New `copystring` function about julia

[KristofferC]

copystring sounds like a string is being copied which is confusing.

[jakobnissen]

One solution could be to make take! and read on IOBuffer not use Base.StringVector, but allocate a normal vector. This will make using IOBuffer for creating strings less efficient, but that is addressed by the new takestring! function in #54372.

I'm willing to pitch a PR if that's an acceptable solution.

[nhz2]

Here is an example where using the String constructor on a Vector{UInt8} can result in mutating a String

io = IOBuffer()
write(io, [0x61,0x62,0x63])
seekstart(io)
a = read(io, 3)
b = reshape(a, 1, 3)
c = reshape(b, 3)
s = String(a)
@show s
c[1] = 0x62
@show s

This changes s from "abc" to "bbc"
This happens on Julia 1.6 as well as nightly.

[nhz2]

Maybe bytes2string is a better name?

[jariji]

Python calls this function bytes.decode.

bytes2string is clear, which is the most important factor imo.

2 names in Base: bytes2hex, deg2rad, hex2bytes, hex2bytes!, rad2deg.
to names in Base: htol, hton, ltoh, ntoh.

It should maybe take a Type{AbstractString} argument so it can support Strs.jl etc.

[nhz2]

There is already a transcode(::Type{String}, src) function.

julia/base/strings/cstring.jl

Line 179 in d01d256

transcode(::Type{String}, src) = String(transcode(UInt8, src))

Maybe this can be changed to avoid mutating the input.

[oscardssmith]

I think that is a good solution.

[KristofferC]

This will make using IOBuffer for creating strings less efficient,

That seems completely unacceptable? Isn't the whole reason not to make this slow pretty much the reason for the current behavior of String?

[oscardssmith]

@KristofferC the point is that with this change takestring!(iobuffer) will be as efficient as String(take!(iobuffer)) is now, while String(take!(iobuffer)) will have a copy. The reason for this change is that a single takestring! fully encapsulates the sketchy part where an immutable String is created from mutable data without a copy, while our current idiom for this exposes the mutable object to the user which creates the possibility of the user accidentally holding on to a reference of the mutable data.

With this change in combination with #54372, existing users will see a regression that is fixed by replacing instances of String(take!(iobuffer)) with takestring!(iobuffer) (we will likely make a Compat PR to make takestring! available in older julia versions).

[KristofferC]

he point is that with this change takestring!(iobuffer) will be as efficient as String(take!(iobuffer)) is now, while String(take!(iobuffer)) will have a copy.

The whole reason we didn't do this X years ago was that adding this type of perf regression to code run everywhere was not acceptable. I don't see why it would be now.

[KristofferC]

This is the comment I was thinking about: #24388 (comment)

[KristofferC]

Those who don't know history are destined to repeat it.

Reading through #24388, #25241, #25846, #26093 is probably a good idea for people interested in this to know a bit about the discussing and history that has lead to the current behavior.

[nhz2]

String(copy(v)) still makes two copies as noted in #26093 (comment) In ZipArchives.jl I would like to avoid the extra copy. This is currently possible with unsafe_string, so an extra function in Base is not needed, but IMO it would be nice to avoid having to deal with pointers.