labels: allow restoring cache-map

E.g. with unexpected termination for mu it is possible that the
labels-cache (i.e., the one that is used for auto-completion) gets disconnected
from reality.

Add a --restore option to `mu label list` to restore the actual labels from the
labels seen in the store.

lib/mu-store-labels.cc

@@ -23,6 +23,25 @@
 
 using namespace Mu;
 
+Result<void>
+Mu::LabelsCache::restore(const Store& store)
+{
+	const auto res{store.run_query("")};
+	if (!res)
+		return Err(Error{Error::Code::Query,
+				"failed to run query: {}",
+				 *res.error().what()});
+	label_map_.clear();
+
+	for (auto&& item: *res) {
+		if (auto &&msg{item.message()}; msg) {
+			for (const auto& label: msg->labels())
+				increase(label);
+		}
+	}
+	return Ok();
+}
+
 namespace {
 constexpr std::string_view	path_key       = "path:";
 constexpr std::string_view	message_id_key = "message-id:";

lib/mu-store-labels.hh

@@ -30,6 +30,8 @@
 
 namespace Mu {
 
+class Store;
+
 /**
  * The cache keeps track of what labels are being used. This can be used
  * for completion etc. and `mu label list`
@@ -143,6 +145,17 @@ public:
 		return map;
 	}
 
+
+	/**
+	 * Restore the labels-cache from the labels seen in the store.
+	 *
+	 * @param store a store
+	 *
+	 * @return Ok() or some error
+	 */
+	Result<void> restore(const Store& store);
+
+
 	/**
 	 * Is the cache "dirty"?
 	 *
@@ -159,8 +172,6 @@ private:
 	mutable bool dirty_{};
 };
 
-class Store;
-
 /**
  * Export labels to a file
  *

lib/mu-store.cc

@@ -694,6 +694,14 @@ Store::clear_labels(Message& message)
 	return Ok(std::move(updates));
 }
 
+Result<void>
+Store::restore_label_map()
+{
+	std::unique_lock lock{priv_->lock_};
+
+	return priv_->labels_cache_.restore(*this);
+}
+
 LabelsCache::Map
 Store::label_map() const
 {

lib/mu-store.hh

@@ -373,6 +373,16 @@ public:
 	 */
 	Result<Labels::DeltaLabelVec> clear_labels(Message& message);
 
+	/**
+	 * Restore label-map from store
+	 *
+	 * Restore the labels list in the store, i.e., restore the cached list of labels which is
+	 * used for e.g. auto-completion in mu4e from the labels in the store.
+	 *
+	 * @return Ok or some error.
+	 */
+	Result<void> restore_label_map();
+
 	/**
 	 * Get a copy of the map of labels in use.
 	 *
@@ -382,6 +392,8 @@ public:
 	 */
 	LabelsCache::Map label_map() const;
 
+
+
 	/**
 	 * Prototype for the ForEachMessageFunc
 	 *