Fewer calls to `GET_EC()` and `GET_THREAD()` (#15506)

The changes are to `io.c` and `thread.c`.
I changed the API of 2 exported thread functions from `internal/thread.h` that
didn't look like they had any use in C extensions:

* rb_thread_wait_for_single_fd
* rb_thread_io_wait

I didn't change the following exported internal function because it's
used in C extensions:

* rb_thread_fd_select

I added a comment to note that this function, although internal, is used
in C extensions.

1 files changed, 13 insertions, 1 deletions

diff --git a/include/ruby/fiber/scheduler.h b/include/ruby/fiber/scheduler.h
index 537a3a7bb2..4d764f68ae 100644
--- a/include/ruby/fiber/scheduler.h
+++ b/include/ruby/fiber/scheduler.h
@@ -27,6 +27,7 @@ RBIMPL_SYMBOL_EXPORT_BEGIN()
 #define RUBY_FIBER_SCHEDULER_VERSION 3
 
 struct timeval;
+struct rb_thread_struct;
 
 /**
  * Wrap a `ssize_t` and `int errno` into a single `VALUE`. This interface should
@@ -118,7 +119,7 @@ VALUE rb_fiber_scheduler_current(void);
 
 /**
  * Identical to rb_fiber_scheduler_current(), except it queries for that of the
- * passed thread instead of the implicit current one.
+ * passed thread value instead of the implicit current one.
  *
  * @param[in]  thread         Target thread.
  * @exception  rb_eTypeError  `thread` is not a thread.
@@ -128,6 +129,17 @@ VALUE rb_fiber_scheduler_current(void);
 VALUE rb_fiber_scheduler_current_for_thread(VALUE thread);
 
 /**
+ * Identical to rb_fiber_scheduler_current_for_thread(), except it expects
+ * a threadptr instead of a thread value.
+ *
+ * @param[in]  thread         Target thread.
+ * @exception  rb_eTypeError  `thread` is not a thread.
+ * @retval     RUBY_Qnil      No scheduler is in effect in `thread`.
+ * @retval     otherwise      The scheduler that is in effect in `thread`.
+ */
+VALUE rb_fiber_scheduler_current_for_threadptr(struct rb_thread_struct *thread);
+
+/**
  * Converts the passed timeout to an expression that rb_fiber_scheduler_block()
  * etc. expects.
  *