Possibility for datagrams to be dropped when not able to send

src/core/datagram.c

@@ -421,7 +421,13 @@ QuicDatagramSendFlush(
         CXPLAT_DBG_ASSERT(!(SendRequest->Flags & QUIC_SEND_FLAG_BUFFERED));
         CXPLAT_TEL_ASSERT(Datagram->SendEnabled);
 
-        if (SendRequest->TotalLength > (uint64_t)Datagram->MaxSendLength || QuicConnIsClosed(Connection)) {
+        // Cancels the sending of a Datagram if:
+        // The length we want to send > the length we can send
+        // The connection is closed
+        // The sending mode requires to send immediately (no datagram in queue)
+        if (SendRequest->TotalLength > (uint64_t)Datagram->MaxSendLength 
+        || QuicConnIsClosed(Connection) 
+        || ((SendRequest->Flags & QUIC_SEND_FLAG_DGRAM_CANCEL_ON_BLOCKED) && Datagram->SendQueue != NULL)) {
             QuicDatagramCancelSend(Connection, SendRequest);
             continue;
         }

src/inc/msquic.h

@@ -241,6 +241,7 @@ typedef enum QUIC_SEND_FLAGS {
     QUIC_SEND_FLAG_DGRAM_PRIORITY           = 0x0008,   // Indicates the datagram is higher priority than others.
     QUIC_SEND_FLAG_DELAY_SEND               = 0x0010,   // Indicates the send should be delayed because more will be queued soon.
     QUIC_SEND_FLAG_CANCEL_ON_LOSS           = 0x0020,   // Indicates that a stream is to be cancelled when packet loss is detected.
+    QUIC_SEND_FLAG_DGRAM_CANCEL_ON_BLOCKED  = 0x0030,   // Indicates that a datagram should be dropped when it can't be sent immediately.
 } QUIC_SEND_FLAGS;
 
 DEFINE_ENUM_FLAG_OPERATORS(QUIC_SEND_FLAGS)