sstables, gdb: Track readers in a linked list

For the purpose of scylla-gdb.py command "scylla
active-sstables". Before the patch, readers were located by scanning
the heap for live objects with vtable pointers corresponding to
readers. It was observed that the test scylla_gdb/test_misc.py::test_active_sstables started failing like this:

  gdb.error: Error occurred in Python: Cannot access memory at address 0x300000000000000

This could be explained by there being a live object on the heap which
used to be a reader but now is a different object, and the _sst field
contains some other data which is not a pointer.

To fix, track readers explicitly in a linked list so that the gdb
script can reliably walk readers.

Fixes #18618.

scylla-gdb.py

@@ -1797,9 +1797,7 @@ def _lookup_type(type_names):
 
     types = []
     try:
-        # For Scylla < 2.1
-        # FIXME: this only finds range readers
-        types = [_lookup_type(['sstable_range_wrapping_reader'])]
+        return intrusive_list(gdb.parse_and_eval('sstables::_reader_tracker._readers'), link='_tracker_link')
     except gdb.error:
         try:
             # for Scylla <= 4.5

sstables/sstable_mutation_reader.cc

@@ -14,9 +14,32 @@
 #include "utils/to_string.hh"
 
 #include <boost/range/algorithm/stable_partition.hpp>
+#include <boost/intrusive/list.hpp>
 
 namespace sstables {
 
+class reader_tracker {
+    using list_type = boost::intrusive::list<mp_row_consumer_reader_base,
+        boost::intrusive::member_hook<mp_row_consumer_reader_base,
+            mp_row_consumer_reader_base::tracker_link_type,
+            &mp_row_consumer_reader_base::_tracker_link>,
+        boost::intrusive::constant_time_size<false>>;
+public:
+    list_type _readers;
+
+    void add(mp_row_consumer_reader_base& reader) {
+        _readers.push_back(reader);
+    }
+};
+
+thread_local reader_tracker _reader_tracker;
+
+mp_row_consumer_reader_base::mp_row_consumer_reader_base(shared_sstable sst)
+    : _sst(std::move(sst))
+{
+    _reader_tracker.add(*this);
+}
+
 atomic_cell make_counter_cell(api::timestamp_type timestamp, fragmented_temporary_buffer::view cell_value) {
     static constexpr size_t shard_size = 32;
 

sstables/sstable_mutation_reader.hh

@@ -24,9 +24,14 @@ namespace mx {
 }
 
 class mp_row_consumer_reader_base {
+public:
+    using tracker_link_type = bi::list_member_hook<bi::link_mode<bi::auto_unlink>>;
+    friend class reader_tracker;
 protected:
     shared_sstable _sst;
 
+    tracker_link_type _tracker_link;
+
     // Whether index lower bound is in current partition
     bool _index_in_current_partition = false;
 
@@ -42,9 +47,7 @@ protected:
 
     std::optional<dht::decorated_key> _current_partition_key;
 public:
-    mp_row_consumer_reader_base(shared_sstable sst)
-        : _sst(std::move(sst))
-    { }
+    mp_row_consumer_reader_base(shared_sstable sst);
 
     // Called when all fragments relevant to the query range or fast forwarding window
     // within the current partition have been pushed.