wallet: add wallet_find_utxo().

This is a new fundamental routine to obtain UTXOs from the database.

It's not the most efficient approach, as it returns a single UTXO at a
time, but it can consolidate all our UTXO handling (becoming more
complex by the reservation timeout logic).

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

wallet/wallet.c

@@ -502,6 +502,79 @@ void wallet_unreserve_utxo(struct wallet *w, struct utxo *utxo, u32 current_heig
 	db_set_utxo(w->db, utxo);
 }
 
+static bool excluded(const struct utxo **excludes,
+		     const struct utxo *utxo)
+{
+	for (size_t i = 0; i < tal_count(excludes); i++) {
+		if (bitcoin_txid_eq(&excludes[i]->txid, &utxo->txid)
+		    && excludes[i]->outnum == utxo->outnum)
+			return true;
+	}
+	return false;
+}
+
+static bool deep_enough(u32 maxheight, const struct utxo *utxo)
+{
+	/* If we require confirmations check that we have a
+	 * confirmation height and that it is below the required
+	 * maxheight (current_height - minconf) */
+	if (maxheight == 0)
+		return true;
+	if (!utxo->blockheight)
+		return false;
+	return *utxo->blockheight <= maxheight;
+}
+
+/* FIXME: Make this wallet_find_utxos, and branch and bound and I've
+ * left that to @niftynei to do, who actually read the paper! */
+struct utxo *wallet_find_utxo(const tal_t *ctx, struct wallet *w,
+			      unsigned current_blockheight,
+			      struct amount_sat *amount_hint,
+			      unsigned feerate_per_kw,
+			      u32 maxheight,
+			      const struct utxo **excludes)
+{
+	struct db_stmt *stmt;
+	struct utxo *utxo;
+
+	stmt = db_prepare_v2(w->db, SQL("SELECT"
+					"  prev_out_tx"
+					", prev_out_index"
+					", value"
+					", type"
+					", status"
+					", keyindex"
+					", channel_id"
+					", peer_id"
+					", commitment_point"
+					", confirmation_height"
+					", spend_height"
+					", scriptpubkey "
+					", reserved_til"
+					" FROM outputs"
+					" WHERE status = ?"
+					" OR (status = ? AND reserved_til <= ?)"
+					"ORDER BY RANDOM();"));
+	db_bind_int(stmt, 0, output_status_in_db(output_state_available));
+	db_bind_int(stmt, 1, output_status_in_db(output_state_reserved));
+	db_bind_u64(stmt, 2, current_blockheight);
+
+	/* FIXME: Use feerate + estimate of input cost to establish
+	 * range for amount_hint */
+
+	db_query_prepared(stmt);
+
+	utxo = NULL;
+	while (!utxo && db_step(stmt)) {
+		utxo = wallet_stmt2output(ctx, stmt);
+		if (excluded(excludes, utxo) || !deep_enough(maxheight, utxo))
+			utxo = tal_free(utxo);
+
+	}
+	tal_free(stmt);
+	return utxo;
+}
+
 bool wallet_add_onchaind_utxo(struct wallet *w,
 			      const struct bitcoin_txid *txid,
 			      u32 outnum,

wallet/wallet.h

@@ -374,6 +374,29 @@ struct utxo **wallet_get_utxos(const tal_t *ctx, struct wallet *w,
 struct utxo **wallet_get_unconfirmed_closeinfo_utxos(const tal_t *ctx,
 						     struct wallet *w);
 
+/**
+ * wallet_find_utxo - Select an available UTXO (does not reserve it!).
+ * @ctx: tal context
+ * @w: wallet
+ * @current_blockheight: current chain length.
+ * @amount_we_are_short: optional amount.
+ * @feerate_per_kw: feerate we are using.
+ * @maxheight: zero (if caller doesn't care) or maximum blockheight to accept.
+ * @excludes: UTXOs not to consider.
+ *
+ * If @amount_we_are_short is not NULL, we try to get something very close
+ * (i.e. when we add this input, we will add => @amount_we_are_short, but
+ * less than @amount_we_are_short + dustlimit).
+ *
+ * Otherwise we give a random UTXO.
+ */
+struct utxo *wallet_find_utxo(const tal_t *ctx, struct wallet *w,
+			      unsigned current_blockheight,
+			      struct amount_sat *amount_we_are_short,
+			      unsigned feerate_per_kw,
+			      u32 maxheight,
+			      const struct utxo **excludes);
+
 /**
  * wallet_add_onchaind_utxo - Add a UTXO with spending info from onchaind.
  *