Summary

Extract the I/O capacity management fields from __wt_connection_impl into a new WT_CONN_CAPACITY 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).

Fields to extract

From lines 887-891 of src/include/connection.h:

• WT_CAPACITY capacity → throttle (I/O capacity rate limits — existing inner sub-struct renamed to avoid collision with the outer field name)
• WT_SESSION_IMPL *capacity_session → session (Capacity thread session)
• wt_thread_t capacity_tid → tid (Capacity thread)
• bool capacity_tid_set → tid_set (Capacity thread set)
• WT_CONDVAR *capacity_cond → cond (Capacity wait mutex)

Access pattern: conn->capacity., e.g. conn->capacity.throttle.ckpt, conn->capacity.session.

Tasks

Define struct __wt_conn_capacity in src/include/connection.h (alphabetically among WT_CONN_* structs)

Add forward declaration and typedef in src/include/wt_internal.h alphabetically among WT_CONN_* entries

Replace the 5 individual fields in __wt_connection_impl with a single WT_CONN_CAPACITY capacity member

Add *wti_conn_capacity_init / *wti_conn_capacity_destroy helper functions following the pattern of __wti_conn_prefetch_init (WT-17496)

Update all references to conn->capacity_session, conn->capacity_tid, conn->capacity_cond, conn->capacity_tid_set

Update all references to the inner WT_CAPACITY fields: conn->capacity.ckpt → conn->capacity.throttle.ckpt, etc.

Regenerate src/include/extern.h via dist/prototypes.py after adding the new helper functions

Run cd dist && bash s_fast and fix any formatting issues (commit separately)

Build with ninja wt and confirm zero warnings

Definition of Done

• WT_CONN_CAPACITY structure defined in connection.h containing all 5 fields above
• All fields removed from __wt_connection_impl and replaced with a single capacity member
• All references updated: conn->capacity_session → conn->capacity.session, conn->capacity.ckpt → conn->capacity.throttle.ckpt, etc.
• Init/destroy helper functions added following the established pattern
• Code compiles cleanly with no warnings
• grep confirms no stale references to old field names remain