insert() held no local strong ref to the prepared modification_statement
across the suspension in execute(). On a single shard:
1. Fiber A suspends inside _insert_stmt->execute().
2. DROP TABLE / DROP KEYSPACE on the target, or LRU eviction, removes
the prepared_statements_cache entry, releasing its strong ref.
3. Fiber B re-enters cache_table_info(), sees _prepared_stmt
(checked_weak_ptr) invalidated, and runs _insert_stmt = nullptr,
releasing the last strong ref. The modification_statement is freed.
4. Fiber A resumes inside execute() and touches freed *this.
Pin strong ref to _insert_stmt locally before the suspension.
Fixes https://scylladb.atlassian.net/browse/SCYLLADB-1667
Backport: all supported branches, it's memory corruption bug, long present
Closesscylladb/scylladb#29588
* github.com:scylladb/scylladb:
test/boost: add dummy case to table_helper_test for non-injection modes
test/boost: add regression test for table_helper insert() UAF
utils/error_injection: add waiters() API
table_helper: fix use-after-free on prepared-statement invalidation
(cherry picked from commit efcc0b6376)
Closesscylladb/scylladb#29747Closesscylladb/scylladb#29802
02c7f44da4