`take!` after `pushfirst!` reads garbage values

[nhz2]

Here is a MWE:

julia> b = pushfirst!([0x02], 0x01)
2-element Vector{UInt8}:
 0x01
 0x02

julia> take!(IOBuffer(b))
5-element Vector{UInt8}:
 0x01
 0x02
 0x69
 0x73
 0x69

julia> versioninfo()
Julia Version 1.12.0-DEV.347
Commit c0611e8c49c (2024-04-15 17:22 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 16 × AMD Ryzen 7 7800X3D 8-Core Processor
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, znver4)
Threads: 1 default, 0 interactive, 1 GC (on 16 virtual cores)

I think the issue is

julia/base/iobuffer.jl

Line 461 in d8b9810

data = copyto!(StringVector(io.size), 1, io.data, io.offset + 1, nbytes)

using StringVector(io.size) instead of StringVector(nbytes).

The pushfirst! causes the IOBuffer to have a nonzero offset field (added in #53192) which makes nbytes and io.size different.

[andrewjradcliffe]

Technically, it's not reading garbage values, but rather, nothing is written to the trailing io.size - nbytes values allocated by StringVector.

[KristofferC]

I made a PR to get the ball rolling (#54129).