view: ignore duplicated key entries in progress virtual reader

Build progress virtual reader uses Scylla-specific
scylla_views_builds_in_progress table in order to represent
legacy views_builds_in_progress rows. The Scylla-specific table contains
additional cpu_id clustering key part, which is trimmed before returning
it to the user. That may cause duplicated clustering row fragments to be
emitted by the reader, which may cause undefined behaviour in consumers.
The solution is to keep track of previous clustering keys for each
partition and drop fragments that would cause duplication. That way if
any shard is still building a view, its progress will be returned,
and if many shards are still building, the returned value will indicate
the progress of a single arbitrary shard.

Fixes #4524
Tests:
unit(dev) + custom monotonicity checks from <tgrabiec@scylladb.com>

(cherry picked from commit 85a3a4b458)

db/view/build_progress_virtual_reader.hh

@@ -44,6 +44,11 @@ namespace db::view {
 // columns. When reading the results from the scylla_views_builds_in_progress
 // table, we adjust the clustering key (we shed the cpu_id column) and map
 // back the regular columns.
+// Since mutation fragment consumers expect clustering_row fragments
+// not to be duplicated for given primary key, previous clustering key
+// is stored between mutation fragments. If the clustering key becomes
+// the same as the previous one (as a result of trimming cpu_id),
+// the duplicated fragment is ignored.
 class build_progress_virtual_reader {
     database& _db;
 
@@ -55,6 +60,7 @@ class build_progress_virtual_reader {
         const query::partition_slice& _legacy_slice;
         query::partition_slice _slice;
         flat_mutation_reader _underlying;
+        std::optional<clustering_key> _previous_clustering_key;
 
         build_progress_reader(
                 schema_ptr legacy_schema,
@@ -79,7 +85,8 @@ class build_progress_virtual_reader {
                         pc,
                         std::move(trace_state),
                         fwd,
-                        fwd_mr)) {
+                        fwd_mr))
+                , _previous_clustering_key() {
         }
 
         const schema& underlying_schema() const {
@@ -127,8 +134,13 @@ class build_progress_virtual_reader {
                                 legacy_in_progress_row.append_cell(_legacy_generation_number_col, std::move(c));
                             }
                         });
+                        auto ck = adjust_ckey(scylla_in_progress_row.key());
+                        if (_previous_clustering_key && ck.equal(*_schema, *_previous_clustering_key)) {
+                            continue;
+                        }
+                        _previous_clustering_key = ck;
                         mf = clustering_row(
-                                adjust_ckey(scylla_in_progress_row.key()),
+                                std::move(ck),
                                 std::move(scylla_in_progress_row.tomb()),
                                 std::move(scylla_in_progress_row.marker()),
                                 std::move(legacy_in_progress_row));
@@ -140,6 +152,8 @@ class build_progress_virtual_reader {
                                 adjust_ckey(scylla_in_progress_rt.end),
                                 scylla_in_progress_rt.end_kind,
                                 scylla_in_progress_rt.tomb);
+                    } else if (mf.is_end_of_partition()) {
+                        _previous_clustering_key.reset();
                     }
                     push_mutation_fragment(std::move(mf));
                 }
@@ -192,4 +206,4 @@ public:
     }
 };
 
-}
+}