scalafmt: add automated update script

Add an update script for scalafmt

pkgs/by-name/sc/scalafmt/package.nix

@@ -47,6 +47,8 @@ stdenv.mkDerivation {
     $out/bin/${baseName} --version | grep -q "${version}"
   '';
 
+  passthru.updateScript = ./update.sh;
+
   meta = {
     description = "Opinionated code formatter for Scala";
     homepage = "http://scalameta.org/scalafmt";

pkgs/by-name/sc/scalafmt/update.sh

@@ -0,0 +1,38 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -I nixpkgs=./. -i bash -p curl jq gnused nix
+
+set -euo pipefail
+
+latest_version=$(curl -s ${GITHUB_TOKEN:+-u ":$GITHUB_TOKEN"} \
+    https://api.github.com/repos/scalameta/scalafmt/releases/latest | jq -r '.tag_name' | sed 's/^v//')
+
+current_version=$(nix-instantiate --eval -A scalafmt.version | jq -r)
+
+if [[ "$current_version" == "$latest_version" ]]; then
+    echo "Already up to date, nothing to do."
+    exit 0
+fi
+
+echo "Updating scalafmt: $current_version -> $latest_version"
+
+package_dir="$(dirname "${BASH_SOURCE[0]}")"
+package_nix="$package_dir/package.nix"
+
+sed -i "s|version = \"$current_version\"|version = \"$latest_version\"|" "$package_nix"
+
+build_output=$(nix-build -A scalafmt 2>&1) || true
+
+if echo "$build_output" | grep -q "hash mismatch"; then
+    new_deps_hash=$(echo "$build_output" | grep "got:" | sed -n 's/.*got:[[:space:]]*\(sha256-[A-Za-z0-9+/=]*\).*/\1/p' | head -1)
+    if [[ -n "$new_deps_hash" ]]; then
+        sed -i "s|outputHash = \"sha256-[A-Za-z0-9+/=]*\"|outputHash = \"$new_deps_hash\"|" "$package_nix"
+    else
+        echo "Could not extract new hash from build output, build output:"
+        echo "$build_output"
+        exit 1
+    fi
+else
+    echo "Unexpected: Build succeeded without hash mismatch, build output:"
+    echo "$build_output"
+    exit 1
+fi