wallet: Implement wallet_invoice_nextpaid.

wallet_invoice_nextpaid is used to iterate over paid invoices,
and will be used to fix waitanyinvoice command.

wallet/wallet.c

@@ -1125,6 +1125,65 @@ bool wallet_htlcs_reconnect(struct wallet *wallet,
 	return true;
 }
 
+int wallet_invoice_nextpaid(const tal_t *cxt,
+			    const struct wallet *wallet,
+			    const char *labelz,
+			    char **outlabel,
+			    struct sha256 *outrhash,
+			    u64 *outmsatoshi)
+{
+	sqlite3_stmt *stmt;
+	int res;
+	u64 pay_index;
+
+	/* Generate query. */
+	if (labelz) {
+		/* Find label. */
+		stmt = db_prepare(wallet->db,
+				"SELECT pay_index FROM invoices WHERE label=?;");
+		sqlite3_bind_text(stmt, 1, labelz, strlen(labelz), SQLITE_TRANSIENT);
+		res = sqlite3_step(stmt);
+		if (res != SQLITE_ROW) {
+			sqlite3_finalize(stmt);
+			return -1;
+		}
+		pay_index = sqlite3_column_int64(stmt, 0);
+		sqlite3_finalize(stmt);
+
+		stmt = db_prepare(wallet->db,
+				"SELECT label, payment_hash, msatoshi FROM invoices"
+				" WHERE pay_index NOT NULL"
+				"   AND pay_index > ?"
+				" ORDER BY pay_index ASC LIMIT 1;");
+		sqlite3_bind_int64(stmt, 1, pay_index);
+	} else {
+		stmt = db_prepare(wallet->db,
+				"SELECT label, payment_hash, msatoshi FROM invoices"
+				" WHERE pay_index NOT NULL"
+				" ORDER BY pay_index ASC LIMIT 1;");
+	}
+
+	res = sqlite3_step(stmt);
+	if (res != SQLITE_ROW) {
+		/* No paid invoice found. */
+		sqlite3_finalize(stmt);
+		return 0;
+	} else {
+		/* Paid invoice found, return data. */
+		*outlabel = tal_strndup(cxt, sqlite3_column_blob(stmt, 0), sqlite3_column_bytes(stmt, 0));
+
+		assert(sqlite3_column_bytes(stmt, 1) == sizeof(struct sha256));
+		memcpy(outrhash, sqlite3_column_blob(stmt, 1), sqlite3_column_bytes(stmt, 1));
+
+		*outmsatoshi = sqlite3_column_int64(stmt, 2);
+
+		sqlite3_finalize(stmt);
+		return 1;
+	}
+
+	return -1;
+}
+
 /* Acquire the next pay_index. */
 static s64 wallet_invoice_next_pay_index(struct db *db)
 {

wallet/wallet.h

@@ -330,6 +330,32 @@ bool wallet_htlcs_reconnect(struct wallet *wallet,
 			    struct htlc_in_map *htlcs_in,
 			    struct htlc_out_map *htlcs_out);
 
+/**
+ * wallet_invoice_nextpaid -- Find a paid invoice.
+ *
+ * Get the details (label, rhash, msatoshi) of the next paid
+ * invoice after the invoice with the given label. If label is
+ * `NULL`, get the details of the first paid invoice. Return -1
+ * if label is non-`NULL` and is not found, 0 if no more paid
+ * invoices after specified invoice (or no paid invoices if label
+ * is `NULL`), 1 if the next paid invoice was found.
+ *
+ * @ctx: Context to create the returned label.
+ * @wallet: Wallet to query
+ * @labelz: The label to be queried (zero-terminated), or
+ * `NULL` if first invoice is to be queried.
+ * @outlabel: Pointer to label of found paid invoice. Caller
+ * must free if this function returns 1.
+ * @outrhash: Pointer to struct rhash to be filled.
+ * @outmsatoshi: Pointer to number of millisatoshis value to pay.
+ */
+int wallet_invoice_nextpaid(const tal_t *cxt,
+			    const struct wallet *wallet,
+			    const char *labelz,
+			    char **outlabel,
+			    struct sha256 *outrhash,
+			    u64 *outmsatoshi);
+
 /**
  * wallet_invoice_save -- Save/update an invoice to the wallet
  *