[Tarantool-patches] [PATCH 1/4] fiber: introduce fiber.arg

[Vladislav Shpilevoy <v.shpilevoy@tarantool.org>]

Struct fiber has a member va_list f_data. It is used to forward
arguments to the fiber function when fiber_start() is called,
right from the caller's stack.

But it is useless when fiber is started asynchronously, with
fiber_new + fiber_wakeup. And there is no way to pass anything
into such a fiber.

This patch adds a new member 'void *arg', which shares memory with
va_list f_data, and can be used to pass something into the fiber.

The feature is going to be used by raft. Currently raft worker
fiber works only with global variables, but soon it will need to
have its own pointer at struct raft object. And it can't be
started with fiber_start(), because raft code does not yield
anywhere in its state machine.

Needed for #5303
---
 src/lib/core/fiber.h | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/src/lib/core/fiber.h b/src/lib/core/fiber.h
index 539e5c8e7..08f1dd662 100644
--- a/src/lib/core/fiber.h
+++ b/src/lib/core/fiber.h
@@ -565,7 +565,19 @@ struct fiber {
 	 * See http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31488
 	 */
 	fiber_func f;
-	va_list f_data;
+	union {
+		/**
+		 * Argument list passed when the fiber is invoked in a blocking
+		 * way, so the caller may pass arguments from its own stack.
+		 */
+		va_list f_data;
+		/**
+		 * Fiber function argument which passed asynchronously. Can be
+		 * used not to call fiber_start to avoid yields, but still pass
+		 * something into the fiber.
+		 */
+		void *arg;
+	};
 	int f_ret;
 	/** Fiber local storage. */
 	struct {
-- 
2.21.1 (Apple Git-122.3)