close #75: fix crash on non-released preemptive resources when capaci…

…ty decreases

NEWS.md

@@ -5,6 +5,7 @@
 * Remove warnings for unused arguments in `seize` and `seize_selected` (1c8c3bb).
 * Fix crash on arrival reneging in non-triggered batches (8713d95).
 * Fix crash on batches triggered before the timer expires (8713d95).
+* Fix crash on non-released preemptive resources when capacity decreases (#75).
 
 # Changes in version 3.4.2
 

src/resource.h

@@ -297,6 +297,8 @@ class PreemptiveRes : public PriorityRes<T> {
 
   bool try_free_server(bool verbose, double time) {
     typename T::iterator first = this->server.begin();
+    if (first == this->server.end())
+      return false;
     first->arrival->deactivate();
     if (verbose) this->verbose_print(time, first->arrival->name, "PREEMPT");
     if (first->arrival->is_monitored()) {

tests/testthat/test-simmer-resource-schedule.R

@@ -129,3 +129,16 @@ test_that("resource's capacity decreases before post-release tasks", {
   expect_equal(arrivals$end_time, c(10, 20, 25))
   expect_equal(arrivals$activity_time, c(5, 5, 5))
 })
+
+test_that("capacity decrease on a non-released preemptive resource does not crash", {
+  t <- create_trajectory() %>%
+    seize("dummy", 1)
+
+  sched <- schedule(c(0, 1), c(1, 0), period = Inf)
+
+  env <- simmer(verbose = TRUE) %>%
+    add_resource("dummy", sched, preemptive = TRUE) %>%
+    add_generator("arrival", t, at(0))
+
+  expect_output(run(env))
+})