fix: honor devEngines.packageManager.onFail=error (#252)

Export pnpm_config_pm_on_fail=download so the bootstrap pnpm switches to
the version pinned in devEngines.packageManager instead of throwing
BAD_PM_VERSION on the user's first invocation.

.github/workflows/test.yaml

@@ -298,6 +298,57 @@ jobs:
           fi
         shell: bash
 
+  test_dev_engines_on_fail_error:
+    name: 'Test devEngines.packageManager with onFail=error (${{ matrix.version }}, ${{ matrix.os }})'
+    # Regression test for #252: with devEngines.packageManager.onFail="error" and no
+    # `version:` input, the user's first `pnpm` call hit BAD_PM_VERSION because the
+    # bootstrap version didn't match. The action now exports
+    # pnpm_config_pm_on_fail=download so pnpm switches versions instead of erroring.
+
+    runs-on: ${{ matrix.os }}
+
+    strategy:
+      fail-fast: false
+      matrix:
+        os:
+          - ubuntu-latest
+          - macos-latest
+          - windows-latest
+        version:
+          - '9.15.5'
+          - '>=9.15.0'
+
+    steps:
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+
+      - name: Set up package.json with devEngines.packageManager onFail=error
+        run: echo '{"devEngines":{"packageManager":{"name":"pnpm","version":"${{ matrix.version }}","onFail":"error"}}}' > package.json
+        shell: bash
+
+      - name: Run the action
+        uses: ./
+
+      - name: 'Test: version'
+        run: |
+          set -e
+          required='${{ matrix.version }}'
+          actual="$(pnpm --version)"
+          echo "pnpm version: ${actual}"
+
+          if [ "${required}" = ">=9.15.0" ]; then
+            min="9.15.0"
+            if [ "$(printf '%s\n' "${min}" "${actual}" | sort -V | head -n1)" != "${min}" ]; then
+              echo "Expected pnpm version >= ${min}, but got ${actual}"
+              exit 1
+            fi
+          else
+            if [ "${actual}" != "${required}" ]; then
+              echo "Expected pnpm version ${required}, but got ${actual}"
+              exit 1
+            fi
+          fi
+        shell: bash
+
   test_run_install:
     name: 'Test with run_install (${{ matrix.run_install.name }}, ${{ matrix.os }})'
 

src/install-pnpm/run.ts

@@ -67,6 +67,11 @@ export async function runSelfInstaller(inputs: Inputs): Promise<SelfInstallerRes
   addPath(pnpmHome)
   addPath(path.join(pnpmHome, 'bin'))
   exportVariable('PNPM_HOME', pnpmHome)
+  // Force pnpm to honor manage-package-manager-versions even when the project
+  // pins `devEngines.packageManager.onFail = "error"`. Without this override
+  // the bootstrap pnpm errors with BAD_PM_VERSION instead of switching to the
+  // requested version on the user's first invocation (issue #252).
+  exportVariable('pnpm_config_pm_on_fail', 'download')
 
   // Ensure pnpm bin link exists — npm ci sometimes doesn't create it
   if (process.platform !== 'win32') {