guard against one descriptor being flagged twice

io/io_canread.c

@@ -32,6 +32,7 @@ int64 io_canread() {
       alt_firstread=r;
       debug_printf(("io_canread: enqueue %ld in alt read queue (next is %ld)\n",alt_firstread,e->next_read));
 #endif
+      e->canread=0;
       return r;
     }
   }

io/io_canwrite.c

@@ -32,6 +32,7 @@ int64 io_canwrite() {
       alt_firstwrite=r;
       debug_printf(("io_canwrite: enqueue %ld in alt write queue (next is %ld)\n",alt_firstwrite,e->next_write));
 #endif
+      e->canwrite=0;
       return r;
     }
   }

io/io_waituntil2.c

@@ -27,12 +27,12 @@ int64 io_waituntil2(int64 milliseconds) {
     for (i=n-1; i>=0; --i) {
       io_entry* e=array_get(&io_fds,sizeof(io_entry),y[i].data.fd);
       if (e) {
-	if (y[i].events&EPOLLIN) {
+	if (!e->canread && (y[i].events&EPOLLIN)) {
 	  e->canread=1;
 	  e->next_read=first_readable;
 	  first_readable=y[i].data.fd;
 	}
-	if (y[i].events&EPOLLOUT) {
+	if (!e->canwrite && (y[i].events&EPOLLOUT)) {
 	  e->canwrite=1;
 	  e->next_write=first_writeable;
 	  first_writeable=y[i].data.fd;
@@ -54,12 +54,12 @@ int64 io_waituntil2(int64 milliseconds) {
     for (i=n-1; i>=0; --i) {
       io_entry* e=array_get(&io_fds,sizeof(io_entry),y[--n].ident);
       if (e) {
-	if (y[n].filter==EVFILT_READ) {
+	if (!e->canread && (y[n].filter==EVFILT_READ)) {
 	  e->canread=1;
 	  e->next_read=first_readable;
 	  first_readable=y[n].ident;
 	}
-	if (y[n].filter==EVFILT_WRITE) {
+	if (!e->canwrite && (y[n].filter==EVFILT_WRITE)) {
 	  e->canwrite=1;
 	  e->next_write=first_writeable;
 	  first_writeable=y[i].ident;
@@ -134,12 +134,12 @@ again:
   }
   for (j=r-1; j>=0; --j) {
     io_entry* e=array_get(&io_fds,sizeof(io_entry),p->fd);
-    if (p->revents&POLLIN) {
+    if (!e->canread && (p->revents&POLLIN)) {
       e->canread=1;
       e->next_read=first_readable;
       first_readable=p->fd;
     }
-    if (p->revents&POLLOUT) {
+    if (!e->canwrite && (p->revents&POLLOUT)) {
       e->canwrite=1;
       e->next_write=first_writeable;
       first_writeable=p->fd;