Don't use wall clock for stats

This uses the local NTP clock for RTCP report block stats.

This code exists in the version that Mozilla is shipping, with a review
here https://phabricator.services.mozilla.com/D127709 .

Bug: webrtc:13484
Change-Id: I2f46ec02acab0bbb09040778b05b248c2d815bd1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/240142
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35787}

audio/channel_receive.cc

@@ -846,14 +846,11 @@ CallReceiveStatistics ChannelReceive::GetRTCPStatistics() const {
   absl::optional<RtpRtcpInterface::SenderReportStats> rtcp_sr_stats =
       rtp_rtcp_->GetSenderReportStats();
   if (rtcp_sr_stats.has_value()) {
-    // Number of seconds since 1900 January 1 00:00 GMT (see
-    // https://tools.ietf.org/html/rfc868).
-    constexpr int64_t kNtpJan1970Millisecs =
-        2208988800 * rtc::kNumMillisecsPerSec;
     stats.last_sender_report_timestamp_ms =
-        rtcp_sr_stats->last_arrival_timestamp.ToMs() - kNtpJan1970Millisecs;
+        rtcp_sr_stats->last_arrival_timestamp.ToMs() -
+        rtc::kNtpJan1970Millisecs;
     stats.last_sender_report_remote_timestamp_ms =
-        rtcp_sr_stats->last_remote_timestamp.ToMs() - kNtpJan1970Millisecs;
+        rtcp_sr_stats->last_remote_timestamp.ToMs() - rtc::kNtpJan1970Millisecs;
     stats.sender_reports_packets_sent = rtcp_sr_stats->packets_sent;
     stats.sender_reports_bytes_sent = rtcp_sr_stats->bytes_sent;
     stats.sender_reports_reports_count = rtcp_sr_stats->reports_count;

modules/rtp_rtcp/source/receive_statistics_impl.cc

@@ -22,16 +22,13 @@
 #include "modules/rtp_rtcp/source/rtp_rtcp_config.h"
 #include "modules/rtp_rtcp/source/time_util.h"
 #include "rtc_base/logging.h"
+#include "rtc_base/time_utils.h"
 #include "system_wrappers/include/clock.h"
 
 namespace webrtc {
 namespace {
 constexpr int64_t kStatisticsTimeoutMs = 8000;
 constexpr int64_t kStatisticsProcessIntervalMs = 1000;
-
-// Number of seconds since 1900 January 1 00:00 GMT (see
-// https://tools.ietf.org/html/rfc868).
-constexpr int64_t kNtpJan1970Millisecs = 2'208'988'800'000;
 }  // namespace
 
 StreamStatistician::~StreamStatistician() {}
@@ -43,7 +40,7 @@ StreamStatisticianImpl::StreamStatisticianImpl(uint32_t ssrc,
       clock_(clock),
       delta_internal_unix_epoch_ms_(clock_->CurrentNtpInMilliseconds() -
                                     clock_->TimeInMilliseconds() -
-                                    kNtpJan1970Millisecs),
+                                    rtc::kNtpJan1970Millisecs),
       incoming_bitrate_(kStatisticsProcessIntervalMs,
                         RateStatistics::kBpsScale),
       max_reordering_threshold_(max_reordering_threshold),

modules/rtp_rtcp/source/rtcp_receiver.cc

@@ -662,7 +662,12 @@ void RTCPReceiver::HandleReportBlock(const ReportBlock& report_block,
   rtcp_report_block.delay_since_last_sender_report =
       report_block.delay_since_last_sr();
   rtcp_report_block.last_sender_report_timestamp = report_block.last_sr();
-  report_block_data->SetReportBlock(rtcp_report_block, rtc::TimeUTCMicros());
+  // Number of seconds since 1900 January 1 00:00 GMT (see
+  // https://tools.ietf.org/html/rfc868).
+  report_block_data->SetReportBlock(
+      rtcp_report_block,
+      (clock_->CurrentNtpInMilliseconds() - rtc::kNtpJan1970Millisecs) *
+          rtc::kNumMicrosecsPerMillisec);
 
   int64_t rtt_ms = 0;
   uint32_t send_time_ntp = report_block.last_sr();

modules/rtp_rtcp/source/rtcp_receiver_unittest.cc

@@ -1573,12 +1573,8 @@ TEST(RtcpReceiverTest,
   const uint32_t kCumulativeLoss = 7;
   const uint32_t kJitter = 9;
   const uint16_t kSequenceNumber = 1234;
-  const int64_t kUtcNowUs = 42;
+  const int64_t kNtpNowMs =
-
+      mocks.clock.CurrentNtpInMilliseconds() - rtc::kNtpJan1970Millisecs;
-  // The "report_block_timestamp_utc_us" is obtained from the global UTC clock
-  // (not the simulcated `mocks.clock`) and requires a scoped fake clock.
-  rtc::ScopedFakeClock fake_clock;
-  fake_clock.SetTime(Timestamp::Micros(kUtcNowUs));
 
   rtcp::ReportBlock rtcp_block;
   rtcp_block.SetMediaSsrc(kReceiverMainSsrc);
@@ -1601,7 +1597,8 @@ TEST(RtcpReceiverTest,
         EXPECT_EQ(rtcp_block.extended_high_seq_num(),
                   report_block.extended_highest_sequence_number);
         EXPECT_EQ(rtcp_block.jitter(), report_block.jitter);
-        EXPECT_EQ(kUtcNowUs, report_block_data.report_block_timestamp_utc_us());
+        EXPECT_EQ(kNtpNowMs * rtc::kNumMicrosecsPerMillisec,
+                  report_block_data.report_block_timestamp_utc_us());
         // No RTT is calculated in this test.
         EXPECT_EQ(0u, report_block_data.num_rtts());
       });

rtc_base/time_utils.h

@@ -31,6 +31,12 @@ static const int64_t kNumNanosecsPerMillisec =
 static const int64_t kNumNanosecsPerMicrosec =
     kNumNanosecsPerSec / kNumMicrosecsPerSec;
 
+// Elapsed milliseconds between NTP base, 1900 January 1 00:00 GMT
+// (see https://tools.ietf.org/html/rfc868), and January 1 00:00 GMT 1970
+// epoch. This is useful when converting between the NTP time base and the
+// time base used in RTCP reports.
+constexpr int64_t kNtpJan1970Millisecs = 2'208'988'800 * kNumMillisecsPerSec;
+
 // TODO(honghaiz): Define a type for the time value specifically.
 
 class ClockInterface {