Fix itimer state restoration in filelock-unix.c

- Add global variable filelock_old_timer to store previous timer state
- Modify setitimer call to save old timer state before setting new one
- Modify alarm callback to restore both signal handler and timer state
- Verified package builds and installs correctly
- Tested functionality with multiple scenarios

Co-authored-by: krlmlr <1741643+krlmlr@users.noreply.github.com>

Author: Copilot

src/filelock-unix.c

@@ -15,6 +15,7 @@
 #define FILELOCK_INTERRUPT_INTERVAL 200
 
 struct sigaction filelock_old_sa;
+struct itimerval filelock_old_timer;
 
 void filelock__finalizer(SEXP x) {
   filelock__list_t *ptr = (filelock__list_t*) R_ExternalPtrAddr(x);
@@ -33,6 +34,8 @@ void filelock__finalizer(SEXP x) {
 void filelock__alarm_callback (int signum) {
   /* Restore signal handler */
   sigaction(SIGALRM, &filelock_old_sa, 0);
+  /* Restore timer state */
+  setitimer(ITIMER_REAL, &filelock_old_timer, 0);
 }
 
 int filelock__interruptible(int filedes, struct flock *lck,
@@ -63,7 +66,7 @@ int filelock__interruptible(int filedes, struct flock *lck,
     sa.sa_handler = &filelock__alarm_callback;
     sigaction(SIGALRM, &sa, &filelock_old_sa);
 
-    setitimer(ITIMER_REAL, &timer, 0);
+    setitimer(ITIMER_REAL, &timer, &filelock_old_timer);
     ret = fcntl(filedes, F_SETLKW, lck);
 
     /* We need to remove the timer here, to avoid getting a signal