perf: lazy-init _callbacks/_errbacks in ResponseFuture (25ns saving, 2x speedup, 112 bytes saved)

[mykaul]

Summary

• Defer list allocation for _callbacks and _errbacks from ResponseFuture.__init__() to first use in add_callback()/add_errback()
• On the synchronous execute path (session.execute()), no callbacks are registered, so both lists are never allocated — saving 112 bytes per request
• All access is under _callback_lock; _set_final_result and _set_final_exception use or () guard to iterate safely when None

Benchmark

Scenario	Per-call	Speedup
Init with [] x2	50 ns	baseline
Init with None x2	25 ns	~2x

Memory saved per request (no callbacks): 112 bytes (2 x 56-byte empty list)

Tests

• 11 focused unit tests covering lazy init, callback/errback invocation, clear_callbacks, add_callback after result
• Full unit test suite passes (656 passed)

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[mykaul]

Rebased onto current origin/master (past the bound_result_metadata / SCYLLA_USE_METADATA_ID changes in ResponseFuture.__init__). The rebase applied cleanly with no manual conflict resolution needed — the lazy-init hunks (class body, __init__, _set_final_result/_set_final_exception guards, add_callback/add_errback lazy-alloc, clear_callbacks) landed in the surrounding new code without overlap.

Given the recent history of ResponseFuture.__init__ collisions on other PRs (#806, #805), I specifically audited every direct ._callbacks/._errbacks access across the whole codebase (not just add_callback/add_errback) to make sure nothing reads them as a list/tuple before they'd be lazily created:

• All access in cassandra/cluster.py is already properly guarded (is None checks before append, or () guards in the two iteration sites, clear_callbacks resets to None).
• The only other ._callbacks in the repo is cqlengine's unrelated BatchQuery._callbacks, which is a separate class, always initializes to [], and is untouched by this change.
• No getattr/hasattr indirection or test mocks touch ResponseFuture._callbacks/._errbacks directly outside of this PR's own add_callback/add_errback/_set_final_result/_set_final_exception paths.

No unresolved review threads or comments existed on the PR prior to this push.

Verified:

• tests/unit/test_response_future.py + tests/unit/test_lazy_init_callbacks.py: 63 passed
• Full tests/unit/: 731 passed, 88 skipped (environment-gated), 0 failed

Force-pushed the rebased commit (same commit, no content changes needed beyond the rebase) to keep this a single amended commit rather than adding new ones. Still a draft.

[Copilot]

Pull request overview

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This PR optimizes ResponseFuture by lazily initializing _callbacks/_errbacks to avoid unnecessary list allocations on the synchronous execute path, while keeping callback behavior consistent and thread-safe.

Changes:

• Change _callbacks/_errbacks initialization from empty lists to None, and lazily allocate on first add_* use
• Guard callback/errback iteration in _set_final_result / _set_final_exception to handle None safely
• Add unit tests and a micro-benchmark to validate and measure the optimization

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
cassandra/cluster.py	Implements lazy init and safe iteration for callbacks/errbacks in ResponseFuture.
tests/unit/test_lazy_init_callbacks.py	Adds focused unit tests validating lazy init and callback/errback behavior.
benchmarks/bench_lazy_init_callbacks.py	Adds a micro-benchmark to quantify time and allocation savings.

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