sync: fix chan.close() while a sending thread is waiting (#9654)

vlib/sync/channel_close_test.v

@@ -1,4 +1,4 @@
-import sync
+import time
 
 fn do_rec(ch chan int, resch chan i64) {
 	mut sum := i64(0)
@@ -71,3 +71,38 @@ fn test_channel_close_unbuffered() {
 	sum += <-resch
 	assert sum == i64(8000) * (8000 - 1) / 2
 }
+
+fn test_channel_send_close_buffered() {
+	ch := chan int{cap: 1}
+	t := go fn (ch chan int) {
+		ch <- 31
+		mut x := 45
+		ch <- 17 or {
+			x = -133
+		}
+		assert x == -133
+	}(ch)
+	time.sleep(100 * time.millisecond)
+	ch.close()
+	mut r := <-ch
+	r = <-ch or { 23 }
+	assert r == 23
+	t.wait()
+}
+
+fn test_channel_send_close_unbuffered() {
+	time.sleep(1 * time.second)
+	ch := chan int{}
+	t := go fn (ch chan int) {
+		mut x := 31
+		ch <- 177 or {
+			x = -71
+		}
+		assert x == -71
+	}(ch)
+	time.sleep(100 * time.millisecond)
+	ch.close()
+	r := <-ch or { 238 }
+	assert r == 238
+	t.wait()
+}

vlib/sync/channels.v

@@ -165,6 +165,9 @@ pub fn (mut ch Channel) close() {
 	}
 	C.atomic_store_u16(&ch.write_sub_mtx, u16(0))
 	ch.writesem.post()
+	if ch.cap == 0 {
+		C.atomic_store_ptr(&ch.read_adr, voidptr(0))
+	}
 	ch.writesem_im.post()
 }