plugins: don't let multifundchannel open a new anchor channel without reserves.

If we're opening a channel with a peer which support anchors (and
we do), we tell fundpsbt/utxopsbt to enforce the emergency reserve;
this matters, as it doesn't know about the channel yet, and thus
won't (if it's our first anchor channel).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Changed: JSON-RPC: `fundchannel` and `multifundchannel` will refuse to spend funds below `min-emergency-msat` if we have any anchor channels (or are opening one).

doc/lightning-fundchannel.7.md

@@ -27,7 +27,7 @@ for the channel.
 *id* is the peer id obtained from **connect**.
 
 *amount* is the amount in satoshis taken from the internal wallet to
-fund the channel. The string *all* can be used to specify all available
+fund the channel (but if we have any anchor channels, this will always leave at least `min-emergency-msat` as change). The string *all* can be used to specify all available
 funds (or 16777215 satoshi if more is available and large channels were not negotiated with the peer). Otherwise, it is in
 satoshi precision; it can be a whole number, a whole number ending in
 *sat*, a whole number ending in *000msat*, or a number with 1 to 8
@@ -100,6 +100,7 @@ The following error codes may occur:
 - 301: There are not enough funds in the internal wallet (including fees) to create the transaction.
 - 302: The output amount is too small, and would be considered dust.
 - 303: Broadcasting of the funding transaction failed, the internal call to bitcoin-cli returned with an error.
+- 313: The `min-emergency-msat` reserve not be preserved (and we have or are opening anchor channels).
 
 Failure may also occur if **lightningd** and the peer cannot agree on
 channel parameters (funding limits, channel reserves, fees, etc.).

doc/lightning-multifundchannel.7.md

@@ -28,7 +28,7 @@ Readiness is indicated by **listpeers** reporting a *state* of
   in a manner understood by **connect**; see lightning-connect(7).
   Each entry in the *destinations* array must have a unique node *id*.
 * *amount* is the amount in satoshis taken from the internal wallet
-  to fund the channel.
+  to fund the channel (but if we have any anchor channels, this will always leave at least `min-emergency-msat` as change).
   The string *all* can be used to specify all available funds
   (or 16,777,215 satoshi if more is available and large channels were
   not negotiated with the peer).
@@ -122,6 +122,7 @@ The following error codes may occur:
 - 301: There are not enough funds in the internal wallet (including fees) to create the transaction.
 - 302: The output amount is too small, and would be considered dust.
 - 303: Broadcasting of the funding transaction failed, the internal call to bitcoin-cli returned with an error.
+- 313: The `min-emergency-msat` reserve not be preserved (and we have or are opening anchor channels).
 
 Failure may also occur if **lightningd** and the peer cannot agree on
 channel parameters (funding limits, channel reserves, fees, etc.).

plugins/spender/multifundchannel.c

@@ -1291,6 +1291,18 @@ fail:
 
 }
 
+static bool any_dest_negotiated_anchors(const struct plugin *plugin,
+					const struct multifundchannel_destination *dests)
+{
+	for (size_t i = 0; i < tal_count(dests); i++) {
+		if (feature_negotiated(plugin_feature_set(plugin),
+				       dests[i].their_features,
+				       OPT_ANCHORS_ZERO_FEE_HTLC_TX))
+			return true;
+	}
+	return false;
+}
+
 static struct command_result *
 perform_fundpsbt(struct multifundchannel_command *mfc, u32 feerate)
 {
@@ -1329,6 +1341,12 @@ perform_fundpsbt(struct multifundchannel_command *mfc, u32 feerate)
 			      dest_count(mfc, OPEN_CHANNEL) > 0);
 	}
 
+	/* If we're about to open an anchor channel, we need emergency funds! */
+	if (any_dest_negotiated_anchors(mfc->cmd->plugin,
+					mfc->destinations)) {
+		json_add_bool(req->js, "opening_anchor_channel", true);
+	}
+
 	/* The entire point is to reserve the inputs. */
 	/*  BOLT #2:
 	 * The sender: