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Fixes a race condition where tablet split can crash the server during truncation.
`truncate_table_on_all_shards()` disables compaction on all existing compaction groups, then later calls `discard_sstables()` which asserts that compaction is disabled. Between these two points, tablet split can call `set_split_mode()`, which creates new compaction groups via `make_empty_group()` — these start with `compaction_disabled_counter == 0`. When `discard_sstables()` checks its assertion, it finds these new groups and fires `on_internal_error`, aborting the server.
In `storage_group::set_split_mode()`, before creating new compaction groups, check whether the main compaction group has compaction disabled. If it does, bail out early and return `false` (not ready). This is safe because the split will be retried once truncation completes and re-enables compaction.
A new regression test `test_split_emitted_during_truncate` reproduces the
exact interleaving using two error injection points:
- **`database_truncate_wait`** — pauses truncation after compaction is disabled but before `discard_sstables()` runs.
- **`tablet_split_monitor_wait`** (new, in `service/storage_service.cc`) — pauses the split monitor at the start of `process_tablet_split_candidate()`.
The test creates a single-tablet table, triggers both operations, uses the injection points to force the problematic ordering, then verifies that truncation completes successfully and the split finishes afterward.
Fixes: SCYLLADB-1035
This needs to be backported to all currently supported version.
Closes scylladb/scylladb#29250
* github.com:scylladb/scylladb:
test: add test_split_emitted_during_truncate
table: fix race between tablet split and truncate
(cherry picked from commit 7fe4ae16f0)
Closes scylladb/scylladb#29478