Remove redundant zero-initialization in Arrow IPC reader hot paths

[pchintar]

Description

In arrow-ipc/src/reader.rs, buffers are currently allocated with MutableBuffer::from_len_zeroed(len) and then passed to read_exact, which fully overwrites the entire buffer contents.

This results in an unnecessary full memory pass:

• first zero-initialization
• then complete overwrite by the incoming data

Since read_exact guarantees that the provided slice is fully written on success, the initial zeroing step is redundant and can be safely avoided by allocating with capacity and setting the length before the read.

Proposed Change

Replace patterns of the form:

let mut buf = MutableBuffer::from_len_zeroed(len);
reader.read_exact(&mut buf)?;

with:

let mut buf = MutableBuffer::with_capacity(len);
unsafe { buf.set_len(len) };
reader.read_exact(buf.as_slice_mut())?;

This removes one full memory write pass per read.

Safety Considerations

The use of unsafe set_len is sound in this context because:

• the buffer is not accessed or observed between set_len and read_exact
• read_exact either fully initializes the buffer or returns an error
• on error, the function returns immediately and the buffer is not used
• there is no exposure of partially initialized data to safe Rust code

Crash scenarios (e.g., panic, early return, process termination) do not introduce unsoundness because:

• if execution stops before read_exact completes, the buffer is never observed
• there is no path where uninitialized memory is read after interruption

This invariant is local and can be maintained by ensuring no intermediate access is introduced between set_len and the successful completion of read_exact.

cc: @alamb