udp: add flag to enable recvmmsg(2) explicitly

Instead of implicitly enabling it by checking the supplied buffer size
to alloc_cb, have a dedicated flag that must be set on `uv_udp_init_ex`.

Fixes: https://github.com/libuv/libuv/issues/2791
Closes: https://github.com/libuv/libuv/pull/2792
PR-URL: https://github.com/libuv/libuv/pull/2799
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

docs/src/udp.rst

@@ -41,12 +41,16 @@ Data types
             * (provided they all set the flag) but only the last one to bind will receive
             * any traffic, in effect "stealing" the port from the previous listener.
             */
-            UV_UDP_REUSEADDR = 4
+            UV_UDP_REUSEADDR = 4,
             /*
              * Indicates that the message was received by recvmmsg, so the buffer provided
              * must not be freed by the recv_cb callback.
              */
-            UV_UDP_MMSG_CHUNK = 8
+            UV_UDP_MMSG_CHUNK = 8,
+            /*
+            * Indicates that recvmmsg should be used, if available.
+            */
+            UV_UDP_RECVMMSG = 256
         };
 
 .. c:type:: void (*uv_udp_send_cb)(uv_udp_send_t* req, int status)
@@ -125,12 +129,17 @@ API
 
 .. c:function:: int uv_udp_init_ex(uv_loop_t* loop, uv_udp_t* handle, unsigned int flags)
 
-    Initialize the handle with the specified flags. At the moment the lower 8 bits
-    of the `flags` parameter are used as the socket domain. A socket will be created
-    for the given domain. If the specified domain is ``AF_UNSPEC`` no socket is created,
-    just like :c:func:`uv_udp_init`.
+    Initialize the handle with the specified flags. The lower 8 bits of the `flags`
+    parameter are used as the socket domain. A socket will be created for the given domain.
+    If the specified domain is ``AF_UNSPEC`` no socket is created, just like :c:func:`uv_udp_init`.
+
+    The remaining bits can be used to set one of these flags:
+
+    * `UV_UDP_RECVMMSG`: if set, and the platform supports it, :man:`recvmmsg(2)` will
+      be used.
 
     .. versionadded:: 1.7.0
+    .. versionchanged:: 1.37.0 added the `UV_UDP_RECVMMSG` flag.
 
 .. c:function:: int uv_udp_open(uv_udp_t* handle, uv_os_sock_t sock)
 
@@ -379,6 +388,9 @@ API
     .. versionchanged:: 1.35.0 added support for :man:`recvmmsg(2)` on supported platforms).
                         The use of this feature requires a buffer larger than
                         2 * 64KB to be passed to `alloc_cb`.
+    .. versionchanged:: 1.37.0 :man:`recvmmsg(2)` support is no longer enabled implicitly,
+                        it must be explicitly requested by passing the `UV_UDP_RECVMMSG` flag to
+                        :c:func:`uv_udp_init_ex`.
 
 .. c:function:: int uv_udp_recv_stop(uv_udp_t* handle)
 

include/uv.h

@@ -610,7 +610,12 @@ enum uv_udp_flags {
    * Indicates that the message was received by recvmmsg, so the buffer provided
    * must not be freed by the recv_cb callback.
    */
-  UV_UDP_MMSG_CHUNK = 8
+  UV_UDP_MMSG_CHUNK = 8,
+
+  /*
+   * Indicates that recvmmsg should be used, if available.
+   */
+  UV_UDP_RECVMMSG = 256
 };
 
 typedef void (*uv_udp_send_cb)(uv_udp_send_t* req, int status);

src/unix/udp.c

@@ -262,11 +262,9 @@ static void uv__udp_recvmsg(uv_udp_t* handle) {
     assert(buf.base != NULL);
 
 #if HAVE_MMSG
+    if (handle->flags & UV_HANDLE_UDP_RECVMMSG) {
       uv_once(&once, uv__udp_mmsg_init);
       if (uv__recvmmsg_avail) {
-      /* Returned space for more than 1 datagram, use it to receive
-       * multiple datagrams. */
-      if (buf.len >= 2 * UV__UDP_DGRAM_MAXSIZE) {
         nread = uv__udp_recvmmsg(handle, &buf);
         if (nread > 0)
           count -= nread;
@@ -949,6 +947,7 @@ static int uv__udp_set_source_membership6(uv_udp_t* handle,
 int uv_udp_init_ex(uv_loop_t* loop, uv_udp_t* handle, unsigned int flags) {
   int domain;
   int err;
+  int extra_flags;
   int fd;
 
   /* Use the lower 8 bits for the domain */
@@ -956,7 +955,9 @@ int uv_udp_init_ex(uv_loop_t* loop, uv_udp_t* handle, unsigned int flags) {
   if (domain != AF_INET && domain != AF_INET6 && domain != AF_UNSPEC)
     return UV_EINVAL;
 
-  if (flags & ~0xFF)
+  /* Use the higher bits for extra flags */
+  extra_flags = flags & ~0xFF;
+  if (extra_flags & ~UV_UDP_RECVMMSG)
     return UV_EINVAL;
 
   if (domain != AF_UNSPEC) {
@@ -977,6 +978,9 @@ int uv_udp_init_ex(uv_loop_t* loop, uv_udp_t* handle, unsigned int flags) {
   QUEUE_INIT(&handle->write_queue);
   QUEUE_INIT(&handle->write_completed_queue);
 
+  if (extra_flags & UV_UDP_RECVMMSG)
+    handle->flags |= UV_HANDLE_UDP_RECVMMSG;
+
   return 0;
 }
 

src/uv-common.h

@@ -104,6 +104,7 @@ enum {
   /* Only used by uv_udp_t handles. */
   UV_HANDLE_UDP_PROCESSING              = 0x01000000,
   UV_HANDLE_UDP_CONNECTED               = 0x02000000,
+  UV_HANDLE_UDP_RECVMMSG                = 0x04000000,
 
   /* Only used by uv_pipe_t handles. */
   UV_HANDLE_NON_OVERLAPPED_PIPE         = 0x01000000,