mu: allow configuring the BatchSize

pkgs/tools/networking/mu/default.nix

@@ -1,7 +1,8 @@
 { stdenv, fetchFromGitHub, sqlite, pkgconfig, autoreconfHook, pmccabe
 , xapian, glib, gmime3, texinfo , emacs, guile
 , gtk3, webkitgtk, libsoup, icu
-, withMug ? false }:
+, withMug ? false
+, batchSize ? null }:
 
 stdenv.mkDerivation rec {
   pname = "mu";
@@ -14,6 +15,11 @@ stdenv.mkDerivation rec {
     sha256 = "1i9chd8ks1q4g5pyscsir6pw4kahkx3k8ckzbi8j3gr6jz1qzzsg";
   };
 
+  postPatch = stdenv.lib.optionalString (batchSize != null) ''
+    sed -i lib/mu-store.cc --regexp-extended \
+      -e 's@(constexpr auto BatchSize).*@\1 = ${toString batchSize};@'
+  '';
+
   buildInputs = [
     sqlite xapian glib gmime3 texinfo emacs guile libsoup icu
   ] ++ stdenv.lib.optionals withMug [ gtk3 webkitgtk ];