.py
+@@ -265,29 +265,35 @@ class DocBuilder(object):
+     # import the customized builder for object.inv files
+     inventory = builder_helper('inventory')
+ 
+-def build_many(target, args):
+-    # Pool() uses an actual fork() to run each new instance. This is important
+-    # for performance reasons, i.e., don't use a forkserver when it becomes
+-    # available with Python 3: Here, sage is already initialized which is quite
+-    # costly, with a forkserver we would have to reinitialize it for every
+-    # document we build. At the same time, don't serialize this by taking the
+-    # pool (and thus the call to fork()) out completely: The call to Sphinx
+-    # leaks memory, so we need to build each document in its own process to
+-    # control the RAM usage.
+-    from multiprocessing import Pool
+-    pool = Pool(NUM_THREADS, maxtasksperchild=1)
+-    # map_async handles KeyboardInterrupt correctly. Plain map and
+-    # apply_async does not, so don't use it.
+-    x = pool.map_async(target, args, 1)
+-    try:
+-        ret = x.get(99999)
+-        pool.close()
+-        pool.join()
+-    except Exception:
+-        pool.terminate()
+-        if ABORT_ON_ERROR:
+-            raise
+-    return ret
++if NUM_THREADS > 1:
++    def build_many(target, args):
++        from multiprocessing import Pool
++        pool = Pool(NUM_THREADS, maxtasksperchild=1)
++        # map_async handles KeyboardInterrupt correctly. Plain map and
++        # apply_async does not, so don't use it.
++        x = pool.map_async(target, args, 1)
++        try:
++            ret = x.get(99999)
++            pool.close()
++            pool.join()
++        except Exception:
++            pool.terminate()
++            if ABORT_ON_ERROR:
++                raise
++        return ret
++else:
++    def build_many(target, args):
++        results = []
++
++        for arg in args:
++            try:
++                results.append(target(arg))
++            except Exception:
++                if ABORT_ON_ERROR:
++                    raise
++
++        return results
++
+ 
+ ##########################################
+ #      Parallel Building Ref Manual      #

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

@@ -27,6 +27,11 @@ stdenv.mkDerivation rec {
     # https://trac.sagemath.org/ticket/25316
     # https://github.com/python/cpython/pull/7476
     ./patches/python-5755-hotpatch.patch
+
+    # Revert the commit that made the sphinx build fork even in the single thread
+    # case. For some yet unknown reason, that breaks the docbuild on nix and archlinux.
+    # See https://groups.google.com/forum/#!msg/sage-packaging/VU4h8IWGFLA/mrmCMocYBwAJ.
+    ./patches/revert-sphinx-always-fork.patch
   ];
 
   packageUpgradePatches = [