buffer: avoid unnecessary copy in Buffer.concat for single FastBuffer

[mertcanaltin]

If there's only one FastBuffer and its length matches the total length, we don't need to manually allocate and copy — we can just return Buffer.from(buf) to keep the semantics (new buffer) but avoid extra work.

my local result:

                                                                            confidence improvement accuracy (*)   (**)  (***)
buffers/buffer-concat-fill.js n=800000 extraSize=1                                         -0.37 %       ±0.78% ±1.04% ±1.36%
buffers/buffer-concat-fill.js n=800000 extraSize=1024                                       0.22 %       ±1.08% ±1.44% ±1.89%
buffers/buffer-concat-fill.js n=800000 extraSize=256                                       -0.88 %       ±1.40% ±1.87% ±2.45%
buffers/buffer-concat.js n=800000 withTotalLength=0 pieceSize=1 pieces=16                  -0.30 %       ±0.52% ±0.69% ±0.89%
buffers/buffer-concat.js n=800000 withTotalLength=0 pieceSize=1 pieces=4                    0.11 %       ±0.66% ±0.88% ±1.14%
buffers/buffer-concat.js n=800000 withTotalLength=0 pieceSize=16 pieces=16                 -0.18 %       ±1.44% ±1.92% ±2.50%
buffers/buffer-concat.js n=800000 withTotalLength=0 pieceSize=16 pieces=4                  -0.30 %       ±0.67% ±0.89% ±1.16%
buffers/buffer-concat.js n=800000 withTotalLength=0 pieceSize=256 pieces=16                -0.38 %       ±0.54% ±0.72% ±0.94%
buffers/buffer-concat.js n=800000 withTotalLength=0 pieceSize=256 pieces=4                 -0.44 %       ±0.65% ±0.87% ±1.13%
buffers/buffer-concat.js n=800000 withTotalLength=1 pieceSize=1 pieces=16           **      1.02 %       ±0.62% ±0.82% ±1.07%
buffers/buffer-concat.js n=800000 withTotalLength=1 pieceSize=1 pieces=4             *     -0.83 %       ±0.69% ±0.92% ±1.19%
buffers/buffer-concat.js n=800000 withTotalLength=1 pieceSize=16 pieces=16                  0.08 %       ±1.18% ±1.58% ±2.07%
buffers/buffer-concat.js n=800000 withTotalLength=1 pieceSize=16 pieces=4                  -0.25 %       ±0.67% ±0.90% ±1.17%
buffers/buffer-concat.js n=800000 withTotalLength=1 pieceSize=256 pieces=16                -0.19 %       ±0.56% ±0.74% ±0.97%
buffers/buffer-concat.js n=800000 withTotalLength=1 pieceSize=256 pieces=4                 -0.25 %       ±0.56% ±0.75% ±0.97%

Be aware that when doing many comparisons the risk of a false-positive result increases.
In this case, there are 15 comparisons, you can thus expect the following amount of false-positive results:
  0.75 false positives, when considering a   5% risk acceptance (*, **, ***),
  0.15 false positives, when considering a   1% risk acceptance (**, ***),
  0.01 false positives, when considering a 0.1% risk acceptance (***)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.18%. Comparing base (264cad7) to head (62de85d).
Report is 25 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #58266   +/-   ##
=======================================
  Coverage   90.18%   90.18%           
=======================================
  Files         629      629           
  Lines      186657   186640   -17     
  Branches    36658    36669   +11     
=======================================
- Hits       168328   168324    -4     
- Misses      11126    11128    +2     
+ Partials     7203     7188   -15     

Files with missing lines	Coverage Δ
lib/buffer.js	100.00% <100.00%> (ø)

... and 38 files with indirect coverage changes

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[jasnell]

If list[0] is a TypedArray (like another Buffer) then this still copies.

[mertcanaltin]

I’m wondering if it makes sense to return list[0] directly. I understand this avoids copying, so I guess I should update the test accordingly?
https://github.com/nodejs/node/blob/main/test/parallel/test-buffer-concat.js#L43

[legendecas]

Buffer.concat should always return a new buffer object:

node/doc/api/buffer.md

Lines 1033 to 1034 in 91d2400

	Returns a new `Buffer` which is the result of concatenating all the `Buffer`
	instances in the `list` together.

[mertcanaltin]

As I understand it, at the end of the day we should always create a buffer.

[mertcanaltin]

I will try an optimization in the inner regions, I am now closing this pr, thank you very much for your comments