Serialize enrollment install with enrolling.lock

Concurrent enroll-url launches could both pass the enrollment
marker check and run overlapping elevated installs, racing on
LoadOrExchangeAPIKey and overwriting agent.key.

Add an exclusive flock on {configDir}/enrolling.lock for the
full install path and re-check IsEnrolled under that lock so
only one install exchanges a token and configures the device.

Signed-off-by: Ludovic Vielle <ludovic@probo.com>

pkg/deviceagent/enrollment_lock_test.go

@@ -0,0 +1,86 @@
+// Copyright (c) 2026 Probo Inc <hello@probo.com>.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+package deviceagent
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestEnrollmentLockPath(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	assert.Equal(
+		t,
+		filepath.Join(EnrollmentRunDir(dir), EnrollmentLockFileName),
+		EnrollmentLockPath(dir),
+	)
+}
+
+func TestAcquireEnrollmentLockConcurrent(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+
+	releaseFirst, err := AcquireEnrollmentLock(dir)
+	require.NoError(t, err)
+
+	acquired := make(chan error, 1)
+
+	go func() {
+		releaseSecond, err := AcquireEnrollmentLock(dir)
+		if err != nil {
+			acquired <- err
+			return
+		}
+
+		releaseSecond()
+
+		acquired <- nil
+	}()
+
+	time.Sleep(100 * time.Millisecond)
+
+	select {
+	case err := <-acquired:
+		require.NoError(t, err)
+		t.Fatal("second install acquired enrollment lock while first still holds it")
+	default:
+	}
+
+	releaseFirst()
+
+	select {
+	case err := <-acquired:
+		require.NoError(t, err)
+	case <-time.After(2 * time.Second):
+		t.Fatal("second install did not acquire enrollment lock after first released it")
+	}
+
+	_, err = os.Stat(EnrollmentLockPath(dir))
+	require.NoError(t, err)
+}