Fix memory leak and dead no-op wait in Lance streaming path

[Copilot]

Two bugs in the Lance streaming write path identified during code review.

Changes

• Memory leak on error in start_stream() (Rust) — libc::free() was called after the ? early-return on ArrowArrayStreamReader::from_raw() failure, leaving the malloc'd stream_ptr unreachable since C++ had already transferred ownership. Fixed by unconditionally freeing before propagating the error:

// Before: free is skipped on from_raw failure
let reader = unsafe { ArrowArrayStreamReader::from_raw(stream_ptr) }
    .map_err(|e| ...)?;
unsafe { libc::free(stream_ptr as *mut c_void) };

// After: always freed regardless of from_raw outcome
let result = unsafe { ArrowArrayStreamReader::from_raw(stream_ptr) };
unsafe { libc::free(stream_ptr as *mut c_void) };
let reader = result.map_err(|e| ...)?;

• Dead no-op else branch in StreamState::push() (C++) — the else path (queue has space) called not_full_cv_.wait() with a predicate guaranteed satisfied immediately, since the mutex is held continuously from the top of push(). This was a no-op that obscured the backpressure logic. Removed the branch entirely.

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[tsafin]

@claude review

[tsafin]

@codex review

[chatgpt-codex-connector]

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Reviewed commit: ee95869d03

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[chatgpt-codex-connector]

Keep stream ownership single on import failure

start_stream now unconditionally calls libc::free(stream_ptr) before propagating from_raw errors, but the C++ caller still frees the same pointer when lance_writer_start_stream returns non-zero (src/writers/lance_writer.cpp:259-262). If ArrowArrayStreamReader::from_raw returns Err (e.g., invalid exported stream), this path double-frees the same allocation and can corrupt the heap or abort the process.

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[tsafin]

Both fixes incorporated into tsafin/lance_stream as commit f5142c6. Thank you for the review — the memory leak on from_raw() failure and the dead else branch are now fixed directly in our working branch.