The main Thread should have report_on_exception=true for consistency

* Adapt test and add specs.
* See [Feature #14143] [ruby-core:84227]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61237 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/ruby/core/thread/report_on_exception_spec.rb

@@ -34,6 +34,16 @@ ruby_version_is "2.4" do
   end
 
   describe "Thread#report_on_exception" do
+    ruby_version_is "2.5" do
+      it "returns true for the main Thread" do
+        Thread.current.report_on_exception.should == true
+      end
+
+      it "returns true for new Threads" do
+        Thread.new { Thread.current.report_on_exception }.value.should == true
+      end
+    end
+
     it "returns whether the Thread will print a backtrace if it exits with an exception" do
       t = Thread.new { Thread.current.report_on_exception = true }
       t.join

test/ruby/test_thread.rb

@@ -382,8 +382,8 @@ class TestThread < Test::Unit::TestCase
 
       assert_equal(true, Thread.report_on_exception,
                    "global flag is true by default")
-      assert_equal(false, Thread.current.report_on_exception,
-                   "the main thread has report_on_exception=false")
+      assert_equal(true, Thread.current.report_on_exception,
+                   "the main thread has report_on_exception=true")
 
       Thread.report_on_exception = true
       Thread.current.report_on_exception = false

thread.c

@@ -743,7 +743,6 @@ thread_create_core(VALUE thval, VALUE args, VALUE (*fn)(ANYARGS))
     RBASIC_CLEAR_CLASS(th->pending_interrupt_mask_stack);
 
     native_mutex_initialize(&th->interrupt_lock);
-    th->report_on_exception = th->vm->thread_report_on_exception;
 
     /* kick thread */
     err = native_thread_create(th);

vm.c

@@ -2518,6 +2518,7 @@ th_init(rb_thread_t *th, VALUE self)
     th->retval = Qundef;
 #endif
     th->name = Qnil;
+    th->report_on_exception = th->vm->thread_report_on_exception;
 }
 
 static VALUE