nix: reduce closure size of default package in flake.nix

Splitting the executable in a separate output avoids distributing all of
the libraries and documentation, which are not needed when just running
PostgREST.

Reduces closure size from 4.3G to 73.9M for the flake exported packages.

Resolves #4149

default.nix

@@ -62,8 +62,10 @@ let
       { name = "postgresql-12"; postgresql = pkgs.postgresql_12.withPackages (p: [ p.postgis p.pg_safeupdate ]); }
     ];
 
+  haskellPackages = pkgs.haskell.packages."${compiler}";
+
   # Dynamic derivation for PostgREST
-  postgrest = pkgs.lib.pipe (pkgs.haskell.packages."${compiler}".callCabal2nix name src { }) [
+  postgrest = pkgs.lib.pipe (haskellPackages.callCabal2nix name src { }) [
     # To allow ghc-datasize to be used.
     lib.disableLibraryProfiling
     # We are never going to use dynamic haskell libraries anyway. "Dynamic" refers to how
@@ -84,9 +86,13 @@ rec {
 
   # Derivation for the PostgREST Haskell package, including the executable,
   # libraries and documentation. We disable running the test suite on Nix
-  # builds, as they require a database to be set up.
-  postgrestPackage =
-    lib.dontCheck postgrest;
+  # builds, as they require a database to be set up. We split the binary
+  # into a separate output, so that the default distribution via flake.nix
+  # has a much smaller closure size.
+  postgrestPackage = pkgs.lib.pipe postgrest [
+    lib.dontCheck
+    lib.enableSeparateBinOutput
+  ];
 
   # Profiled dynamic executable.
   postgrestProfiled = pkgs.lib.pipe postgrestPackage [

flake.nix

@@ -33,8 +33,8 @@
     in
     {
       packages = genSystems (attrs: {
-        default = attrs.postgrestPackage;
-        profiled = attrs.postgrestProfiled;
+        default = attrs.postgrestPackage.bin;
+        profiled = attrs.postgrestProfiled.bin;
       } // nixpkgs.lib.optionalAttrs (attrs ? postgrestStatic) {
         static = attrs.postgrestStatic;
       });
@@ -42,7 +42,7 @@
       apps = genSystems (attrs: {
         default = {
           type = "app";
-          program = "${attrs.postgrestStatic or attrs.postgrestPackage}/bin/postgrest";
+          program = "${attrs.postgrestStatic or attrs.postgrestPackage.bin}/bin/postgrest";
           meta.description = "REST API for any Postgres database";
         };
       });

nix/tools/withTools.nix

@@ -351,12 +351,13 @@ let
         rm -f result
         if [ -z "''${PGRST_BUILD_CABAL:-}" ]; then
           echo -n "Building postgrest (nix)... "
-          nix-build -A postgrestPackage > "$tmpdir"/build.log 2>&1 || {
+          # Using lib.getBin to also make this work with older checkouts, where .bin was not a thing, yet.
+          nix-build -E 'with import ./. {}; pkgs.lib.getBin postgrestPackage' > "$tmpdir"/build.log 2>&1 || {
             echo "failed, output:"
             cat "$tmpdir"/build.log
             exit 1
           }
-          PGRST_CMD=./result/bin/postgrest
+          PGRST_CMD=$(echo ./result*/bin/postgrest)
         else
           echo -n "Building postgrest (cabal)... "
           postgrest-build