bookkeeper: persist currency conversions in the datastore.

diff --git a/plugins/bkpr/bookkeeper.c b/plugins/bkpr/bookkeeper.c
index 06652c7d..ea0b3592 100644
--- a/plugins/bkpr/bookkeeper.c
+++ b/plugins/bkpr/bookkeeper.c
@@ -3,6 +3,7 @@
 #include <ccan/array_size/array_size.h>
 #include <ccan/cast/cast.h>
 #include <ccan/json_escape/json_escape.h>
+#include <ccan/json_out/json_out.h>
 #include <ccan/str/hex/hex.h>
 #include <ccan/tal/str/str.h>
 #include <ccan/tal/tal.h>
@@ -1138,6 +1139,15 @@ struct currency_time {
 	u64 timestamp;
 };
 
+static struct command_result *currencyrate_ds_done(struct command *cmd,
+						   const char *method,
+						   const char *buf,
+						   const jsmntok_t *result,
+						   struct refresh_info *rinfo)
+{
+	return rinfo_one_done(cmd, rinfo);
+}
+
 static struct command_result *currency_done(struct command *cmd,
 					    const char *method,
 					    const char *buf,
@@ -1163,6 +1173,20 @@ static struct command_result *currency_done(struct command *cmd,
 				 ctime->timestamp,
 				 p)) {
 			tal_free(p);
+		} else {
+			const char **key;
+			key = mkdatastorekey(tmpctx,
+					     "bookkeeper",
+					     "currencyrate",
+					     ctime->currency,
+					     take(tal_fmt(NULL, "%"PRIu64,
+							  ctime->timestamp)));
+			jsonrpc_set_datastore_string(cmd, key,
+						     tal_fmt(tmpctx, "%f", rate),
+						     "must-create",
+						     currencyrate_ds_done,
+						     plugin_broken_cb,
+						     use_rinfo(ctime->rinfo));
 		}
 	}
 	return rinfo_one_done(cmd, ctime->rinfo);
@@ -1776,6 +1800,58 @@ static const char *init(struct command *init_cmd, const char *b, const jsmntok_t
 	return NULL;
 }
 
+static void load_currencyrates(struct command *cmd,
+			       struct bkpr *bkpr)
+{
+	struct json_out *params;
+	const jsmntok_t *reply, *ds, *t;
+	const char *buf;
+	size_t i;
+
+	params = json_out_new(NULL);
+	json_out_start(params, NULL, '{');
+	json_out_start(params, "key", '[');
+	json_out_addstr(params, NULL, "bookkeeper");
+	json_out_addstr(params, NULL, "currencyrate");
+	json_out_addstr(params, NULL, bkpr->currency);
+	json_out_end(params, ']');
+	json_out_end(params, '}');
+	json_out_finished(params);
+
+	reply = jsonrpc_request_sync(tmpctx, cmd, "listdatastore",
+				     take(params), &buf);
+	ds = json_get_member(buf, reply, "datastore");
+	json_for_each_arr(i, t, ds) {
+		const jsmntok_t *data, *keytok = json_get_member(buf, t, "key");
+		u64 ts;
+		double rate;
+
+		if (keytok->size != 4)
+			goto weird;
+
+		/* key = ["bookkeeper", "currencyrate", "USD", "timestamp"] */
+		if (!json_to_u64(buf, keytok + 4, &ts))
+			goto weird;
+
+		data = json_get_member(buf, t, "string");
+		if (!data)
+			goto weird;
+		if (!json_to_double(buf, data, &rate))
+			goto weird;
+
+		uintmap_add(bkpr->currency_rates,
+			    ts,
+			    tal_dup(bkpr->currency_rates, double, &rate));
+		continue;
+
+	weird:
+		plugin_log(cmd->plugin, LOG_BROKEN,
+			   "Unexpected datastore rate entry '%.*s'",
+			   json_tok_full_len(t),
+			   json_tok_full(buf, t));
+	}
+}
+
 static char *option_currency(struct command *cmd,
 			     const char *arg,
 			     bool check_only,
@@ -1809,6 +1885,9 @@ static char *option_currency(struct command *cmd,
 	/* Reset this so we get a new message for new currency */
 	bkpr->warned_currency_fail = false;
 
+	/* Start with saved currency rates */
+	load_currencyrates(cmd, bkpr);
+
 	/* Don't do this yet if we're before init! */
 	if (bkpr->accounts)
 		start_waiting_for_currency(bkpr, cmd);
diff --git a/tests/test_currencyrate.py b/tests/test_currencyrate.py
index a0064779..e523e2e9 100644
--- a/tests/test_currencyrate.py
+++ b/tests/test_currencyrate.py
@@ -2,6 +2,7 @@ import logging
 import pytest
 import threading
 import time
+from utils import wait_for, only_one
 from pyln.client import RpcError
 from fixtures import *  # noqa: F401,F403
 from flask import Flask, jsonify
@@ -427,3 +428,63 @@ def test_bkpr_currency_dynamic(node_factory, fake_rateserver):
     events = l1.rpc.bkpr_listaccountevents()["events"]
     assert events
     assert all("currencyrate" not in e for e in events)
+
+
+def test_bkpr_currencyrate_persisted(node_factory, fake_rateserver):
+    opts = {
+        "currencyrate-disable-source": [
+            "bitstamp",
+            "coinbase",
+            "coingecko",
+            "kraken",
+            "blockchain.info",
+            "coindesk",
+            "binance",
+        ],
+        "currencyrate-add-source": [
+            f"fast,{fake_rateserver['url']}/fast,price",
+            f"slow,{fake_rateserver['url']}/slow,price",
+        ],
+        "bkpr-currency": "USD",
+        'may_reconnect': True,
+    }
+    l1, l2 = node_factory.line_graph(2, opts=opts)
+
+    old_median = (fake_rateserver["state"]["fast"] + fake_rateserver["state"]["slow"]) / 2
+
+    inv = l2.rpc.invoice(100000, "test_bkpr_currencyrate_persisted", "desc")
+    l1.rpc.pay(inv["bolt11"])
+    # Make sure it's fully resolved so we get all events now.
+    wait_for(lambda: only_one(l1.rpc.listpeerchannels(l2.info['id'])['channels'])['htlcs'] == [])
+
+    events = l1.rpc.bkpr_listaccountevents()["events"]
+    assert events
+    for e in events:
+        assert e["currencyrate"] == old_median
+
+    l1.restart()
+    l1.connect(l2)
+
+    fake_rateserver["state"]["fast"] = 200_000_000
+    fake_rateserver["state"]["slow"] = 150_000_000
+    new_median = (fake_rateserver["state"]["fast"] + fake_rateserver["state"]["slow"]) / 2
+
+    new_events = l1.rpc.bkpr_listaccountevents()["events"]
+    assert new_events == events
+
+    # And we can add more.
+    inv2 = l2.rpc.invoice(100000, "test_bkpr_currencyrate_persisted2", "desc")
+    l1.rpc.pay(inv2["bolt11"])
+    wait_for(lambda: only_one(l1.rpc.listpeerchannels(l2.info['id'])['channels'])['htlcs'] == [])
+
+    new_events = l1.rpc.bkpr_listaccountevents()["events"]
+    assert new_events[:len(events)] == events
+    assert new_events[len(events):]
+
+    for e in new_events[len(events):]:
+        assert e["currencyrate"] == new_median
+
+    # Underlying check: they should all be human readable timestamp->rate.
+    rates = l1.rpc.listdatastore(['bookkeeper', 'currencyrate', 'USD'])['datastore']
+    assert {r['key'][3] for r in rates} == {str(e['timestamp']) for e in new_events}
+    assert {float(r['string']) for r in rates} == {old_median, new_median}