Add NIV_OVERRIDE_{...}

This updates the sources.nix to replace the `outPath` field of package
`foo` with the content of envionment variable `NIV_OVERRIDE_foo`. In the
environment variable name, all characters outside of `[a-zA-Z0-9_]` are
escaped to `_`.

default.nix

@@ -197,6 +197,7 @@ rec
 
   tests-github = pkgs.callPackage ./tests/github { inherit niv; };
   tests-git = pkgs.callPackage ./tests/git { inherit niv; };
+  tests-eval = pkgs.callPackage ./tests/eval {};
 
   fmt-check =
     pkgs.stdenv.mkDerivation

nix/sources.nix

@@ -73,6 +73,15 @@ let
     else
       abort "ERROR: niv spec ${name} has unknown type ${builtins.toJSON spec.type}";
 
+  # If the environment variable NIV_OVERRIDE_${name} is set, then use
+  # the path directly as opposed to the fetched source.
+  replace = name: drv:
+    let
+      saneName = stringAsChars (c: if isNull (builtins.match "[a-zA-Z0-9]" c) then "_" else c) name;
+      ersatz = builtins.getEnv "NIV_OVERRIDE_${saneName}";
+    in
+      if ersatz == "" then drv else ersatz;
+
   # Ports of functions for older nix versions
 
   # a Nix version of mapAttrs if the built-in doesn't exist
@@ -119,7 +128,7 @@ let
         then abort
           "The values in sources.json should not have an 'outPath' attribute"
         else
-          spec // { outPath = fetch config.pkgs name spec; }
+          spec // { outPath = replace name (fetch config.pkgs name spec); }
     ) config.sources;
 
   # The "config" used by the fetchers
@@ -134,5 +143,6 @@ let
       # The "pkgs" (evaluated nixpkgs) to use for e.g. non-builtin fetchers
       inherit pkgs;
     };
+
 in
 mkSources (mkConfig {}) // { __functor = _: settings: mkSources (mkConfig settings); }

src/Niv/Sources.hs

@@ -160,6 +160,8 @@ data SourcesNixVersion
   | -- prettify derivation name
     -- add 'local' type of sources
     V18
+  | -- add NIV_OVERRIDE_{name}
+    V19
   deriving stock (Bounded, Enum, Eq)
 
 -- | A user friendly version
@@ -183,6 +185,7 @@ sourcesVersionToText = \case
   V16 -> "16"
   V17 -> "17"
   V18 -> "18"
+  V19 -> "19"
 
 latestVersionMD5 :: T.Text
 latestVersionMD5 = sourcesVersionToMD5 maxBound
@@ -213,6 +216,7 @@ sourcesVersionToMD5 = \case
   V16 -> "2d93c52cab8e960e767a79af05ca572a"
   V17 -> "149b8907f7b08dc1c28164dfa55c7fad"
   V18 -> "bc5e6aefcaa6f9e0b2155ca4f44e5a33"
+  V19 -> "543621698065cfc6a4a7985af76df718"
 
 -- | The MD5 sum of ./nix/sources.nix
 sourcesNixMD5 :: IO T.Text

tests/eval/default.nix

@@ -0,0 +1,45 @@
+{ pkgs ? import <nixpkgs> {}
+}:
+
+pkgs.runCommand "foobar" { nativeBuildInputs = [ pkgs.jq pkgs.nix pkgs.moreutils ]; }
+  ''
+    # for nix to run smoothly in multi-user install
+    # https://github.com/NixOS/nix/issues/3258
+    # https://github.com/cachix/install-nix-action/issues/16
+    export NIX_STATE_DIR="$TMPDIR"
+    export NIX_LOG_DIR="$TMPDIR"
+
+    cp ${ ../../nix/sources.nix} sources.nix
+    echo '{}' > sources.json
+
+    update_sources() {
+      cat sources.json | jq -cMe "$1" | sponge sources.json
+    }
+
+    update_sources '.foo = { type: "tarball", url: "foo", sha256: "whocares" }'
+    update_sources '."ba-r" = { type: "tarball", url: "foo", sha256: "whocares" }'
+    update_sources '."ba z" = { type: "tarball", url: "foo", sha256: "whocares" }'
+
+    eval_outPath() {
+      nix eval --raw '(let sources = import ./sources.nix; in sources.'"$1"'.outPath)'
+    }
+
+    eq() {
+      if ! [ "$1" == "$2" ]; then
+        echo "expected"
+        echo "  '$1' == '$2'"
+        exit 1
+      fi
+    }
+
+    res="$(NIV_OVERRIDE_foo="hello" eval_outPath "foo")"
+    eq "$res" "hello"
+
+    res="$(NIV_OVERRIDE_ba_r="hello" eval_outPath "ba-r")"
+    eq "$res" "hello"
+
+    res="$(NIV_OVERRIDE_ba_z="hello" eval_outPath '"ba z"')"
+    eq "$res" "hello"
+
+    touch "$out"
+  ''