compaction: compaction_writer: destroy shared_sstable after the sstable_writer

sstable_writer may depend on the sstable throughout its whole lifecycle.
If the sstable is freed before the sstable_writer we might hit use-after-free
as in the follwing case:
```
std::_Deque_iterator<sstables::compression::segmented_offsets::bucket, sstables::compression::segmented_offsets::bucket&, sstables::compression::segmented_offsets::bucket*>::operator+=(long) at /usr/include/c++/10/bits/stl_deque.h:240
 (inlined by) std::operator+(std::_Deque_iterator<sstables::compression::segmented_offsets::bucket, sstables::compression::segmented_offsets::bucket&, sstables::compression::segmented_offsets::bucket*> const&, long) at /usr/include/c++/10/bits/stl_deque.h:378
 (inlined by) std::_Deque_iterator<sstables::compression::segmented_offsets::bucket, sstables::compression::segmented_offsets::bucket&, sstables::compression::segmented_offsets::bucket*>::operator[](long) const at /usr/include/c++/10/bits/stl_deque.h:252
 (inlined by) std::deque<sstables::compression::segmented_offsets::bucket, std::allocator<sstables::compression::segmented_offsets::bucket> >::operator[](unsigned long) at /usr/include/c++/10/bits/stl_deque.h:1327
 (inlined by) sstables::compression::segmented_offsets::push_back(unsigned long, sstables::compression::segmented_offsets::state&) at ./sstables/compress.cc:214
sstables::compression::segmented_offsets::writer::push_back(unsigned long) at ./sstables/compress.hh:123
 (inlined by) compressed_file_data_sink_impl<crc32_utils, (compressed_checksum_mode)1>::put(seastar::temporary_buffer<char>) at ./sstables/compress.cc:519
seastar::output_stream<char>::put(seastar::temporary_buffer<char>) at table.cc:?
 (inlined by) seastar::output_stream<char>::put(seastar::temporary_buffer<char>) at ././seastar/include/seastar/core/iostream-impl.hh:432
seastar::output_stream<char>::flush() at table.cc:?
seastar::output_stream<char>::close() at table.cc:?
sstables::file_writer::close() at sstables.cc:?
sstables::mc::writer::~writer() at writer.cc:?
 (inlined by) sstables::mc::writer::~writer() at ./sstables/mx/writer.cc:790
sstables::mc::writer::~writer() at writer.cc:?
flat_mutation_reader::impl::consumer_adapter<stable_flattened_mutations_consumer<compact_for_compaction<sstables::compacting_sstable_writer, noop_compacted_fragments_consumer> > >::~consumer_adapter() at compaction.cc:?
 (inlined by) std::_Optional_payload_base<sstables::compaction_writer>::_M_destroy() at /usr/include/c++/10/optional:260
 (inlined by) std::_Optional_payload_base<sstables::compaction_writer>::_M_reset() at /usr/include/c++/10/optional:280
 (inlined by) std::_Optional_payload<sstables::compaction_writer, false, false, false>::~_Optional_payload() at /usr/include/c++/10/optional:401
 (inlined by) std::_Optional_base<sstables::compaction_writer, false, false>::~_Optional_base() at /usr/include/c++/10/optional:474
 (inlined by) std::optional<sstables::compaction_writer>::~optional() at /usr/include/c++/10/optional:659
 (inlined by) sstables::compacting_sstable_writer::~compacting_sstable_writer() at ./sstables/compaction.cc:229
 (inlined by) compact_mutation<(emit_only_live_rows)0, (compact_for_sstables)1, sstables::compacting_sstable_writer, noop_compacted_fragments_consumer>::~compact_mutation() at ././mutation_compactor.hh:468
 (inlined by) compact_for_compaction<sstables::compacting_sstable_writer, noop_compacted_fragments_consumer>::~compact_for_compaction() at ././mutation_compactor.hh:538
 (inlined by) std::default_delete<compact_for_compaction<sstables::compacting_sstable_writer, noop_compacted_fragments_consumer> >::operator()(compact_for_compaction<sstables::compacting_sstable_writer, noop_compacted_fragments_consumer>*) const at /usr/include/c++/10/bits/unique_ptr.h:85
 (inlined by) std::unique_ptr<compact_for_compaction<sstables::compacting_sstable_writer, noop_compacted_fragments_consumer>, std::default_delete<compact_for_compaction<sstables::compacting_sstable_writer, noop_compacted_fragments_consumer> > >::~unique_ptr() at /usr/include/c++/10/bits/unique_ptr.h:361
 (inlined by) stable_flattened_mutations_consumer<compact_for_compaction<sstables::compacting_sstable_writer, noop_compacted_fragments_consumer> >::~stable_flattened_mutations_consumer() at ././mutation_reader.hh:342
 (inlined by) flat_mutation_reader::impl::consumer_adapter<stable_flattened_mutations_consumer<compact_for_compaction<sstables::compacting_sstable_writer, noop_compacted_fragments_consumer> > >::~consumer_adapter() at ././flat_mutation_reader.hh:201
auto flat_mutation_reader::impl::consume_in_thread<stable_flattened_mutations_consumer<compact_for_compaction<sstables::compacting_sstable_writer, noop_compacted_fragments_consumer> >, flat_mutation_reader::no_filter>(stable_flattened_mutations_consumer<compact_for_compaction<sstables::compacting_sstable_writer, noop_compacted_fragments_consumer> >, flat_mutation_reader::no_filter, std::chrono::time_point<seastar::lowres_clock, std::chrono::duration<long, std::ratio<1l, 1000l> > >) at ././flat_mutation_reader.hh:272
 (inlined by) auto flat_mutation_reader::consume_in_thread<stable_flattened_mutations_consumer<compact_for_compaction<sstables::compacting_sstable_writer, noop_compacted_fragments_consumer> >, flat_mutation_reader::no_filter>(stable_flattened_mutations_consumer<compact_for_compaction<sstables::compacting_sstable_writer, noop_compacted_fragments_consumer> >, flat_mutation_reader::no_filter, std::chrono::time_point<seastar::lowres_clock, std::chrono::duration<long, std::ratio<1l, 1000l> > >) at ././flat_mutation_reader.hh:383
 (inlined by) auto flat_mutation_reader::consume_in_thread<stable_flattened_mutations_consumer<compact_for_compaction<sstables::compacting_sstable_writer, noop_compacted_fragments_consumer> > >(stable_flattened_mutations_consumer<compact_for_compaction<sstables::compacting_sstable_writer, noop_compacted_fragments_consumer> >, std::chrono::time_point<seastar::lowres_clock, std::chrono::duration<long, std::ratio<1l, 1000l> > >) at ././flat_mutation_reader.hh:389
 (inlined by) seastar::future<void> sstables::compaction::setup<noop_compacted_fragments_consumer>(noop_compacted_fragments_consumer)::{lambda(flat_mutation_reader)#1}::operator()(flat_mutation_reader)::{lambda()#1}::operator()() at ./sstables/compaction.cc:612
```

What happens here is that:

    compressed_file_data_sink_impl(output_stream<char> out, sstables::compression* cm, sstables::local_compression lc)
            : _out(std::move(out))
            , _compression_metadata(cm)
            , _offsets(_compression_metadata->offsets.get_writer())
            , _compression(lc)
            , _full_checksum(ChecksumType::init_checksum())

_compression_metadata points to a buffer held by the sstable object.
and _compression_metadata->offsets.get_writer returns a writer that keeps
a reference to the segmented_offsets in the sstables::compression
that is used in the ~writer -> close path.

Fixes #7821

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Message-Id: <20201227145726.33319-1-bhalevy@scylladb.com>
(cherry picked from commit 8a745a0ee0)

sstables/compaction.cc

@@ -126,8 +126,8 @@ static std::vector<shared_sstable> get_uncompacting_sstables(column_family& cf,
 class compaction;
 
 struct compaction_writer {
-    sstable_writer writer;
     shared_sstable sst;
+    sstable_writer writer;
 };
 
 class compacting_sstable_writer {
@@ -786,7 +786,8 @@ public:
         cfg.max_sstable_size = _max_sstable_size;
         cfg.monitor = &_active_write_monitors.back();
         cfg.run_identifier = _run_identifier;
-        return compaction_writer{sst->get_writer(*_schema, partitions_per_sstable(), cfg, get_encoding_stats(), priority), sst};
+        auto writer = sst->get_writer(*_schema, partitions_per_sstable(), cfg, get_encoding_stats(), priority);
+        return compaction_writer{std::move(sst), std::move(writer)};
     }
 
     virtual void stop_sstable_writer(compaction_writer* writer) override {
@@ -1268,7 +1269,8 @@ public:
         // sstables generated for a given shard will share the same run identifier.
         cfg.run_identifier = _run_identifiers.at(shard);
         auto&& priority = service::get_local_compaction_priority();
-        return compaction_writer{sst->get_writer(*_schema, partitions_per_sstable(shard), cfg, get_encoding_stats(), priority, shard), sst};
+        auto writer = sst->get_writer(*_schema, partitions_per_sstable(shard), cfg, get_encoding_stats(), priority, shard);
+        return compaction_writer{std::move(sst), std::move(writer)};
     }
 
     void on_new_partition() override {}