[stream] BufferedSender

[clabby]

Overview

Adds a BufferedSender wrapper in stream that reduces memory fragmentation by re-using a buffer that grows to the size of the largest message sent.

send_frame (unbuffered)

BufferedSender

[cursor]

Bug: Reserve calculation incorrectly computes additional capacity needed

The reserve() call uses target_capacity - self.send_buf.capacity() but BytesMut::reserve(n) ensures capacity >= len + n. After clear(), len is 0, so reserve(n) guarantees capacity >= n. When capacity is between half the target and the target (e.g., capacity=150, target=200), calling reserve(50) only guarantees capacity ≥ 50, which is already satisfied, so nothing happens. The intended 2x buffer optimization fails to work, causing the same memory fragmentation the BufferedSender was designed to prevent.

 

[clabby]

This is incorrect; BytesMut::reserve documents:

Reserves capacity for at least additional more bytes to be inserted into the given BytesMut.

More than additional bytes may be reserved in order to avoid frequent reallocations. A call to reserve may result in an allocation.

[clabby]

#2518 potential alternative

[clabby]

It's worth approaching vectored writes instead of this IMO. Yields a performance benefit for sure, but is more of a band-aid when a larger API change is justified.

[codecov]

Codecov Report

❌ Patch coverage is 89.77273% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.50%. Comparing base (2be92d6) to head (008de79).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
stream/src/utils/codec.rs	89.41%	9 Missing ⚠️

@@            Coverage Diff             @@
##             main    #2512      +/-   ##
==========================================
- Coverage   92.51%   92.50%   -0.02%     
==========================================
  Files         340      340              
  Lines       97407    97485      +78     
==========================================
+ Hits        90114    90174      +60     
- Misses       7293     7311      +18     

Files with missing lines	Coverage Δ
stream/src/lib.rs	95.67% <100.00%> (-0.16%)	⬇️
stream/src/utils/codec.rs	95.69% <89.41%> (-4.31%)	⬇️

... and 3 files with indirect coverage changes

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