Improve error messaging when installing an incompatible npm version

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## main
 
+- Improved error messaging when installing an incompatible npm version.
 - Added Node.js version 18.18.0.
 - Added Node.js version 20.7.0.
 

lib/binaries.sh

@@ -123,8 +123,10 @@ install_npm() {
     echo "npm $npm_version already installed with node"
   else
     echo "Bootstrapping npm $version (replacing $npm_version)..."
-    if ! npm install --unsafe-perm --quiet -g "npm@$version" 2>@1>/dev/null; then
-      echo "Unable to install npm $version; does it exist?" && false
+    if ! npm install --unsafe-perm --quiet --no-audit --no-progress -g "npm@$version" >/dev/null; then
+      echo "Unable to install npm $version. " \
+        "Does npm $version exist? " \
+        "Is npm $version compatible with this Node.js version?" && false
     fi
     # Verify npm works before capturing and ensure its stderr is inspectable later
     npm --version 2>&1 1>/dev/null

test/fixtures/npm-version-incompat/README.md

@@ -0,0 +1 @@
+A fake README, to keep npm from polluting stderr.

test/fixtures/npm-version-incompat/package.json

@@ -0,0 +1,13 @@
+{
+  "name": "node-buildpack-test-app",
+  "version": "0.0.1",
+  "description": "node buildpack integration test app",
+  "repository" : {
+    "type" : "git",
+    "url" : "http://github.com/example/example.git"
+  },
+  "engines": {
+    "node": "16.x",
+    "npm": "10.x"
+  }
+}

test/run

@@ -840,6 +840,13 @@ testNpmVersionSpecific() {
   assertCapturedSuccess
 }
 
+testNpmVersionIncompatible() {
+  compile "npm-version-incompat"
+  assertCaptured "Unable to install npm 10.x"
+  assertCaptured "Is npm 10.x compatible with this Node.js version?"
+  assertCapturedError
+}
+
 testFailingBuild() {
   compile "failing-build"
   assertCaptured "Build failed"
@@ -1139,7 +1146,7 @@ testNodeEnvNpmConfigProductionFalse() {
   assertCaptured "preinstall: production"
   assertCaptured "postinstall: production"
   assertCaptured "heroku-postbuild: production"
-  assertCapturedSuccess
+  assertEquals "Expected successful exit code" "0" "${RETURN}"
 }
 
 # Avoid this issue