use upstream page redirects for Nix manual

this currently won't build because the pinned versions don't have the
redirects yet, and bumping them would require an expensive local build
that may fail as it does for Hydra.

Author: fricklerhandwerk

default.nix

@@ -30,22 +30,27 @@ let
         # Various versions of the Nix manuals, grep for (nix-manual)=
         # FIXME: This requires human interaction to update!
         # See ./CONTRIBUTING.md for details.
+        # Using Nix from Nixpkgs could simplify handling, but would requires a bit more setup here
         releases = [
           "2.19"
           "2.18"
           "2.13"
         ];
         inputName = version: pkgs.lib.strings.replaceStrings [ "." ] [ "-" ] version;
-        src = version: (import inputs."nix_${inputName version}").default.doc;
+        src = version: inputs."nix_${inputName version}";
+        manual = version: (import (src version)).default.doc;
         copy = version: ''
-          cp -R ${src version}/share/doc/nix/manual/* $out/manual/nix/${version}
+          cp -Rf ${manual version}/share/doc/nix/manual/* $out/manual/nix/${version}
+          # use upstream page redirects
+          sed 's#^\(.*\) \(.*\) #/manual/nix/${version}\1 /manual/nix/${version}\2 #g' ${src version}/doc/manual/_redirects >> $out/_redirects
         '';
       in
       with pkgs.lib.strings;
       ''
-        mkdir -p $out/manual/nix/{${concatStringsSep "," releases}}
-        ${concatStringsSep "\n" (map copy releases)}
+        # NOTE: the comma in the shell expansion makes it also work for singleton lists
+        mkdir -p $out/manual/nix/{${concatStringsSep "," releases},}
         cp -R build/html/* $out/
+        ${concatStringsSep "\n" (map copy releases)}
       '';
   };