Fix handling of error/errno in io_internal_wait. (#12961)

[Bug #21195]

io.c

@@ -1181,8 +1181,14 @@ io_internal_wait(VALUE thread, rb_io_t *fptr, int error, int events, struct time
         return -1;
     }
 
-    errno = error;
-    return -1;
+    // If there was an error BEFORE we started waiting, return it:
+    if (error) {
+        errno = error;
+        return -1;
+    } else {
+        // Otherwise, whatever error was generated by `nogvl_wait_for` is the one we want:
+        return ready;
+    }
 }
 
 static VALUE

test/ruby/test_io.rb

@@ -4373,4 +4373,30 @@ __END__
       end
     end
   end
+
+  def test_blocking_timeout
+    assert_separately([], <<~'RUBY')
+      IO.pipe do |r, w|
+        trap(:INT) do
+          w.puts "INT"
+        end
+
+        main = Thread.current
+        thread = Thread.new do
+          # Wait until the main thread has entered `$stdin.gets`:
+          Thread.pass until main.status == 'sleep'
+
+          # Cause an interrupt while handling `$stdin.gets`:
+          Process.kill :INT, $$
+        end
+
+        r.timeout = 1
+        assert_equal("INT", r.gets.chomp)
+      rescue IO::TimeoutError
+        # Ignore - some platforms don't support interrupting `gets`.
+      ensure
+        thread&.join
+      end
+    RUBY
+  end
 end