lightningd: another gossip transition corner case.

If we see a close tx at the same time we see the channel reach announce depth (here, 104x1x0 confirms
and we process blocks 109 and 110), we see it go from:

	CGOSSIP_WAITING_FOR_ANNOUNCE_DEPTH->CGOSSIP_CHANNEL_UNANNOUNCED_DYING (Channel closing before 6 confirms)

Then:
	CGOSSIP_CHANNEL_UNANNOUNCED_DYING->CGOSSIP_CHANNEL_ANNOUNCED_DEAD

```
INFO    0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-chan#2: State changed from CHANNELD_NORMAL to CHANNELD_SHUTTING_DOWN
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-chan#2: gossip state: CGOSSIP_WAITING_FOR_ANNOUNCE_DEPTH->CGOSSIP_CHANNEL_UNANNOUNCED_DYING (Channel closing before 6 confirms)
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-channeld-chan#2: billboard: Channel ready for use. They've sent shutdown, waiting for ours
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-channeld-chan#2: Trying commit
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-channeld-chan#2: Can't send commit: nothing to send, feechange not wanted ({ RCVD_ADD_ACK_REVOCATION:3755 }) blockheight not wanted ({ RCVD_ADD_ACK_REVOCATION:109 })
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-channeld-chan#2: peer_out WIRE_SHUTDOWN
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-channeld-chan#2: billboard: Channel ready for use. Shutdown messages exchanged.
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-closingd-chan#2: pid 35692, msgfd 86
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-channeld-chan#2: Status closed, but not exited. Killing
INFO    0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-chan#2: State changed from CHANNELD_SHUTTING_DOWN to CLOSINGD_SIGEXCHANGE
DEBUG   hsmd: new_client: 2
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-closingd-chan#2: Expected closing weight = 772, fee 2895sat (min 1447sat, max 1006268sat)
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-closingd-chan#2: out = 506268sat/500000sat
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-closingd-chan#2: dustlimit = 546sat
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-closingd-chan#2: fee = 2895sat
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-closingd-chan#2: fee negotiation step = 50%
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-closingd-chan#2: billboard perm: Negotiating closing fee between 1447sat and 1006268sat satoshi (ideal 2895sat) using step 50%
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-closingd-chan#2: billboard: Waiting for their initial closing fee offer
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-closingd-chan#2: peer_in WIRE_CLOSING_SIGNED
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-closingd-chan#2: Making close tx at = 506268sat/500000sat fee 2895sat
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-closingd-chan#2: Received fee offer 2895sat
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-closingd-chan#2: ...offer is reasonable
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-chan#2: Their actual closing tx fee is 2895sat vs previous 4220sat: weight is 772
INFO    0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-closingd-chan#2: performing quickclose in range 1447sat-8492sat
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-closingd-chan#2: Making close tx at = 506268sat/500000sat fee 2895sat
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-hsmd: Got WIRE_HSMD_SIGN_MUTUAL_CLOSE_TX
DEBUG   hsmd: Client: Received message 21 from client
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-closingd-chan#2: sending fee offer 2895sat
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-closingd-chan#2: peer_out WIRE_CLOSING_SIGNED
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-closingd-chan#2: billboard perm: We agreed on a closing fee of 2895 satoshi for tx:7685256e6e4632bd601f42bb85b6f22b7a0db8d55c468dfbb1703c3ab2e4d9ce
INFO    0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-chan#2: State changed from CLOSINGD_SIGEXCHANGE to CLOSINGD_COMPLETE
DEBUG   wallet: Owning output 1 506268sat (p2tr) txid 7685256e6e4632bd601f42bb85b6f22b7a0db8d55c468dfbb1703c3ab2e4d9ce
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-chan#2: We have 1 anchor points to use
DEBUG   lightningd: Broadcasting txid 7685256e6e4632bd601f42bb85b6f22b7a0db8d55c468dfbb1703c3ab2e4d9ce
DEBUG   lightningd: sendrawtransaction: 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
DEBUG   lightningd: Adding block 108: 4e6ba3059c6890797a29acd306e82d6f4e85ad62a6aa6d0ee5bc6ead3e6329cd
DEBUG   hsmd: Client: Received message 5 from client
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-connectd: peer_downgrade
DEBUG   plugin-bcli: sendrawtx exit 0 (bitcoin-cli -regtest -datadir=/tmp/ltests-laf51vlg/test_buy_liquidity_ad_check_bookkeeping_1/lightning-2/ -rpcclienttimeout=60 -rpcport=41847 -rpcuser=... -stdinrpcpass sendrawtransaction 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) 
DEBUG   lightningd: Adding block 109: 540a47d6b316aec8d1b56efca22ffb384403659e3b29cddd3d68f87df72065a6
DEBUG   lightningd: Adding block 110: 1931e9ea62324d0fb50188df525e84ee7021e1910cf3534b343db05cc4639851
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-chan#2: Got UTXO spend for beff505170fbccbbc8f72ae16f07b3a99dcf1460de9d71ed8224bf1a48e5506f:0: 7685256e6e4632bd601f42bb85b6f22b7a0db8d55c468dfbb1703c3ab2e4d9ce
UNUSUAL 0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-chan#2: Peer permanent failure in CLOSINGD_COMPLETE: Funding transaction spent: onchain txid 7685256e6e4632bd601f42bb85b6f22b7a0db8d55c468dfbb1703c3ab2e4d9ce (reason=unknown)
UNUSUAL 0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-chan#2: Not dropping our unilateral close onchain since we already saw 7685256e6e4632bd601f42bb85b6f22b7a0db8d55c468dfbb1703c3ab2e4d9ce confirm.
INFO    0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-chan#2: State changed from CLOSINGD_COMPLETE to FUNDING_SPEND_SEEN
**BROKEN** 0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-chan#2: Illegal gossip state transition: CGOSSIP_CHANNEL_UNANNOUNCED_DYING->CGOSSIP_CHANNEL_ANNOUNCED_DEAD
DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-onchaind-chan#2: pid 35721, msgfd 86
```

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

lightningd/channel_gossip.c

@@ -118,6 +118,8 @@ static struct state_transition allowed_transitions[] = {
 	  "Unannounced channel closed onchain." },
 	{ CGOSSIP_CHANNEL_UNANNOUNCED_DYING, CGOSSIP_CHANNEL_ANNOUNCED_DYING,
 	  "Unannounced closing channel reached announce depth." },
+	{ CGOSSIP_CHANNEL_UNANNOUNCED_DYING, CGOSSIP_CHANNEL_ANNOUNCED_DEAD,
+	  "Unannounced closing channel reached announce depth and was closed in same block." },
 	{ CGOSSIP_WAITING_FOR_ANNOUNCE_DEPTH, CGOSSIP_ANNOUNCED,
 	  "Channel fully announced" },
 	{ CGOSSIP_WAITING_FOR_MATCHING_PEER_SIGS, CGOSSIP_ANNOUNCED,