fix(storage): optimize gRPC writer with zero-copy and lazy allocation

[cpriti-os]

This change eliminates unnecessary double-buffering in the gRPC writer,
reducing memory usage by up to 50% for Parallel Composite Uploads (PCU)
and large single-shot uploads.

Key changes:

• Implements a Zero-Copy path in gRPCWriter: If the input slice is
  large enough (>= chunk size) or if in one-shot mode, the writer now
  passes the user's memory directly to the gRPC transport layer instead
  of copying it into an internal buffer.
• Implements Lazy Allocation: The internal buffer (w.buf) is now
  initialized to nil and is only allocated if the "Slow Path" (buffering)
  is triggered. This allows purely zero-copy workloads to run with
  near-zero overhead.
• Adds chunkSize to gRPCWriter to support logic when w.buf is nil.
• Adds comprehensive tests verifying memory addresses to ensure the
  optimization is active and strictly correct during retries.

[gemini-code-assist]

Summary of Changes

Hello @cpriti-os, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly optimizes the gRPC writer by eliminating unnecessary double-buffering, which can reduce memory usage by up to 50% for large file uploads and parallel composite uploads. The core improvements involve introducing a zero-copy path for efficient data transfer and implementing lazy allocation for the internal write buffer, ensuring that memory is only used when strictly necessary.

Highlights

• Zero-Copy Path: Implemented a zero-copy mechanism in gRPCWriter that directly passes user memory to the gRPC transport layer for large inputs (>= chunk size) or one-shot uploads, avoiding internal buffer copies.
• Lazy Buffer Allocation: The internal buffer (w.buf) is now initialized to nil and only allocated if buffering is required, reducing overhead for purely zero-copy workloads.
• chunkSize Field: Added chunkSize to the gRPCWriter struct to support the new zero-copy and lazy allocation logic.
• Comprehensive Testing: New tests were added to verify the zero-copy optimization, including scenarios for one-shot uploads, buffering fallback, and correct memory address handling during retries.

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[gemini-code-assist]

Code Review

This pull request introduces a significant optimization to the gRPC writer by implementing a zero-copy path and lazy allocation of the internal buffer. The changes are well-structured, and the logic for handling zero-copy writes, including partial sends and retries, appears robust. The addition of comprehensive tests that verify memory addresses is excellent and provides strong confidence in the correctness of the optimization. I've found one potential issue regarding integer overflow on 32-bit systems, for which I've left a specific comment. Overall, this is a great improvement for memory efficiency.

[tritone]

Great optimization!

Looks like there are some failing emulator tests involving progress func callbacks, race detector issues, and a deadlock. LMK if you have any questions about any of these. Probably you'll need to work on thread safety between the buffer alloc and the pipelined sends.

In addition we'd probably want to think about how to message this -- does this change how the writer "should" be used for gRPC? Do we want to suggest buffer pooling in the user application?

Finally, it'd be useful to get a pprof CPU/memory profile for before/after on this.