Add test case for switching out of desched syscall

This adds a test case to model #3285, where the test case pokes the sigframe
to force sigreturn to switch to a different function than that which incurred
the signal.

CMakeLists.txt

@@ -912,6 +912,7 @@ set(BASIC_TESTS
   daemon
   desched_blocking_poll
   desched_sigkill
+  desched_sigreturn
   detach_state
   detach_threads
   detach_sigkill

src/test/desched_sigreturn.c

@@ -0,0 +1,85 @@
+/* -*- Mode: C; tab-width: 8; c-basic-offset: 2; indent-tabs-mode: nil; -*- */
+
+#include "util.h"
+
+pid_t main_thread_tid = 0;
+int fds[2];
+char zeros[8192];
+
+void sigproc_and_hang(void)
+{
+  sigset_t sigs;
+  sigfillset(&sigs);
+  sigdelset(&sigs, SIGUSR2);
+  sigprocmask(SIG_SETMASK, &sigs, NULL);
+  write(fds[1], &zeros, 8192);
+  write(fds[1], &zeros, 8192);
+  test_assert(0);
+}
+
+void print_and_exit(void)
+{
+  atomic_printf("EXIT-SUCCESS\n");
+  exit(0);
+}
+
+volatile int counter = 0;
+void usr2_handler(__attribute__((unused)) int signum,
+             __attribute__((unused)) siginfo_t* siginfo_ptr,
+             void* ucontext_ptr) {
+  uintptr_t target = counter == 0 ? (uintptr_t)&sigproc_and_hang : (uintptr_t)&print_and_exit;
+  counter += 1;
+#if defined(__i386__)
+  ucontext_t* ctx = (ucontext_t*)ucontext_ptr;
+  ctx->uc_mcontext.gregs[REG_EIP] = (uint32_t)target;
+#elif defined(__x86_64__)
+  ucontext_t* ctx = (ucontext_t*)ucontext_ptr;
+  ctx->uc_mcontext.gregs[REG_RIP] = (long long)target;
+#elif defined(__aarch64__)
+  ucontext_t* ctx = (ucontext_t*)ucontext_ptr;
+  ctx->uc_mcontext.pc = (long)target;
+#else
+  #error "Unsupported architecture"
+#endif
+}
+
+static void* signaler_thread(__attribute__((unused)) void* p) {
+  for (int i = 0; i < 10; ++i)
+    sched_yield();
+  syscall(SYS_tgkill, getpid(), main_thread_tid, SIGUSR2);
+  // Technically should use atomics, but volatile is good enough for this test,
+  // since we're doing a syscall in the loop.
+  while (counter == 0)
+    sched_yield();
+  for (int i = 0; i < 10; ++i)
+    sched_yield();
+  syscall(SYS_tgkill, getpid(), main_thread_tid, SIGUSR2);
+  return NULL;
+}
+
+int main(void) {
+  int err = pipe(fds);
+  test_assert(err == 0);
+
+  err = fcntl(fds[1], F_SETPIPE_SZ, 4096);
+  test_assert(err > 0);
+
+  struct sigaction sa;
+  memset(&sa, 0, sizeof(sa));
+  sa.sa_sigaction = usr2_handler;
+  sa.sa_flags = SA_ONSTACK | SA_SIGINFO | SA_RESTART;
+  err = sigaction(SIGUSR2, &sa, NULL);
+  test_assert(err == 0);
+
+  main_thread_tid = sys_gettid();
+
+  pthread_t thread;
+  pthread_create(&thread, NULL, signaler_thread, NULL);
+
+  // Block on pipe read.
+  int ch = 0;
+  err = read(fds[0], &ch, sizeof(int));
+  test_assert(0);
+
+  return 0;
+}