avoid nodiscard compiler warnings

We got a lot of the warnings for unchecked Result / tl::expected; let's
handle those.

lib/mu-config.hh

@@ -429,6 +429,21 @@ public:
 		return Ok();
 	}
 
+	/**
+	 * Set a new value for some property; unlike set(), merely log a
+	 * warning if that fails. For callers that cannot meaningfully
+	 * handle the error.
+	 *
+	 * @param prop_id property-id
+	 * @param val the new value (of the correct type)
+	 */
+	template<Id ID, typename T>
+	void checked_set(const T& val) {
+		if (const auto res{set<ID>(val)}; !res)
+			mu_warning("failed to set config property '{}': {}",
+				   property<ID>().name, res.error());
+	}
+
 	/**
 	 * Is this a read-only Config?
 	 *

lib/mu-contacts-cache.cc

@@ -162,8 +162,11 @@ ContactsCache::Private::serialize() const
 			       ci.message_date, SepaChar2,
 			       ci.frequency);
 	}
-	config_db_.set<Config::Id::Contacts>(s);
-	dirty_ = 0;
+	if (const auto res{config_db_.set<Config::Id::Contacts>(s)}; res)
+		dirty_ = 0;
+	else
+		mu_error("failed to serialize contacts: {}",
+			 res.error());
 }
 
 ContactsCache::ContactsCache(Config& config_db)

lib/mu-indexer.cc

@@ -427,7 +427,7 @@ Indexer::Private::scan_worker()
 	if (!aborted && conf_.scan) {
 		// Store started time, not ending time, so that next time we run we know to scan
 		// anything that appeared during our scan.
-		store_.config().set<Mu::Config::Id::LastIndex>(started_.value());
+		store_.config().checked_set<Mu::Config::Id::LastIndex>(started_.value());
 	}
 
 	completed_ = ::time({});

lib/mu-labels-cache.cc

@@ -30,9 +30,12 @@ Mu::LabelsCache::serialize() const
 	for (const auto&[label, n]: label_map_)
 		s += mu_format("{}{}{}\n", label, SepaChar2, n);
 
-	config_.set<Config::Id::Labels>(s);
-	mu_debug("labels: serialized {} change(s)", dirty_);
-	dirty_ = 0;
+	if (const auto res{config_.set<Config::Id::Labels>(s)}; res) {
+		mu_debug("labels: serialized {} change(s)", dirty_);
+		dirty_ = 0;
+	} else
+		mu_warning("labels: fail to store labels: {}",
+			   res.error());
 }
 
 Mu::LabelsCache::Map

lib/mu-scanner.cc

@@ -429,7 +429,10 @@ main (int argc, char *argv[])
 
 	Scanner scanner{argv[1], on_path, Mode::MaildirsOnly};
 
-	scanner.start();
+	if (const auto res{scanner.start()}; !res) {
+		mu_printerrln("scan failed: {}", res.error());
+		return 1;
+	}
 
 	return 0;
 }

lib/mu-server.cc

@@ -141,7 +141,9 @@ struct Server::Private {
 		if (have_indexer_)
 			indexer().stop();
 		if (!tmp_dir_.empty())
-			remove_directory(tmp_dir_);
+			if (const auto res{remove_directory(tmp_dir_)}; !res)
+				mu_warning("failed to remove '{}': {}",
+					   tmp_dir_, res.error());
 	}
 	//
 	// construction helpers
@@ -552,13 +554,15 @@ Server::Private::contacts_handler(const Command& cmd)
 	auto       n{0};
 	auto&& out{make_output_stream()};
 	mu_print(out, "(");
-	store().contacts_cache().for_each([&](const Contact& ci) {
+	const auto res = store().contacts_cache().for_each([&](const Contact& ci) {
 		if (!match_contact(ci))
 			return true; // continue
 		mu_println(out.out(), "{}", quote(ci.display_name()));
 		++n;
 		return maxnum == 0 || n <  maxnum;
 	});
+	if (!res)
+		mu_warning("contacts: {}", res.error());
 	mu_print(out, ")");
 	output(mu_format("(:contacts {}\n:tstamp \"{}\")",
 			 out.to_string(), g_get_monotonic_time()));
@@ -896,7 +900,8 @@ Server::Private::index_handler(const Command& cmd)
 		throw Error{Error::Code::Xapian, "indexer is already running"};
 
 	do_index(conf);
-	store().serialize();
+	if (const auto res{store().serialize()}; !res)
+		mu_warning("failed to serialize store: {}", res.error());
 }
 
 void

lib/mu-store.cc

@@ -103,8 +103,10 @@ struct Store::Private {
 		if (conf)
 			config.import_configurable(*conf);
 
-		config.set<Config::Id::RootMaildir>(remove_slash(root_maildir));
-		config.set<Config::Id::SchemaVersion>(ExpectedSchemaVersion);
+		if (const auto res{config.set<Config::Id::RootMaildir>(remove_slash(root_maildir))}; !res)
+			throw res.error();
+		if (const auto res{config.set<Config::Id::SchemaVersion>(ExpectedSchemaVersion)}; !res)
+			throw res.error();
 
 		return config;
 	}
@@ -420,8 +422,11 @@ Store::remove_message(const std::string& path)
 		for (auto&& label : doc.string_vec_value(Field::Id::Labels))
 			priv_->labels_cache_.decrease(label);
 
-		xapian_db().delete_document(term);
-		mu_debug("deleted message @ {} from store", path);
+		if (const auto res{xapian_db().delete_document(term)}; res)
+			mu_debug("deleted message @ {} from store", path);
+		else
+			mu_warning("failed to delete message @ {}: {}",
+				   path, res.error());
 
 		return true;
 	}
@@ -432,7 +437,8 @@ Store::remove_messages(const std::vector<Store::Id>& ids)
 {
 	std::lock_guard guard{priv_->lock_};
 
-	xapian_db().request_transaction();
+	if (const auto res{xapian_db().request_transaction()}; !res)
+		mu_warning("failed to request transaction: {}", res.error());
 
 	for (auto&& id : ids)
 		priv_->remove_message_by_id_unlocked(id);
@@ -464,7 +470,10 @@ Store::remove_messages_by_term(std::span<const std::string> terms,
 	std::vector<Xapian::Query> qvec;
 	std::vector<Store::Id> ids_to_remove;
 
-	xapian_db().request_transaction();
+	if (const auto res{xapian_db().request_transaction()}; !res) {
+		mu_error("failed to request transaction: {}", res.error());
+		return 0;
+	}
 
 	while (!terms.empty()) {
 		auto chunk = terms.subspan(0, std::min<size_t>(terms.size(), 1024));
@@ -810,7 +819,11 @@ Store::maildirs() const
 	};
 
 	Scanner scanner{root_maildir(), handler, Scanner::Mode::MaildirsOnly};
-	scanner.start();
+	if (const auto res{scanner.start()}; !res) {
+		mu_warning("failed to start scanner: {}", res.error());
+		return {};
+	}
+
 	std::ranges::sort(mdirs);
 
 	return mdirs;

lib/utils/mu-error.hh

@@ -1,5 +1,5 @@
 /*
-** Copyright (C) 2019-2025 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+** Copyright (C) 2019-2026 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
 **
 ** This program is free software; you can redistribute it and/or modify it
 ** under the terms of the GNU General Public License as published by the
@@ -107,7 +107,7 @@ struct Error final : public std::exception {
 	 *
 	 * @return
 	 */
-	virtual const char* what() const noexcept override { return what_.c_str(); }
+	const char* what() const noexcept override { return what_.c_str(); }
 
 	/**
 	 * Get the error-code for this error
@@ -183,7 +183,10 @@ private:
 	std::string		hint_;
 };
 
-inline auto
+/**
+ * Make Error formattable with libfmt
+ */
+inline std::string
 format_as(const Error& err) {
 	return mu_format("<{} ({}:{})>",
 			 err.what(),

mu/mu-cmd-init.cc

@@ -53,15 +53,15 @@ Mu::mu_cmd_init(const Options& opts)
 		Config conf{mdb};
 
 		if (opts.init.max_msg_size)
-			conf.set<Config::Id::MaxMessageSize>(*opts.init.max_msg_size);
+			conf.checked_set<Config::Id::MaxMessageSize>(*opts.init.max_msg_size);
 		if (opts.init.batch_size && *opts.init.batch_size != 0)
-			conf.set<Config::Id::BatchSize>(*opts.init.batch_size);
+			conf.checked_set<Config::Id::BatchSize>(*opts.init.batch_size);
 		if (!opts.init.personal_addresses.empty())
-			conf.set<Config::Id::PersonalAddresses>(opts.init.personal_addresses);
+			conf.checked_set<Config::Id::PersonalAddresses>(opts.init.personal_addresses);
 		if (!opts.init.ignored_addresses.empty())
-			conf.set<Config::Id::IgnoredAddresses>(opts.init.ignored_addresses);
+			conf.checked_set<Config::Id::IgnoredAddresses>(opts.init.ignored_addresses);
 		if (opts.init.support_ngrams)
-			conf.set<Config::Id::NgramsEnabled>(true);
+			conf.checked_set<Config::Id::NgramsEnabled>(true);
 
 		return Store::make_new(opts.runtime_path(RuntimePath::XapianDb),
 				       opts.init.maildir, conf);
@@ -77,7 +77,9 @@ Mu::mu_cmd_init(const Options& opts)
 		// mildly hacky
 		Options opts_copy{opts};
 		opts_copy.info.topic = "store";
-		mu_cmd_info(*store, opts_copy);
+		if (const auto res{mu_cmd_info(*store, opts_copy)}; !res)
+			mu_warning("failed to show store info: {}",
+				   res.error());
 
 		mu_println("Database is empty. You can use 'mu index' to fill it.");
 	}

mu/mu-cmd-labels.cc

@@ -67,7 +67,9 @@ label_update(Mu::Store& store, const Options& opts)
 				mu_println("labels: apply {} to {}", labelstr, msg->path());
 
 			if (!opts.labels.dry_run) {
-				store.update_labels(*msg, deltas);
+				if (const auto res{store.update_labels(*msg, deltas)}; !res)
+					mu_warning("failed to update labels for {}: {}",
+						   msg->path(), res.error());
 			}
 			++n;
 		}
@@ -101,7 +103,9 @@ label_clear(Mu::Store& store, const Options& opts)
 				mu_println("labels: clear all from {}", msg->path());
 
 			if (!opts.labels.dry_run) {
-				store.clear_labels(*msg);
+				if (const auto res{store.clear_labels(*msg)}; !res)
+					mu_warning("failed to clear labels for {}: {}",
+						   msg->path(), res.error());
 			}
 			++n;
 		}

scm/mu-scm-store.cc

@@ -104,11 +104,13 @@ subr_cc_store_cfind(SCM store_scm, SCM pattern_scm, SCM personal_scm, SCM after_
 	// 0 means "unlimited"
 	const size_t maxnum = from_scm_with_default(max_results_scm, 0U, func, 5);
 
-	to_store(store_scm, func, 1).contacts_cache().for_each(
+	const auto res = to_store(store_scm, func, 1).contacts_cache().for_each(
 		[&](const auto& contact)->bool {
 			contacts = scm_cons(to_scm(contact), contacts);
 			return true;
 		}, pattern, personal, after, maxnum);
+	if (!res)
+		throw ScmError{ScmError::Id::WrongArg, func, 2, pattern_scm, "pattern"};
 
 	return scm_reverse_x(contacts, SCM_EOL);
 } catch (const ScmError& scm_err) {