coroutine: add qemu_coroutine_entered() function

See the doc comments for a description of this new coroutine API.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 1474989516-18255-2-git-send-email-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

include/qemu/coroutine.h

@@ -92,6 +92,19 @@ Coroutine *coroutine_fn qemu_coroutine_self(void);
  */
 bool qemu_in_coroutine(void);
 
+/**
+ * Return true if the coroutine is currently entered
+ *
+ * A coroutine is "entered" if it has not yielded from the current
+ * qemu_coroutine_enter() call used to run it.  This does not mean that the
+ * coroutine is currently executing code since it may have transferred control
+ * to another coroutine using qemu_coroutine_enter().
+ *
+ * When several coroutines enter each other there may be no way to know which
+ * ones have already been entered.  In such situations this function can be
+ * used to avoid recursively entering coroutines.
+ */
+bool qemu_coroutine_entered(Coroutine *co);
 
 
 /**

util/qemu-coroutine.c

@@ -146,3 +146,8 @@ void coroutine_fn qemu_coroutine_yield(void)
     self->caller = NULL;
     qemu_coroutine_switch(self, to, COROUTINE_YIELD);
 }
+
+bool qemu_coroutine_entered(Coroutine *co)
+{
+    return co->caller;
+}