Only emit circular dependency warning for owned thread shields

[Bug #19415]

If multiple threads attemps to load the same file concurrently
it's not a circular dependency issue.

So we check that the existing ThreadShield is owner by the current
fiber before warning about circular dependencies.

internal/thread.h

@@ -29,6 +29,7 @@ VALUE rb_get_coverages(void);
 int rb_get_coverage_mode(void);
 VALUE rb_default_coverage(int);
 VALUE rb_thread_shield_new(void);
+bool rb_thread_shield_owned(VALUE self);
 VALUE rb_thread_shield_wait(VALUE self);
 VALUE rb_thread_shield_release(VALUE self);
 VALUE rb_thread_shield_destroy(VALUE self);

load.c

@@ -850,7 +850,8 @@ load_lock(rb_vm_t *vm, const char *ftptr, bool warn)
         st_insert(loading_tbl, (st_data_t)ftptr, data);
         return (char *)ftptr;
     }
-    if (warn) {
+
+    if (warn && rb_thread_shield_owned((VALUE)data)) {
         VALUE warning = rb_warning_string("loading in progress, circular require considered harmful - %s", ftptr);
         rb_backtrace_each(rb_str_append, warning);
         rb_warning("%"PRIsVALUE, warning);

spec/ruby/core/kernel/shared/require.rb

@@ -237,6 +237,17 @@
       }.should complain(/circular require considered harmful/, verbose: true)
       ScratchPad.recorded.should == [:loaded]
     end
+
+    ruby_bug "#17340", ''...'3.3' do
+      it "loads a file concurrently" do
+        path = File.expand_path "concurrent_require_fixture.rb", CODE_LOADING_DIR
+        ScratchPad.record(@object)
+        -> {
+          @object.require(path)
+        }.should_not complain(/circular require considered harmful/, verbose: true)
+        ScratchPad.recorded.join
+      end
+    end
   end
 
   describe "(non-extensioned path)" do

spec/ruby/fixtures/code/concurrent_require_fixture.rb

@@ -0,0 +1,4 @@
+object = ScratchPad.recorded
+thread = Thread.new { object.require(__FILE__) }
+thread.wakeup unless thread.stop?
+ScratchPad.record(thread)

test/ruby/test_require.rb

@@ -562,9 +562,6 @@ class << (output = "")
 
       assert_equal(true, (t1_res ^ t2_res), bug5754 + " t1:#{t1_res} t2:#{t2_res}")
       assert_equal([:pre, :post], scratch, bug5754)
-
-      assert_match(/circular require/, output)
-      assert_match(/in #{__method__}'$/o, output)
     }
   ensure
     $VERBOSE = verbose

thread.c

@@ -4920,6 +4920,17 @@ rb_thread_shield_new(void)
     return thread_shield;
 }
 
+bool
+rb_thread_shield_owned(VALUE self)
+{
+    VALUE mutex = GetThreadShieldPtr(self);
+    if (!mutex) return false;
+
+    rb_mutex_t *m = mutex_ptr(mutex);
+
+    return m->fiber == GET_EC()->fiber_ptr;
+}
+
 /*
  * Wait a thread shield.
  *