Add a test to check that we can use the specified stack size

Actual test of the guard pages can be done, but is hard to fit in the test framework.
To test manually decrease the headroom const to 0

diff --git a/pdns/recursordist/test-mtasker.cc b/pdns/recursordist/test-mtasker.cc
index fb678897be4113b0c5891d2bc150ab5433ba11b7..83d0930d8ea361c42fa279737fc82e5b6961c034 100644 (file)
--- a/pdns/recursordist/test-mtasker.cc
+++ b/pdns/recursordist/test-mtasker.cc
@@ -6,6 +6,7 @@
 #endif
 #include <boost/test/unit_test.hpp>
 #include "mtasker.hh"
+#include <fcntl.h>
 
 BOOST_AUTO_TEST_SUITE(mtasker_cc)
 
@@ -40,6 +41,43 @@ BOOST_AUTO_TEST_CASE(test_Simple)
   BOOST_CHECK_EQUAL(g_result, o);
 }
 
+static const size_t stackSize = 8 * 1024;
+static const size_t headroom = 256; // Decrease to hit stackoverflow
+
+static void doAlmostStackoverflow(void* arg)
+{
+  auto* mt = reinterpret_cast<MTasker<>*>(arg);
+  int localvar[stackSize / sizeof(int) - headroom]; // expermimentally derived headroom
+  localvar[0] = 0;
+  localvar[sizeof(localvar) / sizeof(localvar[0]) - 1] = 12;
+  if (mt->waitEvent(localvar[sizeof(localvar) / sizeof(localvar[0]) - 1], &localvar[0]) == 1) {
+    g_result = localvar[0];
+  }
+}
+
+BOOST_AUTO_TEST_CASE(test_AlmostStackOverflow)
+{
+  MTasker<> mt(stackSize);
+  mt.makeThread(doAlmostStackoverflow, &mt);
+  struct timeval now;
+  gettimeofday(&now, 0);
+  bool first = true;
+  int o = 25;
+  for (;;) {
+    while (mt.schedule(&now)) {
+      ;
+    }
+    if (first) {
+      mt.sendEvent(12, &o);
+      first = false;
+    }
+    if (mt.noProcesses()) {
+      break;
+    }
+  }
+  BOOST_CHECK_EQUAL(g_result, o);
+}
+
 #if defined(HAVE_FIBER_SANITIZER) && defined(__APPLE__) && defined(__arm64__)
 
 // This test is buggy on MacOS when compiled with asan. It also causes subsequents tests to report spurious issues.