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.go

@@ -0,0 +1,84 @@
+// 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 (
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+const (
+	// EnrollmentLockFileName serializes concurrent LoadOrExchangeAPIKey
+	// calls that exchange enrollment tokens and write agent.key. It lives
+	// under EnrollmentRunDir so a reboot clears it with the run tree.
+	EnrollmentLockFileName = "enrolling.lock"
+
+	// enrollmentLockMode is owner-only; only the elevated install path
+	// creates and holds this lock.
+	enrollmentLockMode = 0o600
+)
+
+// EnrollmentLockPath returns the absolute path of the enrollment lock file
+// under EnrollmentRunDir for the given state directory.
+func EnrollmentLockPath(configDir string) string {
+	return filepath.Join(EnrollmentRunDir(configDir), EnrollmentLockFileName)
+}
+
+// AcquireEnrollmentLock takes an exclusive lock on enrolling.lock under
+// EnrollmentRunDir(configDir) for the duration of credential exchange. The
+// returned release function unlocks and closes the lock file. The path is
+// left in place so waiters that already opened the inode stay serialized
+// with later acquirers; the kernel releases the lock on close (including
+// after crashes). Callers must not unlink this path while a holder may
+// exist.
+func AcquireEnrollmentLock(configDir string) (release func(), err error) {
+	runDir := EnrollmentRunDir(configDir)
+
+	if err := os.MkdirAll(runDir, EnrollmentRunDirMode); err != nil {
+		return nil, fmt.Errorf("cannot create enrollment run dir: %w", err)
+	}
+
+	if err := os.Chmod(runDir, EnrollmentRunDirMode); err != nil {
+		return nil, fmt.Errorf("cannot set enrollment run dir permissions: %w", err)
+	}
+
+	path := EnrollmentLockPath(configDir)
+
+	file, err := os.OpenFile(
+		path,
+		os.O_RDWR|os.O_CREATE,
+		enrollmentLockMode,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("cannot open enrollment lock: %w", err)
+	}
+
+	if err := lockFileExclusive(file); err != nil {
+		_ = file.Close()
+		return nil, fmt.Errorf("cannot acquire enrollment lock: %w", err)
+	}
+
+	return func() {
+		_ = unlockFile(file)
+		_ = file.Close()
+	}, nil
+}