callHackage: make 'all-cabal-hashes' repository overridable

This commit changes callHackage to use a deterministic version of the Hackage
checkout from https://github.com/commercialhaskell/all-cabal-hashes by default.
This means that packages uploaded to Hackage after today will be available to
callHackage only after "pkgs/data/misc/hackage/default.nix" has been updated.

People who want the previous behavior where we always had the latest version of
Hackage available -- at the cost of frequent downloads from Github --, can add
the following override to their "~/.nixpkgs/config.nix" file:

  {
    packageOverrides = super: {
      all-cabal-hashes = builtins.fetchTarball "https://github.com/commercialhaskell/all-cabal-hashes/archive/hackage.tar.gz";
    };
  }

pkgs/data/misc/hackage/default.nix

@@ -0,0 +1,11 @@
+{ fetchFromGitHub }:
+
+# Use builtins.fetchTarball "https://github.com/commercialhaskell/all-cabal-hashes/archive/hackage.tar.gz"
+# instead if you want the latest Hackage automatically at the price of frequent re-downloads.
+
+fetchFromGitHub {
+  owner = "commercialhaskell";
+  repo = "all-cabal-hashes";
+  rev = "ee101d34ff8bd59897aa2eb0a124bcd3fb47ceec";
+  sha256 = "1hky0s2c1rv1srfnhbyi3ny14rnfnnp2j9fsr4ylz76xyxgjf5wm";
+}

pkgs/development/haskell-modules/default.nix

@@ -1,4 +1,4 @@
-{ pkgs, stdenv, ghc
+{ pkgs, stdenv, ghc, all-cabal-hashes
 , compilerConfig ? (self: super: {})
 , packageSetConfig ? (self: super: {})
 , overrides ? (self: super: {})
@@ -6,14 +6,6 @@
 
 let
 
-  allCabalFiles = stdenv.mkDerivation {
-    name = "all-cabal-hashes-0";
-    buildCommand = ''
-      mkdir -p $out
-      tar -C $out --strip-components=1 -x -f ${builtins.fetchurl "https://github.com/commercialhaskell/all-cabal-hashes/archive/hackage.tar.gz"}
-    '';
-  };
-
   inherit (stdenv.lib) fix' extends;
 
   haskellPackages = self:
@@ -69,8 +61,8 @@ let
         installPhase = ''
           export HOME="$TMP"
           mkdir $out
-          hash=$(sed -e 's/.*"SHA256":"//' -e 's/".*$//' ${allCabalFiles}/${name}/${version}/${name}.json)
-          cabal2nix --compiler=${self.ghc.name} --system=${stdenv.system} --sha256=$hash ${allCabalFiles}/${name}/${version}/${name}.cabal >$out/default.nix
+          hash=$(sed -e 's/.*"SHA256":"//' -e 's/".*$//' ${all-cabal-hashes}/${name}/${version}/${name}.json)
+          cabal2nix --compiler=${self.ghc.name} --system=${stdenv.system} --sha256=$hash ${all-cabal-hashes}/${name}/${version}/${name}.cabal >$out/default.nix
         '';
       };
 

pkgs/top-level/all-packages.nix

@@ -4821,6 +4821,8 @@ in
     postFixup = "rm -rf $out/lib $out/nix-support $out/share/doc";
   });
 
+  all-cabal-hashes = callPackage ../data/misc/hackage/default.nix { };
+
   inherit (ocamlPackages) haxe;
 
   hxcpp = callPackage ../development/compilers/haxe/hxcpp.nix { };