Rewrite sources.nix to support several use cases

sources.nix now supports the following use cases:
- nixpkgs is not set in the NIX_PATH
- nixpkgs is set to ./.
- sources.nix is evaluated with the restricted mode (no builtins)

Main.hs

@@ -668,10 +668,48 @@ initNixSourcesNixContent :: String
 initNixSourcesNixContent = [s|
 # This file has been generated by Niv.
 
-# A record, from name to path, of the third-party packages
+# These expressions are in charge of fetching sources defined in the
+# sources.json file.
+#
+# We try to use as much as possilbe fetchers from `nixpkgs` because
+# - they are supported by the Nix restricted evaluation mode
+# - they provide error messages cleaner than builtins fetchers.
+# When nixpkgs can not be found, we fallback to builtins fetchers.
+#
+# - If '<nixpkgs>' is defined and not set to the current path,
+#   it is imported and fetchers come from it.
+# - If '<nixpkgs>' is not defined or set to the current path, we try to
+#   find a `nixpkgs` attribute in `source.json`.
+#      - If the `nixpkgs` attribute exists, it is fetched with `builtins.fetchTarball`
+#        and it is imported. It then provides fetchers for other sources.
+#      - If the `nixpkgs` attribute is not found, we still try to use builtins
+#        fetchers when possible.
+
 with rec
 {
-  pkgs = import <nixpkgs> {};
+  nixPathHasNixpkgs = (builtins.tryEval <nixpkgs>).success;
+  # If nixpkgs is defined in sources.json
+  sourcesHasNixpkgs = builtins.hasAttr "nixpkgs" sources;
+  # `<nixpkgs> != ./.` leads to infinite recursion
+  useNixpkgsFromNixPath = nixPathHasNixpkgs && <nixpkgs> != ./.;
+  nixpkgsTypeIsTarball = (
+    # For backward compatibility
+    ! builtins.hasAttr "type" sources.nixpkgs
+    || (builtins.hasAttr "type" sources.nixpkgs && sources.nixpkgs.type == "tarball"));
+
+  pkgs =
+    if useNixpkgsFromNixPath
+    then import <nixpkgs> {}
+    else if sourcesHasNixpkgs
+      then pkgsFromSources
+      else abort "nixpkgs is not found while it is required. You should either set nixpkgs in your NIX_PATH or niv add nixpkgs.";
+
+  # Import nixpkgs from sources.json by using `builtins.fetchTarball`
+  # to fetch it.
+  pkgsFromSources =
+    if nixpkgsTypeIsTarball
+    then import (builtins.fetchTarball { inherit (sources.nixpkgs) url sha256; }) {}
+    else abort "The type of nixpkgs must be either tarball or not specified";
 
   sources = builtins.fromJSON (builtins.readFile ./sources.json);
 
@@ -679,28 +717,40 @@ with rec
     (f: set: with builtins;
       listToAttrs (map (attr: { name = attr; value = f attr set.${attr}; }) (attrNames set)));
 
+  fetchers = {
+    file =
+      if useNixpkgsFromNixPath || sourcesHasNixpkgs
+      then pkgs.fetchurl
+      else builtins.fetchurl;
+    tarball =
+      if useNixpkgsFromNixPath || sourcesHasNixpkgs
+      then pkgs.fetchzip
+      else builtins.fetchTarball;
+  };
+
   getFetcher = spec:
     let fetcherName =
       if builtins.hasAttr "type" spec
       then builtins.getAttr "type" spec
+      # For backward compatibility
       else "tarball";
     in builtins.getAttr fetcherName {
-      "tarball" = pkgs.fetchzip;
-      "file" = pkgs.fetchurl;
+      "tarball" = fetchers.tarball;
+      "file" = fetchers.file;
     };
-};
-# NOTE: spec must _not_ have an "outPath" attribute
-mapAttrs (_: spec:
+
+  outPath = spec:
     if builtins.hasAttr "outPath" spec
     then abort
       "The values in sources.json should not have an 'outPath' attribute"
     else
       if builtins.hasAttr "url" spec && builtins.hasAttr "sha256" spec
-    then
-      spec //
+      then spec //
         { outPath = getFetcher spec { inherit (spec) url sha256; } ; }
-    else spec
-  ) sources
+      else spec;
+
+};
+mapAttrs (_: outPath) sources
 |]
 
 -- | @nix/default.nix@

script/test-sources

@@ -0,0 +1,38 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -i bash
+#!nix-shell -I nixpkgs=./nix
+#!nix-shell -p nix
+#!nix-shell --pure
+
+set -euo pipefail
+
+echo "*** Building Niv"
+NIV=$(nix-build -A niv)/bin/niv
+
+echo "*** Running niv init"
+TEMPDIR=$(mktemp -d)
+pushd $TEMPDIR
+$NIV init
+
+echo "*** Fetching niv sources: classical"
+echo "      - nixpkgs is not in the NIX_PATH"
+echo "      - nixpkgs in sources.json"
+nix-build nix/sources.nix -A niv
+
+echo "*** Fetching niv sources: restricted evaluation mode"
+echo "      - nixpkgs is not in the NIX_PATH"
+echo "      - nixpkgs in sources.json"
+echo "      - restrict-eval is true"
+export NIX_PATH=./
+nix-build nix/sources.nix -A niv \
+  --option restrict-eval true \
+  --option allowed-uris https://github.com
+
+echo "*** Fetching niv sources: only builtins"
+echo "      - nixpkgs is not in the NIX_PATH"
+echo "      - nixpkgs not in sources.json"
+$NIV drop nixpkgs
+nix-build nix/sources.nix -A niv
+
+echo "*** Tear down"
+rm -rf $TEMPDIR