haskellPackages.pandoc: build with -f+embed_data_files (#505790)

pkgs/by-name/pa/pandoc/package.nix

@@ -15,8 +15,6 @@ let
 
 in
 (haskell.lib.compose.overrideCabal (drv: {
-  configureFlags = drv.configureFlags or [ ] ++ [ "-fembed_data_files" ];
-  buildDepends = drv.buildDepends or [ ] ++ [ pandoc-cli.scope.file-embed ];
   buildTools = (drv.buildTools or [ ]) ++ [
     removeReferencesTo
     installShellFiles

pkgs/development/haskell-modules/configuration-nix.nix

@@ -347,6 +347,26 @@ builtins.intersectAttrs super {
   # Heist's test suite requires system pandoc
   heist = addTestToolDepend pkgs.pandoc super.heist;
 
+  pandoc = lib.pipe super.pandoc [
+    # pandoc can't do I/O (including reading data files). See
+    # <https://pandoc.org/pandoc-server.html#description>.
+    # It's simpler to just enable this globally rather than building multiple pandocs.
+    (enableCabalFlag "embed_data_files")
+    # pandoc still references these data files and we can't prevent their installation.
+    # pkgs.pandoc removes the reference to $out, so having everything in one place is best.
+    (overrideCabal { enableSeparateDataOutput = false; })
+  ];
+
+  # So pandoc-server can be used:
+  # https://pandoc.org/MANUAL.html#running-pandoc-as-a-web-server
+  # TODO(@sternenseemann): provide pandoc-server.cgi symlink?
+  pandoc-cli = overrideCabal (drv: {
+    postInstall = ''
+      ${drv.postInstall or ""}
+      ln -s "''${!outputBin}/bin/pandoc" "''${!outputBin}/bin/pandoc-server"
+    '';
+  }) super.pandoc-cli;
+
   # Use Nixpkgs' double-conversion library
   double-conversion = disableCabalFlag "embedded_double_conversion" (
     addBuildDepends [ pkgs.double-conversion ] super.double-conversion