bookkeeper: make bookkeeper-currency dynamic.

diff --git a/doc/lightningd-config.5.md b/doc/lightningd-config.5.md
index c64cbf67..25721771 100644
--- a/doc/lightningd-config.5.md
+++ b/doc/lightningd-config.5.md
@@ -573,7 +573,7 @@ command, so they invoices can also be paid onchain.
 
 * **bkpr-currency**=*name* [plugin `bookkeeper`, *dynamic*]
 
-  The *name* is an ISO-4217 name (e.g. USD), which will be passed to *currencyrate* to fetch the exchange rate for that currency on each bookkeeper event.
+  The *name* is an ISO-4217 name (e.g. USD), which will be passed to *currencyrate* to fetch the exchange rate for that currency on each bookkeeper event.  Setting *name* to the empty string is equivalent not setting it.
 
 ### Networking options
 
diff --git a/plugins/bkpr/bookkeeper.c b/plugins/bkpr/bookkeeper.c
index 0dda6ccb..73283f0a 100644
--- a/plugins/bkpr/bookkeeper.c
+++ b/plugins/bkpr/bookkeeper.c
@@ -1131,6 +1131,7 @@ static struct command_result *lookup_invoice_desc(struct command *cmd,
 
 struct currency_time {
 	struct refresh_info *rinfo;
+	const char *currency;
 	u64 timestamp;
 };
 
@@ -1151,7 +1152,8 @@ static struct command_result *currency_done(struct command *cmd,
 			   "Invalid currencyrate return '%.*s': %s",
 			   json_tok_full_len(result),
 			   json_tok_full(buf, result), err);
-	} else {
+	/* Make sure they didn't change currency while we were out! */
+	} else if (bkpr->currency && streq(ctime->currency, bkpr->currency)) {
 		double *p = tal_dup(bkpr->currency_rates, double, &rate);
 		/* Can fail if we raced and asked twice */
 		if (!uintmap_add(bkpr->currency_rates,
@@ -1197,6 +1199,7 @@ static void lookup_currency(struct command *cmd,
 
 	ctime = tal(cmd, struct currency_time);
 	ctime->timestamp = timestamp;
+	ctime->currency = tal_strdup(ctime, bkpr->currency);
 	ctime->rinfo = use_rinfo(rinfo);
 	req = jsonrpc_request_start(cmd,
 				    "currencyrate",
@@ -1753,14 +1756,46 @@ static const char *init(struct command *init_cmd, const char *b, const jsmntok_t
 	} else
 		bkpr->chainmoves_index = 0;
 
-	/* If we're supposed to do currency conversions, we refresh
-	 * all the time. */
-	if (bkpr->currency) {
-		struct command *auxcmd = aux_command(init_cmd);
-		currency_chainmoves_wait(auxcmd, NULL);
-		currency_channelmoves_wait(auxcmd, NULL);
+	return NULL;
+}
+
+static char *option_currency(struct command *cmd,
+			     const char *arg,
+			     bool check_only,
+			     char **p)
+{
+	struct bkpr *bkpr = bkpr_of(cmd->plugin);
+
+	assert(p == &bkpr->currency);
+
+	/* FIXME: Check for valid currency? */
+	if (check_only)
+		return NULL;
+
+	/* Changed?  Clean up old one! */
+	if (bkpr->currency != NULL) {
+		/* Stop refreshes */
+		bkpr->currency_cmds = tal_free(bkpr->currency_cmds);
+		/* Clear existing values and free contents.*/
+		tal_free(bkpr->currency_rates);
+		bkpr->currency_rates = tal(bkpr, currencymap_t);
+		uintmap_init(bkpr->currency_rates);
+		memleak_add_helper(bkpr->currency_rates, memleak_scan_currencyrates);
+		bkpr->currency = tal_free(bkpr->currency);
 	}
 
+	/* Explicit empty string means unset. */
+	if (streq(arg, ""))
+		return NULL;
+
+	bkpr->currency = tal_strdup(bkpr, arg);
+	/* Reset this so we get a new message for new currency */
+	bkpr->warned_currency_fail = false;
+	/* If we're supposed to do currency conversions, we refresh
+	 * all the time. */
+	bkpr->currency_cmds = aux_command(cmd);
+	currency_chainmoves_wait(bkpr->currency_cmds, NULL);
+	currency_channelmoves_wait(bkpr->currency_cmds, NULL);
 	return NULL;
 }
 
@@ -1772,7 +1807,6 @@ int main(int argc, char *argv[])
 	/* No datadir is default */
 	bkpr = tal(NULL, struct bkpr);
 	bkpr->currency = NULL;
-	bkpr->warned_currency_fail = false;
 	bkpr->currency_rates = tal(bkpr, currencymap_t);
 	uintmap_init(bkpr->currency_rates);
 	memleak_add_helper(bkpr->currency_rates, memleak_scan_currencyrates);
@@ -1781,10 +1815,10 @@ int main(int argc, char *argv[])
 		    notifs, ARRAY_SIZE(notifs),
 		    NULL, 0,
 		    NULL, 0,
-		    plugin_option("bkpr-currency",
-				  "string",
-				  "Look up and record this currency on each event",
-				  charp_option, charp_jsonfmt, &bkpr->currency),
+		    plugin_option_dynamic("bkpr-currency",
+					  "string",
+					  "Look up and record this currency on each event",
+					  option_currency, charp_jsonfmt, &bkpr->currency),
 		    NULL);
 
 	return 0;
diff --git a/plugins/bkpr/bookkeeper.h b/plugins/bkpr/bookkeeper.h
index a6c125c6..1e724ac7 100644
--- a/plugins/bkpr/bookkeeper.h
+++ b/plugins/bkpr/bookkeeper.h
@@ -30,6 +30,8 @@ struct bkpr {
 	currencymap_t *currency_rates;
 	/* True if we've warned about currency failures */
 	bool warned_currency_fail;
+	/* aux_command and parent of currency queries */
+	struct command *currency_cmds;
 };
 
 /* Add optional currencyrate for this timestamp */
diff --git a/tests/test_currencyrate.py b/tests/test_currencyrate.py
index 2181d4e5..a0064779 100644
--- a/tests/test_currencyrate.py
+++ b/tests/test_currencyrate.py
@@ -367,3 +367,63 @@ def test_bkpr_listaccountevents_realtime(node_factory, fake_rateserver):
     assert events
     for e in events:
         assert e["currencyrate"] == old_median
+
+
+def test_bkpr_currency_dynamic(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",
+        ],
+    }
+    l1, l2 = node_factory.line_graph(2, opts=opts)
+
+    median_rate = (fake_rateserver["state"]["fast"] + fake_rateserver["state"]["slow"]) / 2
+
+    inv1 = l2.rpc.invoice(100000, "test_bkpr_currency_dynamic_1", "desc")
+    l1.rpc.pay(inv1["bolt11"])
+    # We want this event in the list, so wait until it's totally closed.
+    wait_for(lambda: only_one(l1.rpc.listpeerchannels()['channels'])['htlcs'] == [])
+
+    # No bookkeeker-currency, no currencyrate
+    events = l1.rpc.bkpr_listaccountevents()["events"]
+    assert events
+    assert all("currencyrate" not in e for e in events)
+    num_events_1 = len(events)
+
+    l1.rpc.setconfig("bkpr-currency", "USD")
+
+    inv2 = l2.rpc.invoice(100000, "test_bkpr_currency_dynamic_2", "desc")
+    l1.rpc.pay(inv2["bolt11"])
+    wait_for(lambda: only_one(l1.rpc.listpeerchannels()['channels'])['htlcs'] == [])
+
+    events = l1.rpc.bkpr_listaccountevents()["events"]
+    assert len(events) > num_events_1
+
+    old_events = events[:num_events_1]
+    new_events = events[num_events_1:]
+
+    assert all("currencyrate" not in e for e in old_events)
+    assert all(e["currencyrate"] == median_rate for e in new_events)
+
+    # Disables all currency conversions.
+    l1.rpc.setconfig("bkpr-currency", "")
+
+    inv3 = l2.rpc.invoice(100000, "test_bkpr_currency_dynamic_3", "desc")
+    l1.rpc.pay(inv3["bolt11"])
+    # If we don't wait here, we can get a spurious error from
+    # cln-currencyrate as fixture gets torn down!
+    wait_for(lambda: only_one(l1.rpc.listpeerchannels()['channels'])['htlcs'] == [])
+
+    events = l1.rpc.bkpr_listaccountevents()["events"]
+    assert events
+    assert all("currencyrate" not in e for e in events)