When performing a timer based retransmission, clear the BBR_RWND_COLLAPSED flag. If it sticks, t_rxtshift is never incremented resulting in unlimited retransmissions in the LAST-ACK state.

Reported by: Illia Vynohradov

Test Plan

Run the following packetdrill script:

//
// Copyright (c) 2026 Michael Tuexen
// All rights reserved.
//
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// SUCH DAMAGE.
//
--tolerance_usecs=25000
// Ensure that all relevant sysctl variables have their default values.
 0.000 `sysctl -w net.inet.tcp.sack.enable=1`
// Ensure the BBR stack is available.
+0.000 `kldload -n tcp_bbr`

// Get the TCP connection is LAST-ACK state.
+0.000 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+0.000 setsockopt(3, IPPROTO_TCP, TCP_FUNCTION_BLK, {function_set_name="bbr",
                                                     pcbcnt=0}, 36) = 0
+0.000 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+0.000 bind(3, ..., ...) = 0
+0.000 listen(3, 1) = 0
+0.000 < S   0:0(0)          win 65535 <mss 1460,sackOK,eol,eol>
+0.000 > S.  0:0(0)   ack  1 win 65535 <mss 1460,sackOK,eol,eol>
+0.050 <  .  1:1(0)   ack  1 win 65535
+0.000 accept(3, ..., ...) = 4
+0.000 close(3) = 0
+0.000 setsockopt(4, IPPROTO_TCP, TCP_LOG, [TCP_LOG_STATE_CONTINUAL], 4) = 0
+0.020 < F.  1:25(24) ack  1 win 65535
+0.000 recv(4, ..., 100, 0) = 24
+0.000 >  .  1:1(0)   ack 26 win 65535
+0.000 send(4, ..., 24, 0) = 24
+0.000 > P.  1:25(24) ack 26 win 65535
+0.050 < .  26:26(0)  ack 25 win 65535
+0.000 close(4) = 0
+0.000 > F. 25:25(0)  ack 26 win 65535 // Now we are in LAST-ACK
+0.150 > F. 25:25(0)  ack 26 win 65535 // RTX
+0.000 <  . 26:26(0)  ack 25 win 0     // This is the strange segment
+0.300 > F. 25:25(0)  ack 26 win 65535
+0.600 > F. 25:25(0)  ack 26 win 65535
+1.200 > F. 25:25(0)  ack 26 win 65535
+2.400 > F. 25:25(0)  ack 26 win 65535
+4.000 > F. 25:25(0)  ack 26 win 65535
+4.000 > F. 25:25(0)  ack 26 win 65535
+4.000 > F. 25:25(0)  ack 26 win 65535
+4.000 > F. 25:25(0)  ack 26 win 65535
+4.000 > F. 25:25(0)  ack 26 win 65535
+4.000 > F. 25:25(0)  ack 26 win 65535
+4.000 > F. 25:25(0)  ack 26 win 65535
+4.000 > R. 25:25(0)  ack 26 win 65535