wallet: be a little more flexible with change for emergency reserve.

We used to look for either other outputs which are sufficient for
reserve, *or* be able to create sufficient change to meet the
emergency reserve.  Allow the sum of both to meet the requirements:
otherwise test_funder_contribution_limits can flake.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

wallet/reservation.c

@@ -461,40 +461,40 @@ static bool change_for_emergency(struct lightningd *ld,
 				 struct amount_sat *excess,
 				 struct amount_sat *change)
 {
-	struct amount_sat fee;
+	struct amount_sat needed = ld->emergency_sat, fee;
 
 	/* Only needed for anchor channels */
 	if (!have_anchor_channel)
 		return true;
 
-	/* Fine if rest of wallet has funds. */
+	/* Fine if rest of wallet has funds.  Otherwise it may reduce
+	 * needed amount. */
 	if (wallet_has_funds(ld->wallet,
 			     cast_const2(const struct utxo **, utxos),
 			     get_block_height(ld->topology),
-			     ld->emergency_sat))
+			     &needed))
 		return true;
 
-	/* If we can afford with existing change output, great (or
+	/* If we can afford the rest with existing change output, great (or
 	 * ld->emergency_sat is 0) */
 	if (amount_sat_greater_eq(change_amount(*change,
 						feerate_per_kw, weight),
-				  ld->emergency_sat))
+				  needed))
 		return true;
 
 	/* Try splitting excess to add to change. */
 	fee = change_fee(feerate_per_kw, weight);
 	if (!amount_sat_sub(excess, *excess, fee)
-	    || !amount_sat_sub(excess, *excess, ld->emergency_sat))
+	    || !amount_sat_sub(excess, *excess, needed))
 		return false;
 
 	if (!amount_sat_add(change, *change, fee)
-	    || !amount_sat_add(change, *change, ld->emergency_sat))
+	    || !amount_sat_add(change, *change, needed))
 		abort();
 
 	/* We *will* get a change output now! */
-	assert(amount_sat_eq(change_amount(*change, feerate_per_kw,
-						weight),
-			     ld->emergency_sat));
+	assert(amount_sat_eq(change_amount(*change, feerate_per_kw, weight),
+			     needed));
 	return true;
 }
 

wallet/wallet.c

@@ -674,10 +674,9 @@ struct utxo *wallet_find_utxo(const tal_t *ctx, struct wallet *w,
 bool wallet_has_funds(struct wallet *w,
 		      const struct utxo **excludes,
 		      u32 current_blockheight,
-		      struct amount_sat sats)
+		      struct amount_sat *needed)
 {
 	struct db_stmt *stmt;
-	struct amount_sat total = AMOUNT_SAT(0);
 
 	stmt = db_prepare_v2(w->db, SQL("SELECT"
 					"  prev_out_tx"
@@ -712,17 +711,9 @@ bool wallet_has_funds(struct wallet *w,
 			continue;
 		}
 
-		/* Overflow Should Not Happen */
-		if (!amount_sat_add(&total, total, utxo->amount)) {
-			db_fatal(w->db, "Invalid value for %s: %s",
-				 type_to_string(tmpctx,
-						struct bitcoin_outpoint,
-						&utxo->outpoint),
-				 fmt_amount_sat(tmpctx, utxo->amount));
-		}
-
 		/* If we've found enough, answer is yes. */
-		if (amount_sat_greater_eq(total, sats)) {
+		if (!amount_sat_sub(needed, *needed, utxo->amount)) {
+			*needed = AMOUNT_SAT(0);
 			tal_free(stmt);
 			return true;
 		}

wallet/wallet.h

@@ -482,15 +482,15 @@ struct utxo *wallet_find_utxo(const tal_t *ctx, struct wallet *w,
  * @w: the wallet
  * @excludes: the utxos not to count (tal_arr or NULL)
  * @current_blockheight: current chain length.
- * @sats: the target
+ * @needed: the target, reduced if we find some funds
  *
  * This is a gross estimate, since it doesn't take into account the fees we
  * would need to actually spend these utxos!
  */
 bool wallet_has_funds(struct wallet *wallet,
 		      const struct utxo **excludes,
 		      u32 current_blockheight,
-		      struct amount_sat sats);
+		      struct amount_sat *needed);
 
 /**
  * wallet_add_onchaind_utxo - Add a UTXO with spending info from onchaind.