perf: use LRU cache for session contexts in get_session_context

[wjones127]

Motivation

We have a performance issue when LANCE_MEM_POOL_SIZE: when this is set (to something different than the default), the current cache always misses here:

lance/rust/lance-datafusion/src/exec.rs

Lines 398 to 410 in 445dd5b

	pub fn get_session_context(options: &LanceExecutionOptions) -> SessionContext {
	if options.mem_pool_size() == DEFAULT_LANCE_MEM_POOL_SIZE
	&& options.max_temp_directory_size() == DEFAULT_LANCE_MAX_TEMP_DIRECTORY_SIZE
	&& options.target_partition.is_none()
	{
	return if options.use_spilling() {
	DEFAULT_SESSION_CONTEXT_WITH_SPILLING.clone()
	} else {
	DEFAULT_SESSION_CONTEXT.clone()
	};
	}
	new_session_context(options)
	}

Summary

• Replace static LazyLock session contexts with LRU cache (size 4)
• Cache key uses resolved configuration values (after env var lookup)
• Fixes cache misses when LANCE_MEM_POOL_SIZE env var is set

Test plan

• cargo test -p lance-datafusion passes
• cargo clippy -p lance-datafusion passes

🤖 Generated with Claude Code

[github-actions]

Code Review

P1: Missing test coverage for new caching logic

The new LRU cache implementation has no unit tests. Given this code manages shared state (global cache with mutex), I'd recommend adding tests to verify:

• Cache hits return the same context
• LRU eviction works correctly when cache exceeds MAX_CACHE_SIZE
• Different options produce different cache keys

P1: Consider using lru crate instead of manual implementation

The manual LRU implementation using HashMap + Instant requires iterating all entries to find the LRU item on eviction (O(n) instead of O(1)). While n=4 is small, using the lru crate would be cleaner and avoid potential bugs. The current implementation also holds the mutex lock during the O(n) eviction scan.

Minor observation

The change looks functionally correct and solves the stated problem (env var values not being considered for caching). The cache key using resolved values is the right approach.

[codecov]

Codecov Report

❌ Patch coverage is 96.87500% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-datafusion/src/exec.rs	96.87%	0 Missing and 3 partials ⚠️

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