Summary

Extract the five hot-backup fields from _wt_connection_impl into a new WT_CONN_BACKUP sub-structure. This follows the purpose-based modularisation pattern established by the SPM-3483 epic (see WT-17491, WT-17493, WT-17495 as prior examples). incr_granularity and incr_backups are incremental-backup specific and remain on _wt_connection_impl.

Fields to extract

From lines 843-848 of src/include/connection.h:

• WT_RWLOCK hot_backup_lock → lock (Hot backup serialization)
• wt_shared uint64_t hot_backup_start → start (Clock value of most recent checkpoint needed by hot backup)
• wt_timestamp_t hot_backup_timestamp → timestamp (Stable timestamp of checkpoint for the open backup)
• char **hot_backup_list → list (Hot backup file list)
• uint32_t *partial_backup_remove_ids → partial_remove_ids (Remove btree id list for partial backup)

Access pattern: conn->backup.<field>

Tasks

1. Define struct _wt_conn_backup in src/include/connection.h (alphabetically among WT_CONN* structs)
2. Add forward declaration and typedef in src/include/wt_internal.h alphabetically among WT_CONN_* entries
3. Replace the 5 individual fields in __wt_connection_impl with a single WT_CONN_BACKUP backup member
4. Update src/conn/conn_handle.c: add init/destroy helpers _wti_conn_backup_init / wti_conn_backup_destroy following the pattern of _wti_conn_prefetch_init (WT-17496)
5. Update src/conn/conn_api.c and all backup-related source files to use conn->backup.<field>
6. Update the macro in connection.h (around line 640) that references hot_backup_timestamp, hot_backup_start, and hot_backup_list
7. Regenerate src/include/extern.h via dist/prototypes.py after adding the new helper functions
8. Run cd dist && bash s_fast and fix any formatting issues (commit separately)
9. Build with ninja wt and confirm zero warnings

Definition of Done

• WT_CONN_BACKUP structure defined in connection.h containing the 5 hot-backup fields above
• All fields removed from __wt_connection_impl and replaced with a single backup member
• All references updated: conn->hot_backup_lock → conn->backup.lock, conn->hot_backup_list → conn->backup.list, etc.
• _wti_conn_backup_init / _wti_conn_backup_destroy helpers added following the established pattern
• Code compiles cleanly with no warnings
• grep confirms no stale references to old field names remain