all: replace collide mode by async call property

Replace the currently existing straightforward approach to race triggering
(that was almost entirely implemented inside syz-executor) with a more
flexible one.

The `async` call property instructs syz-executor not to block until the
call has completed execution and proceed immediately to the next call.
The decision on what calls to mark with `async` is made by syz-fuzzer.

Ultimately this should let us implement more intelligent race provoking
strategies as well as make more fine-grained reproducers.

Author: a-nogikh

executor/common.h

@@ -514,10 +514,6 @@ static void loop(void)
 	fprintf(stderr, "### start\n");
 #endif
 	int i, call, thread;
-#if SYZ_COLLIDE
-	int collide = 0;
-again:
-#endif
 	for (call = 0; call < /*{{{NUM_CALLS}}}*/; call++) {
 		for (thread = 0; thread < (int)(sizeof(threads) / sizeof(threads[0])); thread++) {
 			struct thread_t* th = &threads[thread];
@@ -534,8 +530,8 @@ static void loop(void)
 			th->call = call;
 			__atomic_fetch_add(&running, 1, __ATOMIC_RELAXED);
 			event_set(&th->ready);
-#if SYZ_COLLIDE
-			if (collide && (call % 2) == 0)
+#if SYZ_ASYNC
+			if (/*{{{ASYNC_CONDITIONS}}}*/)
 				break;
 #endif
 			event_timedwait(&th->done, /*{{{CALL_TIMEOUT_MS}}}*/);
@@ -547,12 +543,6 @@ static void loop(void)
 #if SYZ_HAVE_CLOSE_FDS
 	close_fds();
 #endif
-#if SYZ_COLLIDE
-	if (!collide) {
-		collide = 1;
-		goto again;
-	}
-#endif
 }
 #endif