Fix locking when writing to connections

There are 2 race conditions when writing to connections:
- Since we don't hold the semaphore during iteration, a connection can
  be removed by another thread while we try to use it, which will lead
  to use after free.
- Multiple threads may try to write to the same connection socket,
  corrupting the packets (lima-vm#39).

Both issues are fixed by holding the semaphore during iteration and
writing to the socket. This is not the most efficient way but
socket_vmnet crashes daily and we must stop the bleeding first. We can
to add more fine grain locking later.

main.c

@@ -169,9 +169,7 @@ static void _on_vmnet_packets_available(interface_ref iface, int64_t buf_count,
            dest_mac[5], src_mac[0], src_mac[1], src_mac[2], src_mac[3],
            src_mac[4], src_mac[5]);
     dispatch_semaphore_wait(state->sem, DISPATCH_TIME_FOREVER);
-    struct conn *conns = state->conns;
-    dispatch_semaphore_signal(state->sem);
-    for (struct conn *conn = conns; conn != NULL; conn = conn->next) {
+    for (struct conn *conn = state->conns; conn != NULL; conn = conn->next) {
       // FIXME: avoid flooding
       DEBUGF("[Handler i=%d] Sending to the socket %d: 4 + %ld bytes [Dest "
              "%02X:%02X:%02X:%02X:%02X:%02X]",
@@ -197,6 +195,7 @@ static void _on_vmnet_packets_available(interface_ref iface, int64_t buf_count,
         goto done;
       }
     }
+    dispatch_semaphore_signal(state->sem);
   }
 done:
   if (pdv != NULL) {
@@ -518,9 +517,7 @@ static void on_accept(struct state *state, int accept_fd, interface_ref iface) {
     // (Not handled by vmnet)
     // FIXME: avoid flooding
     dispatch_semaphore_wait(state->sem, DISPATCH_TIME_FOREVER);
-    struct conn *conns = state->conns;
-    dispatch_semaphore_signal(state->sem);
-    for (struct conn *conn = conns; conn != NULL; conn = conn->next) {
+    for (struct conn *conn = state->conns; conn != NULL; conn = conn->next) {
       if (conn->socket_fd == accept_fd)
         continue;
       DEBUGF("[Socket-to-Socket i=%lld] Sending from socket %d to socket %d: "
@@ -545,6 +542,7 @@ static void on_accept(struct state *state, int accept_fd, interface_ref iface) {
         continue;
       }
     }
+    dispatch_semaphore_signal(state->sem);
   }
 done:
   INFOF("Closing a connection (fd %d)", accept_fd);