pay crashes because "cannot find <unannounced peer>"

[fiatjaf]

Issue and Steps to Reproduce

You can use the invoice below to reproduce (but be sure to apply the parameters):

~> lightning-cli pay -k bolt11=lnbc3252500p1p087he3pp57t693zw9adv0a8lvv99zwvs7cl8ed0pkgul27jp09cra355vme9qhp5cv9ldqyn558mp063ays4n2af99cx2w787mlaqy0re75mn4mkpfeqxqyz5vqrzjq2ldrqfd8qj00nzve5ud5htx520ulmq5dl54ufkd9cxnlycdv5ag6zx804nsx6grqqqqqyy6qqqqqqqqqgsp5qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqq9qy9qqqmxrzcgwy7qadlmgv3dmrrsl2e87dcxg7wdhpfs5vevkl4sy34ljpajmq7u7cjvj2r3cxxvxcktputmhmed7908jxqwfsxhgzluqnrcqp74xjh9 riskfactor=3 maxfeepercent=0.4 exemptfee=3000

That causes the following error:

2020-03-28T14:30:25.570Z INFO gossipd: find_route: cannot find 032cbe7d2b0147b494424505e17e6878dfe18bc6e6e68d7b33ed89e525094c6cdc
pay: FATAL SIGNAL 11 (version v0.8.0-371-g8c984fb)
0x55e5f542511b send_backtrace
        common/daemon.c:39
0x55e5f54251c1 crashdump
        common/daemon.c:52
0x7f83e1561f1f ???
        ???:0
0x55e5f5426b92 json_get_member
        common/json.c:299
0x55e5f5412e2e maybe_exclude
        plugins/pay.c:701
0x55e5f541350c getroute_done
        plugins/pay.c:793
0x55e5f5417c7c handle_rpc_reply
        plugins/libplugin.c:542
0x55e5f5418244 rpc_read_response_one
        plugins/libplugin.c:645
0x55e5f5418335 rpc_conn_read_response
        plugins/libplugin.c:664
0x55e5f5438de7 next_plan
        ccan/ccan/io/io.c:59
0x55e5f5439964 do_plan
        ccan/ccan/io/io.c:407
0x55e5f54399a2 io_ready
        ccan/ccan/io/io.c:417
0x55e5f543bb68 io_loop
        ccan/ccan/io/poll.c:445
0x55e5f541a119 plugin_main
        plugins/libplugin.c:1194
0x55e5f5416842 main
        plugins/pay.c:1708
0x7f83e1544b96 ???
        ???:0
0x55e5f5410f49 ???
        ???:0
0xffffffffffffffff ???
        ???:0

However, calling just pay <bolt11> will succeed. Even removing just the exemptfee param will make the command above succeed.

getinfo output

{
   "id": "02c16cca44562b590dd279c942200bdccfd4f990c3a69fad620c10ef2f8228eaff",
   "alias": "@lntxbot",
   "color": "296683",
   "num_peers": 40,
   "num_pending_channels": 1,
   "num_active_channels": 39,
   "num_inactive_channels": 2,
   "address": [
      {
         "type": "ipv6",
         "address": "2a01:4f8:c0c:7b31::1",
         "port": 9735
      }
   ],
   "binding": [
      {
         "type": "ipv6",
         "address": "::",
         "port": 9735
      }
   ],
   "version": "v0.8.0-371-g8c984fb",
   "blockheight": 623329,
   "network": "bitcoin",
   "msatoshi_fees_collected": 870838,
   "fees_collected_msat": "870838msat",
   "lightning-dir": "/home/fiatjaf/.lightning/bitcoin"
}

[niftynei]

Looks like it's caused by no route being found to try excluding when the fees are above the exemptfee amount you've set. I patched it to check for nulls before attempting an exclude, if you've got a testcase to try it out against that'd be great.