Feature: Immutable Buffer `buffer.readonly()`

[mikeal]

Is your feature request related to a problem? Please describe.

I have an object with a buffer attached and I need to be able to pass around that Buffer but ensure that it won’t mutate.

Describe the solution you'd like

A method on Buffer instances to put it into a “read only mode” would be ideal. I’d prefer it not be in the constructor so that I don’t have to perform a memcopy in order to get it.

Describe alternatives you've considered

There isn’t much you can do except force a full copy every time you pass it around, which is pretty bad. Object.freeze() won’t work because all the mutations happen through methods that are effectively invisible to Object.freeze().

However, Object.freeze() has some negative performance implications while implementing this in the Buffer object itself would not have the same problems. This would be a nice features of Node.js Core.

[addaleax]

I think this would have to be a language feature, because the only difference between Buffer and Uint8Array is that the former has a few more prototype methods.

[addaleax]

/cc @nodejs/open-standards (I guess?)

[jasnell]

This would definitely be interesting but I agree we might want to push to tc39 first

[benjamingr]

You can also define a proxy over the buffer though that would probably be expensive since it'd be hit on every access. I wonder if a proxy with only a set trap (and not a get trap) would be slow or not.

[devsnek]

you'd also want to intercept .buffer I assume

[addaleax]

Yeah, that’s a good point to figure out when bringing this to TC39 – would we want read-only ArrayBufferViews, or read-only ArrayBuffers, or both?

[tniessen]

I think read-only ArrayBuffers would be a useful feature to represent memory that should not or cannot be written to. However, if we still need to write to the memory through a different object, we might need two ArrayBuffers, which would technically act like a View I guess?

[ljharb]

Borrowing ArrayBuffer.prototype methods and .calling them on a Proxy would throw, though, wouldn’t it? (since internal slots don’t tunnel)

[littledan]

This is an interesting problem, and I agree that it would probably be best solved at the engine level and across Javascript as a whole.

Is the idea for the API that you would start with a mutable buffer and at some point freeze it?

Do you want a read-only view on something which may still change out from under you? If so, a Proxy-based solution may work, if the performance can be good enough. Otherwise, we need to freeze the underlying ArrayBuffer somehow.

For context, would it be reasonable to copy the Buffer into an immutable structure, or would that be too slow for your needs?

[jasnell]

Copying would certainly be too slow, at least for the cases where I think this would be useful. A read only view on an otherwise mutable structure would work for most cases I could imagine but @mikeal may have other requirements. The most common case I would imagine for this is some bit of code that creates/mutates the buffer then passes it off to some other context, after which it would no longer be modified.

[tniessen]

I agree with @jasnell. Would it be possible to allow freezing both buffers and views? For example,

• ArrayBufferView.freeze() marks the view as "frozen", preventing all future write accesses through the view.
• ArrayBuffer.freeze() marks the buffer as "frozen", preventing all future write accesses to the buffer, including write accesses through views.

Would this work and cover all use cases? It would certainly cause some overhead, but still much less than copying the data into a new buffer.

[mikeal]

A read-only view would be fine, as long as it didn't cost much and looked to the reader like any other binary type.

…

On Sat, Apr 6, 2019, 3:13 PM Tobias Nießen ***@***.***> wrote: I agree with @jasnell <https://github.com/jasnell>. Would it be possible to allow freezing both buffers and views? For example, - ArrayBufferView.freeze() marks the view as "frozen", preventing all future write accesses through the view. - ArrayBuffer.freeze() marks the buffer as "frozen", preventing all future write accesses to the buffer, including write accesses through views. Would this work and cover all use cases? It would certainly cause some overhead, but still much less than copying the data into a new buffer. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#27080 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAACQ_-dPvJR3z1Ke1yys7ZlZn2s8tFLks5veRwagaJpZM4cbpZV> .