cgen, sync: implement separate read/write locks for rwshared types (#5687)

vlib/v/tests/shared_lock_3_test.v

@@ -0,0 +1,58 @@
+import sync
+import time
+
+struct St {
+mut:
+	a int
+}
+
+fn f(rwshared x St, shared z St) {
+	for _ in 0..reads_per_thread {
+		rlock x { // other instances may read at the same time
+			time.sleep_ms(1)
+			assert x.a == 7 || x.a == 5
+		}
+	}
+	lock z {
+		z.a--
+	}
+}
+
+const (
+	reads_per_thread = 30
+	read_threads = 10
+	writes = 5
+)
+
+fn test_shared_lock() {
+	// object with separate read/write lock
+	rwshared x := &St{
+		a: 5
+	}
+	shared z := &St{
+		a: read_threads
+	}
+	for _ in 0..read_threads {
+		go f(rwshared x, shared z)
+	}
+	for i in 0..writes {
+		lock x { // wait for ongoing reads to finish, don't start new ones
+			x.a = 17 // this should never be read
+			time.sleep_ms(50)
+			x.a = if (i&1) == 0 { 7 } else { 5 }
+		} // now new reads are possible again
+		time.sleep_ms(20)
+	}
+	// wait until all read threads are finished
+	for finished := false; ; {
+		mut rr := 0
+		lock z {
+			rr = z.a
+			finished = z.a == 0
+		}
+		if finished {
+			break
+		}
+		time.sleep_ms(100)
+	}
+}