buffer: hard-deprecate SlowBuffer

[ChALkeR]

Checklist

• make -j4 test (UNIX), or vcbuild test nosign (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

buffer

Description of change

This emits a warning first time SlowBuffer instance is created and instructs the users to switch to Buffer.alloc() or Buffer.allocUnsafeSlow(), both of which are not pooled.

Refs: #8169
Refs: #7152

Not sure if we should do this in v7 or v8. It looks like SlowBuffer is used only by a small number of modules (I found 416, only 330 of which — outside of tests, and some of those are false positives). Module authors could use safe-buffer to polyfill for older Node.js versions, though.

Labelling as in progress because this lacks tests.

/cc @nodejs/ctc, @seishun

[seishun]

If we aren't hard-deprecating Buffer constructor in v7.0 due to v0.12 support, then the same reasoning should apply here. I don't think the number of users should be the deciding factor.

[ChALkeR]

@seishun That is true, but the difference here is that SlowBuffer was documented as being for internal use in v0.10: https://nodejs.org/docs/latest-v0.10.x/api/buffer.html#buffer_class_slowbuffer

In v0.12 it got properly documented, though: https://nodejs.org/docs/latest-v0.12.x/api/buffer.html#buffer_class_slowbuffer

[jasnell]

This would need a second argument set to 'DeprecationWarning'... that is:

process.emitWarning(
  'SlowBuffer is deprecated. ' +
  'Use `Buffer.alloc()` or `Buffer.allocUnsafeSlow()` instead.',
  'DeprecationWarning');

[jasnell]

this would also need a test case verifying that the deprecation warning was emitted.

[trevnorris]

Since SlowBuffer only ever accepted a number the polyfill may not be more complicated than

require('buffer').SlowBuffer = Buffer.unsafeAllocSlow;

[ChALkeR]

This should probably be paired with #7152, so targeting 8.0 likewise.

[ChALkeR]

This won't happen soon, afaik.
There is probably no reason to deprecate SlowBuffer without deprecating Buffer(arg).
It could be reopened or replaced with a more up-to-date one when the time is right.