core: allow spurious wakeups in coio_waitpid

Currently it's possible to wakeup a fiber, which is waiting for a
child process termination, using Tarantool C API. This will leave
a zombie process behind. This patch reworks `coio_waitpid` in such
a way that it yields until `cw.data` is set to NULL in the process
status change callback.

Part of #7166

NO_DOC=refactoring
NO_CHANGELOG=refactoring

src/lib/core/coio.c

@@ -491,25 +491,36 @@ coio_stat_stat_timeout(ev_stat *stat, ev_tstamp timeout)
 	return 0;
 }
 
-typedef void (*ev_child_cb)(ev_loop *, ev_child *, int);
+/**
+ * The process status change callback.
+ * Similar to fiber_schedule_cb, but also set watcher->data to NULL to indicate
+ * that the fiber has been woken up on the child process termination.
+ */
+static void
+coio_status_change_cb(ev_loop *loop, ev_child *watcher, int revents)
+{
+	(void)loop;
+	(void)revents;
+	struct fiber *fiber = watcher->data;
+	assert(fiber() == &cord()->sched);
+	watcher->data = NULL;
+	fiber_wakeup(fiber);
+}
 
 int
 coio_waitpid(pid_t pid, int *status)
 {
 	assert(cord_is_main());
 	ev_child cw;
-	ev_init(&cw, (ev_child_cb) fiber_schedule_cb);
+	ev_init(&cw, coio_status_change_cb);
 	ev_child_set(&cw, pid, 0);
 	cw.data = fiber();
 	ev_child_start(loop(), &cw);
-	/*
-	 * It's not safe to spuriously wakeup this fiber since
-	 * in this case the server will leave a zombie process
-	 * behind.
-	 */
-	bool allow_cancel = fiber_set_cancellable(false);
-	fiber_yield();
-	fiber_set_cancellable(allow_cancel);
+
+	do {
+		fiber_yield();
+	} while (cw.data != NULL);
+
 	ev_child_stop(loop(), &cw);
 	if (fiber_is_cancelled()) {
 		diag_set(FiberIsCancelled);

test/unit/coio.cc

@@ -8,6 +8,7 @@
 
 #include <fcntl.h>
 #include <sys/uio.h>
+#include <sys/wait.h>
 #include <sys/errno.h>
 
 int
@@ -171,6 +172,29 @@ test_writev_f(va_list ap)
 	return 0;
 }
 
+static int
+test_waitpid_f(va_list ap)
+{
+	/* Flush buffers to avoid multiple output. */
+	fflush(stdout);
+	fflush(stderr);
+
+	pid_t pid = fork();
+
+	/* Child process: do something for 10ms and exit. */
+	if (pid == 0) {
+		usleep(10000);
+		exit(0);
+	}
+
+	fail_if(pid == -1);
+	int status, rc = coio_waitpid(pid, &status);
+	fail_if(rc != 0);
+	fail_if(WIFEXITED(status) == 0);
+
+	return 0;
+}
+
 static void
 fill_pipe(int fd)
 {
@@ -211,11 +235,13 @@ read_write_test(void)
 		test_read_f,
 		test_write_f,
 		test_writev_f,
+		test_waitpid_f
 	};
 	const char *descr[] = {
 		"read",
 		"write",
 		"writev",
+		"waitpid"
 	};
 
 	int num_tests = sizeof(test_funcs) / sizeof(test_funcs[0]);

test/unit/coio.result

@@ -20,11 +20,13 @@ ok 3 - bad ipv6 host name - error
 ok 4 - bad ipv6 host name - error message
 	*** test_connect: done ***
 	*** read_write_test ***
-1..6
+1..8
 ok 1 - coio_read handle spurious wakeup
 ok 2 - coio_read success after a spurious wakeup
 ok 3 - coio_write handle spurious wakeup
 ok 4 - coio_write success after a spurious wakeup
 ok 5 - coio_writev handle spurious wakeup
 ok 6 - coio_writev success after a spurious wakeup
+ok 7 - coio_waitpid handle spurious wakeup
+ok 8 - coio_waitpid success after a spurious wakeup
 	*** read_write_test: done ***