lightningd: redo closing negotiation even if we consider channel closed.

This corner case started triggering on my machine with latest Bitcoind.

This test sabotages the closing negotiation, and as a result l1
doesn't see l2's CLOSING_SIGNED.  l2 is happy, however, and it is in
CLOSINGD_COMPLETE.  When l1 reconnects, it gets an error, and this causes
it to drop the unilateral tx to chain.

This unilateral tx from l1 replaces or races the mutual close tx from
l2, causing a unilateral close, which breaks our test.

Though this is a corner case, it's much friendlier to allow the
closing negotiation again until we actually see the close onchain.
This fixes the tests here, too.

```
    def test_closing_negotiation_reconnect(node_factory, bitcoind):
        disconnects = ['-WIRE_CLOSING_SIGNED',
                       '+WIRE_CLOSING_SIGNED']
        l1, l2 = node_factory.line_graph(2, opts=[{'disconnect': disconnects,
                                                   'may_reconnect': True},
                                                  {'may_reconnect': True}])
        l1.pay(l2, 200000000)
    
        assert bitcoind.rpc.getmempoolinfo()['size'] == 0
    
        l1.rpc.close(l2.info['id'])
        l1.daemon.wait_for_log(r'State changed from CHANNELD_NORMAL to CHANNELD_SHUTTING_DOWN')
        l2.daemon.wait_for_log(r'State changed from CHANNELD_NORMAL to CHANNELD_SHUTTING_DOWN')
    
        # Now verify that the closing tx is in the mempool.
        bitcoind.generate_block(6, wait_for_mempool=1)
        sync_blockheight(bitcoind, [l1, l2])
        for n in [l1, l2]:
            # Ensure we actually got a mutual close.
>           n.daemon.wait_for_log(r'Resolved FUNDING_TRANSACTION/FUNDING_OUTPUT by MUTUAL_CLOSE')

tests/test_closing.py:275: 
```

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Fixed: Protocol: We now renegotiate an interrupted close, even if we don't need it, instead of sending an error.

Author: rustyrussell

lightningd/peer_control.c

@@ -1869,8 +1869,26 @@ void peer_spoke(struct lightningd *ld, const u8 *msg)
 	if (channel) {
 		/* In this case, we'll send an error below, but send reestablish reply first
 		 * in case they lost their state and need it */
-		if (msgtype == WIRE_CHANNEL_REESTABLISH && channel_state_closed(channel->state))
+		if (msgtype == WIRE_CHANNEL_REESTABLISH && channel_state_closed(channel->state)) {
+			/* Maybe we know it's closed, but they don't?  Happy to negotiate again. */
+			if (channel->state == CLOSINGD_COMPLETE) {
+				pfd = sockpair(tmpctx, channel, &other_fd, &error);
+				if (!pfd)
+					goto send_error;
+
+				/* Tell channeld to handle reestablish, then it will call closingd */
+				if (peer_start_channeld(channel,
+							pfd,
+							NULL, true,
+							NULL)) {
+					goto tell_connectd;
+				}
+				error = towire_warningfmt(tmpctx, &channel_id,
+							  "Trouble in paradise?");
+				goto send_error;
+			}
 			send_reestablish(ld, channel);
+		}
 
 		/* If we have a canned error for this channel, send it now */
 		if (channel->error) {

lightningd/test/run-invoice-select-inchan.c

@@ -908,6 +908,10 @@ bool peer_start_channeld(struct channel *channel UNNEEDED,
 			 bool reconnected UNNEEDED,
 			 bool reestablish_only UNNEEDED)
 { fprintf(stderr, "peer_start_channeld called!\n"); abort(); }
+/* Generated stub for peer_start_closingd */
+void peer_start_closingd(struct channel *channel UNNEEDED,
+			 struct peer_fd *peer_fd UNNEEDED)
+{ fprintf(stderr, "peer_start_closingd called!\n"); abort(); }
 /* Generated stub for peer_start_dualopend */
 bool peer_start_dualopend(struct peer *peer UNNEEDED, struct peer_fd *peer_fd UNNEEDED,
 			  struct channel *channel UNNEEDED)

wallet/test/run-wallet.c

@@ -929,6 +929,10 @@ bool peer_start_channeld(struct channel *channel UNNEEDED,
 			 bool reconnected UNNEEDED,
 			 bool reestablish_only UNNEEDED)
 { fprintf(stderr, "peer_start_channeld called!\n"); abort(); }
+/* Generated stub for peer_start_closingd */
+void peer_start_closingd(struct channel *channel UNNEEDED,
+			 struct peer_fd *peer_fd UNNEEDED)
+{ fprintf(stderr, "peer_start_closingd called!\n"); abort(); }
 /* Generated stub for peer_start_dualopend */
 bool peer_start_dualopend(struct peer *peer UNNEEDED, struct peer_fd *peer_fd UNNEEDED,
 			  struct channel *channel UNNEEDED)