Gracefully handle error when trying to read package.json in detect (#…

…733)

* Gracefully handle error when trying to read `package.json` in detect

This change prevents the detection routine from raising an error during detect if:
- there is no `package.json`
- there is a `package.json` but it cannot be read for some reason

Both error cases will be handled and report a failed detect.

* Update CHANGELOG.md

buildpacks/nodejs-npm-install/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Detection will report a failure instead of throwing an error when there is no `package.json` file in the application directory. ([#733](https://github.com/heroku/buildpacks-nodejs/pull/733))
+
 ## [2.3.0] - 2023-11-09
 
 - No changes.

buildpacks/nodejs-npm-install/src/main.rs

@@ -46,20 +46,21 @@ impl Buildpack for NpmInstallBuildpack {
             .try_exists()
             .map_err(NpmInstallBuildpackError::Detect)?;
 
-        let package_json = PackageJson::read(context.app_dir.join("package.json"))
-            .map_err(NpmInstallBuildpackError::PackageJson)?;
-
-        if npm_lockfile_exists || package_json.has_dependencies() {
-            DetectResultBuilder::pass()
-                .build_plan(
-                    BuildPlanBuilder::new()
-                        .provides("node_modules")
-                        .requires("npm")
-                        .requires("node_modules")
-                        .requires("node")
-                        .build(),
-                )
-                .build()
+        if let Ok(package_json) = PackageJson::read(context.app_dir.join("package.json")) {
+            if npm_lockfile_exists || package_json.has_dependencies() {
+                DetectResultBuilder::pass()
+                    .build_plan(
+                        BuildPlanBuilder::new()
+                            .provides("node_modules")
+                            .requires("npm")
+                            .requires("node_modules")
+                            .requires("node")
+                            .build(),
+                    )
+                    .build()
+            } else {
+                DetectResultBuilder::fail().build()
+            }
         } else {
             DetectResultBuilder::fail().build()
         }

buildpacks/nodejs-npm-install/tests/integration_test.rs

@@ -172,6 +172,20 @@ fn test_dependencies_and_missing_lockfile_errors() {
     );
 }
 
+#[test]
+#[ignore]
+fn detect_rejects_non_npm_project() {
+    nodejs_integration_test_with_config(
+        "./fixtures/empty",
+        |config| {
+            config.expected_pack_result(PackResult::Failure);
+        },
+        |ctx| {
+            assert_contains!(ctx.pack_stdout, "fail: heroku/nodejs-npm-install");
+        },
+    );
+}
+
 fn add_lockfile_entry(app_dir: &Path, package_name: &str, lockfile_entry: serde_json::Value) {
     update_json_file(&app_dir.join("package-lock.json"), |json| {
         let dependencies = json["dependencies"].as_object_mut().unwrap();