From 89429598bf897a34d61027cf7f68f5e35fcfff4b Mon Sep 17 00:00:00 2001
From: Toby <tobyxdd@gmail.com>
Date: Fri, 6 Oct 2023 18:27:30 -0700
Subject: [PATCH] fix: BBR bandwidth estimation edge case

---
 core/internal/congestion/bbr/bbr_sender.go | 18 ++++++++++++++----
 core/internal/congestion/common/pacer.go   |  3 +++
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/core/internal/congestion/bbr/bbr_sender.go b/core/internal/congestion/bbr/bbr_sender.go
index 48e6e4b..9cf9cf6 100644
--- a/core/internal/congestion/bbr/bbr_sender.go
+++ b/core/internal/congestion/bbr/bbr_sender.go
@@ -21,6 +21,8 @@ import (
 //
 
 const (
+	minBps = 65536 // 64 kbps
+
 	invalidPacketNumber            = -1
 	initialCongestionWindowPackets = 32
 
@@ -283,10 +285,7 @@ func newBbrSender(
 		maxCongestionWindowWithNetworkParametersAdjusted: initialMaxCongestionWindow,
 		maxDatagramSize: initialMaxDatagramSize,
 	}
-	b.pacer = common.NewPacer(func() congestion.ByteCount {
-		// Pacer wants bytes per second, but Bandwidth is in bits per second.
-		return congestion.ByteCount(float64(b.bandwidthEstimate()) * b.congestionWindowGain / float64(BytesPerSecond))
-	})
+	b.pacer = common.NewPacer(b.bandwidthForPacer)
 
 	/*
 		if b.tracer != nil {
@@ -536,6 +535,17 @@ func (b *bbrSender) bandwidthEstimate() Bandwidth {
 	return b.maxBandwidth.GetBest()
 }
 
+func (b *bbrSender) bandwidthForPacer() congestion.ByteCount {
+	bps := congestion.ByteCount(float64(b.bandwidthEstimate()) * b.congestionWindowGain / float64(BytesPerSecond))
+	if bps < minBps {
+		// We need to make sure that the bandwidth value for pacer is never zero,
+		// otherwise it will go into an edge case where HasPacingBudget = false
+		// but TimeUntilSend is before, causing the quic-go send loop to go crazy and get stuck.
+		return minBps
+	}
+	return bps
+}
+
 // Returns the current estimate of the RTT of the connection.  Outside of the
 // edge cases, this is minimum RTT.
 func (b *bbrSender) getMinRtt() time.Duration {
diff --git a/core/internal/congestion/common/pacer.go b/core/internal/congestion/common/pacer.go
index b2b301b..4e089a3 100644
--- a/core/internal/congestion/common/pacer.go
+++ b/core/internal/congestion/common/pacer.go
@@ -43,6 +43,9 @@ func (p *Pacer) Budget(now time.Time) congestion.ByteCount {
 		return p.maxBurstSize()
 	}
 	budget := p.budgetAtLastSent + (p.getBandwidth()*congestion.ByteCount(now.Sub(p.lastSentTime).Nanoseconds()))/1e9
+	if budget < 0 { // protect against overflows
+		budget = congestion.ByteCount(1<<62 - 1)
+	}
 	return minByteCount(p.maxBurstSize(), budget)
 }