Eliminate a redundant alloc+copy of each frame

[cgutman]

Description

The streaming code is copying the entire frame into a new std::vector (using the slow std::back_inserter() too) just to prepend the frame header, right before it copies it a second time to insert the packet headers. We can easily consolidate these operations into one to prevent a useless allocation and copy of every frame we encode.

This isn't the only redundant copy left (fec::encode() does one), but it's the only one that's easy to get rid of. Removing more copies requires scatter/gather I/O support in platf::send()/platf::send_batch() to allow us to submit separate buffers for the header and payload data of each packet. It's not that hard to support S/G, but it's tricky to actually use due to the myriad frame/packet headers (some of which participate in FEC, while others don't).

I also included another change to optimize the copy in fec::encode(). It turns out GCC was generating horrible assembly that was doing a byte-by-byte copy of each frame rather than using the optimized __builtin_memmove() as was the case for the std::copy() in concat_and_insert().

Screenshot

Issues Fixed or Closed

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Dependency update (updates to dependencies)
• Documentation update (changes to documentation)
• Repository update (changes to repository files, e.g. .github/...)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the in code docstring/documentation-blocks for new or existing methods/components

Branch Updates

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must be updated before it can be merged. You must also
Allow edits from maintainers.

• I want maintainers to keep my branch updated

[codecov]

Codecov Report

Attention: Patch coverage is 76.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 9.24%. Comparing base (18e7dfb) to head (fe75e01).

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #2852      +/-   ##
=========================================
+ Coverage    9.12%   9.24%   +0.12%     
=========================================
  Files          97      97              
  Lines       17433   17433              
  Branches     8314    8314              
=========================================
+ Hits         1590    1611      +21     
+ Misses      13021   12936      -85     
- Partials     2822    2886      +64     

Flag	Coverage Δ
Linux	6.99% <60.00%> (+0.15%)	⬆️
Windows	4.53% <76.00%> (+0.18%)	⬆️
macOS-12	10.32% <44.00%> (+0.14%)	⬆️
macOS-13	10.23% <44.00%> (+0.15%)	⬆️
macOS-14	10.55% <44.00%> (+0.15%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/stream.cpp	3.42% <76.00%> (+2.31%)	⬆️

... and 25 files with indirect coverage changes