Revert "Send first probe packet directly instead of enqueuing it."

This reverts commit 8088aad5ac0154d8fdc252de9e8ab4e0172d7da2.

Reason for revert: Downstream test failure

Original change's description:
> 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}

Bug: webrtc:11340
Change-Id: Ib2a4bdad3ca82108e150feefb2680b880bc316e5
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/259763
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Emil Lundmark <lndmrk@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36605}

modules/pacing/pacing_controller.cc

@@ -456,7 +456,6 @@ 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
@@ -473,8 +472,8 @@ void PacingController::ProcessPackets() {
         // If no RTP modules sending media are registered, we may not get a
         // padding packet back.
         if (!padding.empty()) {
-          // Send packet immediately to avoid priority inversions.
-          data_sent += SendPacket(std::move(padding[0]), pacing_info, now);
+          // Insert with high priority so larger media packets don't preempt it.
+          EnqueuePacketInternal(std::move(padding[0]), kFirstPriority);
           // We should never get more than one padding packets with a requested
           // size of 1 byte.
           RTC_DCHECK_EQ(padding.size(), 1u);
@@ -486,18 +485,13 @@ 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 =
@@ -524,9 +518,33 @@ void PacingController::ProcessPackets() {
       // Can't fetch new packet and no padding to send, exit send loop.
       break;
     } else {
-      data_sent += SendPacket(std::move(rtp_packet), pacing_info, now);
+      RTC_DCHECK(rtp_packet);
+      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) {
@@ -640,31 +658,6 @@ 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

@@ -167,9 +167,6 @@ 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);