db/view: add clustering_or_static_row

Adds a `clustering_or_static_row`, which is a common, immutable
representation of either a static or clustering row. It will allow to
handle view update generation based on static or clustering rows in a
uniform way.

db/view/view.cc

@@ -233,6 +233,20 @@ bool view_info::has_base_non_pk_columns_in_view_pk() const {
     return _base_info->has_base_non_pk_columns_in_view_pk;
 }
 
+clustering_row db::view::clustering_or_static_row::as_clustering_row(const schema& s) const {
+    if (!is_clustering_row()) {
+        on_internal_error(vlogger, "Tried to interpret a static row as a clustering row");
+    }
+    return clustering_row(*_key, tomb(), marker(), row(s, column_kind::regular_column, cells()));
+}
+
+static_row db::view::clustering_or_static_row::as_static_row(const schema& s) const {
+    if (!is_static_row()) {
+        on_internal_error(vlogger, "Tried to interpret a clustering row as a static row");
+    }
+    return static_row(s, cells());
+}
+
 namespace db {
 
 namespace view {

db/view/view.hh

@@ -80,6 +80,46 @@ struct view_and_base {
     base_info_ptr base;
 };
 
+// An immutable representation of a clustering or static row of the base table.
+struct clustering_or_static_row {
+private:
+    std::optional<clustering_key_prefix> _key;
+    deletable_row _row;
+
+public:
+    explicit clustering_or_static_row(clustering_row&& cr)
+            : _key(std::move(cr.key()))
+            , _row(std::move(cr).as_deletable_row())
+    {}
+
+    explicit clustering_or_static_row(static_row&& sr)
+            : _key()
+            , _row(row_tombstone(), row_marker(), std::move(sr.cells()))
+    {}
+
+    bool is_static_row() const { return !_key.has_value(); }
+    bool is_clustering_row() const { return _key.has_value(); }
+
+    const std::optional<clustering_key_prefix>& key() const { return _key; }
+
+    row_tombstone tomb() const { return _row.deleted_at(); }
+    const row_marker& marker() const { return _row.marker(); }
+    const row& cells() const { return _row.cells(); }
+
+    bool empty() const { return _row.empty(); }
+    bool is_live(const schema& s, tombstone base_tombstone = tombstone(), gc_clock::time_point now = gc_clock::time_point::min()) const {
+        return _row.is_live(s, column_kind(), base_tombstone, now);
+    }
+
+    column_kind column_kind() const {
+        return _key.has_value()
+                ? column_kind::regular_column : column_kind::static_column;
+    }
+
+    clustering_row as_clustering_row(const schema& s) const;
+    static_row as_static_row(const schema& s) const;
+};
+
 /**
  * Whether the view filter considers the specified partition key.
  *