This is an odd corner case, but happens specifically with the gdb testcase make check TESTS=gdb.base/valgrind-infcall-2.exp. At the end valgrind gets killed with SIGKILL (-9) which cannot be blocked. But vgdb at the time is inside waitstopped. It sees the process wasn't exited (WIFEXITED(status) is false) and so assumes the process was stopped by a signal. Which it asserts:

      assert (WIFSTOPPED(status));
      signal_received = WSTOPSIG(status);
      if (signal_received == signal_expected)
         break;

But the assert fails and vgdb dumps core. The gdb testcase doesn't care, because it already finished its test and just makes sure all processes are gone. But it slowly fills your disk with core files (if you have enabled them) when running the testsuite.

Proposed fix is to simply check first whether the program has termined normally or by getting a fatal signal:

diff --git a/coregrind/vgdb-invoker-ptrace.c b/coregrind/vgdb-invoker-ptrace.c
index cb37677d5..389748960 100644
--- a/coregrind/vgdb-invoker-ptrace.c
+++ b/coregrind/vgdb-invoker-ptrace.c
@@ -267,7 +267,8 @@ Bool waitstopped (pid_t pid, int signal_expected, const char *msg)
          return False;
       }
 
-      if (WIFEXITED(status)) {
+      /* The process either exited or was terminated by a (fatal) signal. */
+      if (WIFEXITED(status) || WIFSIGNALED(status)) {
          shutting_down = True;
          return False;
       }