Send first probe packet directly instead of enqueuing it.

This avoids potentially creating needless containers in the packet
queue and removes usage of the packet prio, allowing it to be moved in
an upcoming CL.

Bug: webrtc:11340
Change-Id: Iddd9e7e4e73c97ab25a85e42bcc0094d61fd60d3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/259524
Reviewed-by: Emil Lundmark <lndmrk@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36602}

modules/pacing/pacing_controller.cc

@@ -456,6 +456,7 @@ void PacingController::ProcessPackets() {
 
   PacedPacketInfo pacing_info;
   DataSize recommended_probe_size = DataSize::Zero();
+  DataSize data_sent = DataSize::Zero();
   bool is_probing = prober_.is_probing();
   if (is_probing) {
     // Probe timing is sensitive, and handled explicitly by BitrateProber, so
@@ -472,8 +473,8 @@ void PacingController::ProcessPackets() {
         // If no RTP modules sending media are registered, we may not get a
         // padding packet back.
         if (!padding.empty()) {
-          // Insert with high priority so larger media packets don't preempt it.
+          // Send packet immediately to avoid priority inversions.
-          EnqueuePacketInternal(std::move(padding[0]), kFirstPriority);
+          data_sent += SendPacket(std::move(padding[0]), pacing_info, now);
           // We should never get more than one padding packets with a requested
           // size of 1 byte.
           RTC_DCHECK_EQ(padding.size(), 1u);
@@ -485,13 +486,18 @@ void PacingController::ProcessPackets() {
     }
   }
 
-  DataSize data_sent = DataSize::Zero();
   // Circuit breaker, making sure main loop isn't forever.
   static constexpr int kMaxIterations = 1 << 16;
   int iteration = 0;
   int packets_sent = 0;
   int padding_packets_generated = 0;
   for (; iteration < kMaxIterations; ++iteration) {
+    // If we are currently probing, we need to stop the send loop when we have
+    // reached the send target.
+    if (is_probing && data_sent >= recommended_probe_size) {
+      break;
+    }
+
     // Fetch packet, so long as queue is not empty or budget is not
     // exhausted.
     std::unique_ptr<RtpPacketToSend> rtp_packet =
@@ -518,33 +524,9 @@ void PacingController::ProcessPackets() {
       // Can't fetch new packet and no padding to send, exit send loop.
       break;
     } else {
-      RTC_DCHECK(rtp_packet);
+      data_sent += SendPacket(std::move(rtp_packet), pacing_info, now);
-      RTC_DCHECK(rtp_packet->packet_type().has_value());
-      const RtpPacketMediaType packet_type = *rtp_packet->packet_type();
-      DataSize packet_size = DataSize::Bytes(rtp_packet->payload_size() +
-                                             rtp_packet->padding_size());
-
-      if (include_overhead_) {
-        packet_size += DataSize::Bytes(rtp_packet->headers_size()) +
-                       transport_overhead_per_packet_;
-      }
-
-      packet_sender_->SendPacket(std::move(rtp_packet), pacing_info);
-      for (auto& packet : packet_sender_->FetchFec()) {
-        EnqueuePacket(std::move(packet));
-      }
-      data_sent += packet_size;
       ++packets_sent;
 
-      // Send done, update send time.
-      OnPacketSent(packet_type, packet_size, now);
-
-      // If we are currently probing, we need to stop the send loop when we
-      // have reached the send target.
-      if (is_probing && data_sent >= recommended_probe_size) {
-        break;
-      }
-
       // Update target send time in case that are more packets that we are late
       // in processing.
       if (mode_ == ProcessMode::kDynamic) {
@@ -658,6 +640,31 @@ std::unique_ptr<RtpPacketToSend> PacingController::GetPendingPacket(
   return packet_queue_.Pop();
 }
 
+DataSize PacingController::SendPacket(std::unique_ptr<RtpPacketToSend> packet,
+                                      const PacedPacketInfo& pacing_info,
+                                      Timestamp now) {
+  RTC_DCHECK(packet);
+  RTC_DCHECK(packet->packet_type().has_value());
+  const RtpPacketMediaType packet_type = *packet->packet_type();
+  DataSize packet_size =
+      DataSize::Bytes(packet->payload_size() + packet->padding_size());
+
+  if (include_overhead_) {
+    packet_size += DataSize::Bytes(packet->headers_size()) +
+                   transport_overhead_per_packet_;
+  }
+
+  packet_sender_->SendPacket(std::move(packet), pacing_info);
+  for (std::unique_ptr<RtpPacketToSend>& packet : packet_sender_->FetchFec()) {
+    EnqueuePacket(std::move(packet));
+  }
+
+  // Sending complete, update send time.
+  OnPacketSent(packet_type, packet_size, now);
+
+  return packet_size;
+}
+
 void PacingController::OnPacketSent(RtpPacketMediaType packet_type,
                                     DataSize packet_size,
                                     Timestamp send_time) {

modules/pacing/pacing_controller.h

@@ -167,6 +167,9 @@ class PacingController {
       const PacedPacketInfo& pacing_info,
       Timestamp target_send_time,
       Timestamp now);
+  DataSize SendPacket(std::unique_ptr<RtpPacketToSend> packet,
+                      const PacedPacketInfo& pacing_info,
+                      Timestamp now);
   void OnPacketSent(RtpPacketMediaType packet_type,
                     DataSize packet_size,
                     Timestamp send_time);