thread.c (call_without_gvl): spawn thread for UBF iff single-threaded

We need another native thread to call some unblocking functions
which aren't RUBY_UBF_IO or RUBY_UBF_PROCESS.  Instead of a
permanent thread in <= 2.5, we can now rely on the thread cache
feature to perform interrupts.

[ruby-core:90865] [Bug #15499]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66708 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

thread.c

@@ -1429,11 +1429,15 @@ call_without_gvl(void *(*func)(void *), void *data1,
     rb_execution_context_t *ec = GET_EC();
     rb_thread_t *th = rb_ec_thread_ptr(ec);
     int saved_errno = 0;
+    VALUE ubf_th = Qfalse;
 
     if (ubf == RUBY_UBF_IO || ubf == RUBY_UBF_PROCESS) {
 	ubf = ubf_select;
 	data2 = th;
     }
+    else if (ubf && vm_living_thread_num(th->vm) == 1) {
+	ubf_th = rb_thread_start_unblock_thread();
+    }
 
     BLOCKING_REGION(th, {
 	val = func(data1);
@@ -1444,6 +1448,10 @@ call_without_gvl(void *(*func)(void *), void *data1,
 	RUBY_VM_CHECK_INTS_BLOCKING(ec);
     }
 
+    if (ubf_th != Qfalse) {
+	thread_value(rb_thread_kill(ubf_th));
+    }
+
     errno = saved_errno;
 
     return val;

thread_pthread.c

@@ -2206,4 +2206,23 @@ timer_pthread_fn(void *p)
     return 0;
 }
 #endif /* UBF_TIMER_PTHREAD */
+
+static VALUE
+ubf_caller(const void *ignore)
+{
+    rb_thread_sleep_forever();
+
+    return Qfalse;
+}
+
+/*
+ * Called if and only if one thread is running, and
+ * the unblock function is NOT async-signal-safe
+ * This assumes USE_THREAD_CACHE is true for performance reasons
+ */
+static VALUE
+rb_thread_start_unblock_thread(void)
+{
+    return rb_thread_create(ubf_caller, 0);
+}
 #endif /* THREAD_SYSTEM_DEPENDENT_IMPLEMENTATION */

thread_win32.c

@@ -708,6 +708,12 @@ rb_thread_wakeup_timer_thread(int sig)
     /* do nothing */
 }
 
+static VALUE
+rb_thread_start_unblock_thread(void)
+{
+    return Qfalse; /* no-op */
+}
+
 static void
 rb_thread_create_timer_thread(void)
 {