stream-service: Avoid race condition when accepting sockets

Even if `poll()` indicates that the socket is ready it might block if
it's in blocking mode. This change avoids blocking in such cases (accept
will fail with EAGAIN/EWOULDBLOCK and `watch()` will return TRUE).

As the non-blocking mode is inherited on Windows (on Linux, the man page
documents the non-inheritance as a Linux specialty), we set the mode for
the accepted socket explicitly to blocking to match the expectations of
`stream_t`.

Fixes: daf1880b39 ("stream: add a stream service class abstracting services using BSD sockets")

src/libstrongswan/networking/streams/stream_service.c

@@ -25,6 +25,7 @@
 #include <errno.h>
 #include <unistd.h>
 #include <sys/stat.h>
+#include <fcntl.h>
 
 typedef struct private_stream_service_t private_stream_service_t;
 
@@ -149,6 +150,22 @@ static void destroy_async_data(async_data_t *data)
 	free(data);
 }
 
+/**
+ * Enable/disable non-blocking mode on a socket.
+ */
+static bool set_blocking(int fd, bool block)
+{
+#ifdef WIN32
+	u_long on = block ? 0 : 1;
+	return ioctlsocket(fd, FIONBIO, &on) == 0;
+#else
+	int flags = fcntl(fd, F_GETFL);
+	return flags != -1 &&
+		   fcntl(fd, F_SETFL, block ? (flags & ~O_NONBLOCK)
+									: (flags |  O_NONBLOCK)) != -1;
+#endif
+}
+
 /**
  * Reduce running counter
  */
@@ -180,6 +197,14 @@ static job_requeue_t accept_async(async_data_t *data)
 	this->running++;
 	this->mutex->unlock(this->mutex);
 
+	/* accepted socket may inherit non-blocking mode from listening socket
+	 * depending on the implementation, so we normalize this to blocking mode */
+	if (!set_blocking(data->fd, TRUE))
+	{
+		reduce_running(data);
+		return JOB_REQUEUE_NONE;
+	}
+
 	stream = stream_create_from_fd(data->fd);
 	if (stream)
 	{
@@ -296,6 +321,12 @@ stream_service_t *stream_service_create_from_fd(int fd)
 {
 	private_stream_service_t *this;
 
+	if (!set_blocking(fd, FALSE))
+	{
+		close(fd);
+		return NULL;
+	}
+
 	INIT(this,
 		.public = {
 			.on_accept = _on_accept,

src/libstrongswan/networking/streams/stream_service.h

@@ -76,10 +76,12 @@ struct stream_service_t {
 /**
  * Create a service from a file descriptor.
  *
- * The file descriptor MUST be a socket.
+ * The file descriptor MUST be a socket. It is set to non-blocking mode to
+ * safely accept() connections from the watcher thread. For consistency, the
+ * accepted connection sockets are set to blocking mode on all platforms.
  *
- * @param fd		file descriptor to wrap into a stream_service_t
- * @return			stream_service instance
+ * @param fd		file descriptor to wrap (adopted, closed on failure)
+ * @return			service, NULL if fd can't be switched to non-blocking mode
  */
 stream_service_t *stream_service_create_from_fd(int fd);