IOBuffer does not respect maxsize after the first take!

[Liozou]

The maxsize keyword argument of an IOBuffer is documented to limit its internal size. For example:

julia> io = IOBuffer(; maxsize=3)
IOBuffer(data=UInt8[...], readable=true, writable=true, seekable=true, append=false, size=0, maxsize=3, ptr=1, mark=-1)

julia> print(io, "foobar")  # attempt to write 6 characters

julia> io                   # `size` ends up being only 3
IOBuffer(data=UInt8[...], readable=true, writable=true, seekable=true, append=false, size=3, maxsize=3, ptr=4, mark=-1)

julia> take!(io)            # and indeed, only 3 characters were kept
3-element Vector{UInt8}:
 0x66
 0x6f
 0x6f

The issue is that, after performing this first take! operation, any subsequent write will not respect the maxsize argument anymore. Re-using the same io:

julia> io                   # just after `take!`, the state of `io` is reinitialized so its size is 0
IOBuffer(data=UInt8[...], readable=true, writable=true, seekable=true, append=false, size=0, maxsize=3, ptr=1, mark=-1)

julia> print(io, "And now, qux!")  # attempt to write 13 characters

julia> io                   # notice the inconsistency: size = 13, maxsize = 3
IOBuffer(data=UInt8[...], readable=true, writable=true, seekable=true, append=false, size=13, maxsize=3, ptr=14, mark=-1)

julia> String(take!(io))    # all 13 characters were kept
"And now, qux!"

This bug does not exist in v1.10.8 but it occurs in v1.11.0-alpha1 and is still present in 1.11.4, v1.12.0-DEV.2047 (49263b7) and on nightly (4a6ada6)

[Liozou]

I'm marking as a good first issue because I believe this should not be too hard to identify and fix.

If you're new at this: call @edit print(io, "foobar") (or @functionloc or @which) to identify the call chain until you find the bug. Fix it in your branch, don't forget to add a test, you should probably run the modified test file to check that the rest is still valid, and then submit a PR.

More detailed instructions on how to contribute to the julia repo are written in the CONTRIBUTING file: https://github.com/JuliaLang/julia/blob/master/CONTRIBUTING.md

Note: I'm suggesting to look into print although the bug could also be in take! but since maxsize in still correctly set after the take! call, I'm more inclined to believe the bug lies in print. I could be wrong though.