gradle-env.nix: Support fetchers per URL scheme

gradle-env.nix

@@ -19,7 +19,15 @@
 #         '';
 #       }
 
-{ stdenv, buildEnv, fetchurl, gradleGen, writeText, writeTextDir }:
+{ lib
+, stdenv
+, buildEnv
+, fetchs3
+, fetchurl
+, gradleGen
+, writeText
+, writeTextDir
+}:
 
 {
 # Path to the environment spec generated by gradle2nix (e.g. gradle-env.json).
@@ -38,29 +46,68 @@
 , extraInit ? ""
 # Override the default JDK used to run Gradle itself.
 , buildJdk ? null
+# Override functions which fetch dependency artifacts.
+# Keys in this set are URL schemes such as "https" or "s3".
+# Values are functions which take a dependency in the form
+# `{ urls, sha256 }` and fetch into the Nix store. For example:
+#
+#   {
+#     s3 = { urls, sha256 }: fetchs3 {
+#       s3url = builtins.head urls;
+#       inherit sha256;
+#       region = "us-west-2";
+#       credentials = {
+#         access_key_id = "foo";
+#         secret_access_key = "bar";
+#       };
+#     };
+#   }
+, fetchers ? { }
 , ... } @ args:
 
 let
   inherit (builtins)
-    attrValues concatStringsSep filter fromJSON match replaceStrings sort;
+    attrValues concatStringsSep filter fromJSON getAttr head match
+    replaceStrings sort;
 
-  inherit (stdenv.lib)
-    assertMsg concatMapStringsSep groupBy' hasSuffix last mapAttrs
+  inherit (lib)
+    assertMsg concatMapStringsSep groupBy' hasSuffix hasPrefix last mapAttrs
     mapAttrsToList optionalString readFile removeSuffix unique versionAtLeast
     versionOlder;
 
-  mkDep = depSpec: stdenv.mkDerivation {
-    inherit (depSpec) name;
+  fetchers' = {
+    http = fetchurl;
+    https = fetchurl;
+    s3 = { urls, sha256 }: fetchs3 {
+      s3url = head urls;
+      inherit sha256;
+    };
+  } // fetchers;
 
-    src = fetchurl {
-      inherit (depSpec) urls sha256;
+  # Fetch urls using the scheme for the first entry only; there isn't a
+  # straightforward way to tell Nix to try multiple fetchers in turn
+  # and short-circuit on the first successful fetch.
+  fetch = { urls, sha256 }:
+    let
+      first = head urls;
+      scheme = head (builtins.match "([a-z0-9+.-]+)://.*" first);
+      fetch' = getAttr scheme fetchers';
+      urls' = filter (hasPrefix scheme) urls;
+    in
+      fetch' { urls = urls'; inherit sha256; };
+
+  mkDep = { name, path, urls, sha256, ... }: stdenv.mkDerivation {
+    inherit name;
+
+    src = fetch {
+      inherit urls sha256;
     };
 
     phases = "installPhase";
 
     installPhase = ''
-      mkdir -p $out/${depSpec.path}
-      ln -s $src $out/${depSpec.path}/${depSpec.name}
+      mkdir -p $out/${path}
+      ln -s $src $out/${path}/${name}
     '';
   };