MOD-15579 Lazy initialization of the shared SVS thread pool

[dor-forer]

Make initialization of the shared SVS thread pool lazy: OS worker threads are spawned only when the first SVS index is created, not when VecSim_UpdateThreadPoolSize is called. This eliminates the cost of pre-allocated SVS workers in deployments that configure RediSearch workers but never create an SVS index (today, every Redis with WORKERS=N configured spawns N-1 SVS worker threads at module init regardless of whether SVS is ever used).

The change extends the existing deferred-resize protocol to cover a second "safe point". VecSimSVSThreadPoolImpl::resize() now defers application in two cases:

• No SVS index attached yet → recorded; applied on first onIndexAttached() (lazy thread spawn).
• Shrink while jobs are in flight (pending_jobs_ > 0) → recorded; applied when endScheduledJob() drops the counter to 0 (existing behaviour).
• Otherwise → applied immediately via resize_locked().

The two deferral cases share deferred_size_ and never overlap (no jobs can be in flight before the first index attaches). One new gate field, has_attached_index_, distinguishes the two states. Set on the first onIndexAttached() call from the per-index VecSimSVSThreadPool ctor.

A test-only resetForTest() hook (under BUILD_TESTS) restores the singleton to a clean baseline by releasing the slots vector capacity and clearing the new gate; intended for unit tests that need to observe lazy semantics in a process where prior tests may have already attached indexes.

VecSim_UpdateThreadPoolSize continues to set the write mode (InPlace/Async) eagerly — only the SVS pool sizing is now deferred. The C API surface is unchanged.

RediSearch integration impact: none required — VecSim_UpdateThreadPoolSize(N) is still the single entry point for both module init and CONFIG SET search-workers M. Pre-index calls now just record the size; post-index calls behave exactly as before.

Main objects this PR modified

1. VecSimSVSThreadPoolImpl::resize() — now lazy until first index attaches.
2. VecSimSVSThreadPoolImpl::onIndexAttached() — new entry point called from per-index VecSimSVSThreadPool ctor; flips has_attached_index_ and applies any deferred size.
3. VecSimSVSThreadPoolImpl::has_attached_index_ — new member field gating spawn vs. defer.
4. VecSimSVSThreadPoolImpl::resetForTest() — new BUILD_TESTS-only hook.
5. VecSim_UpdateThreadPoolSize() — comment updated; behaviour now lazy via the new resize() semantics.

Tests

• SVSTest.ThreadPoolLazyInit (new) — verifies VecSim_UpdateThreadPoolSize does not allocate worker threads before any SVS index exists, and that the first SVS index creation triggers the deferred spawn at the previously requested size.
• SVSThreadPoolTest.UpdateThreadPoolSizeModeTransitions — rewritten to validate the lazy → eager transition (size stays at 1 until an index attaches; subsequent transitions are immediate).
• SVSThreadPoolTest fixture — SetUp() calls onIndexAttached() so the existing pool-isolation tests retain eager resize() semantics.
• SVSTest.NumThreadsParamIgnored — calls onIndexAttached() upfront for the same reason.
• SVSTest.debugInfoGlobalMemoryEqualsSharedSVSThreadPoolMemory — ASSERT_GT on global memory moved to after CreateNewIndex (the lazy spawn fires there).

Mark if applicable

• This PR introduces API changes
• This PR introduces serialization changes

Supersedes #969 — ownership transferred to @dor-forer.

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Note

Medium Risk
Changes when OS threads and pool memory appear relative to module init and CONFIG SET search-workers, but the public C API is unchanged and shrink-while-jobs-in-flight deferral is preserved; risk is mainly operational surprise if something assumed threads existed before any SVS index.

Overview
Lazy initialization defers spawning OS worker threads for the shared SVS thread pool until the first SVS index attaches. VecSim_UpdateThreadPoolSize still sets write mode immediately and still calls resize(), but before any index exists that call only records the target size in deferred_size_ instead of creating threads—so Redis deployments with search-workers configured but no SVS index no longer pay for unused SVS workers at module init.

The existing deferred-shrink path (shrink while pending_jobs_ > 0) is unchanged inside resize_locked(). A new gate, has_attached_index_, splits behavior: pre-attach resizes are record-only; post-attach resizes go through resize_locked() as before. The first VecSimSVSThreadPool construction calls onIndexAttached(), which flips the gate and applies any recorded size.

A BUILD_TESTS-only resetForTest() clears slots, deferred_size_, and has_attached_index_ so unit tests can observe lazy semantics on the process-wide singleton. Tests were added/updated (ThreadPoolLazyInit, lazy→eager UpdateThreadPoolSize transitions, fixture onIndexAttached for isolated pool tests).

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[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.97%. Comparing base (fda6a43) to head (f278e8d).

Additional details and impacted files

@@                             Coverage Diff                              @@
##           dor-forer-MOD-15578-track-svs-thpool-memory     #971   +/-   ##
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  Coverage                                        96.97%   96.97%           
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  Files                                              130      130           
  Lines                                             7829     7845   +16     
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+ Hits                                              7592     7608   +16     
  Misses                                             237      237           

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