Writing a String always prints a warning

[johnomotani]

Describe the bug

If a String variable is written using NCDatasets, a warning is printed.

The warning makes sense as String is a variable-length data type, but as there does not seem to be anything to do about it, it is unhelpful.

The warning comes from DiskArrays.jl, but posting here as I'm guessing that the solution may be some sort of special handling for String here. Maybe a specialised version of setindex_disk!(), like

function setindex_disk!(a::NCDatasets.Variable{String, 0, NCDataset{Nothing, Missing}}, v::AbstractArray, i...)
    setindex_disk_nobatch!(a,v,i)
    v   
end

?

To Reproduce

using NCDatasets
f = NCDataset("test.nc", "c")
v = defVar(f, "foo", String, ())
v[] = "bar"

prints

┌ Warning: Can not determine size of element type. Using DiskArrays.fallback_element_size[] = 100 bytes
└ @ DiskArrays ~/.julia/packages/DiskArrays/ny95C/src/chunks.jl:354
┌ Warning: Can not determine size of element type. Using DiskArrays.fallback_element_size[] = 100 bytes
└ @ DiskArrays ~/.julia/packages/DiskArrays/ny95C/src/chunks.jl:354 

Expected behavior

The string is written with no warning.

Environment

• operating system: Linux Mint 21.1
• Julia version: julia 1.10.6, official binaries from https://julialang.org/downloads/
• Output of the julia command versioninfo()

  Julia Version 1.10.6
  Commit 67dffc4a8ae (2024-10-28 12:23 UTC)
  Build Info:
    Official https://julialang.org/ release
  Platform Info:
    OS: Linux (x86_64-linux-gnu)
    CPU: 16 × 11th Gen Intel(R) Core(TM) i7-11800H @ 2.30GHz
    WORD_SIZE: 64
    LIBM: libopenlibm
    LLVM: libLLVM-15.0.7 (ORCJIT, tigerlake)
  Threads: 1 default, 0 interactive, 1 GC (on 16 virtual cores)
  

• NCDatasets version: v0.14.6
• Output of using Pkg; Pkg.status(mode=PKGMODE_MANIFEST)

Status `~/physics/moment_kinetics-master-clean/Manifest.toml`
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⌅ [3da002f7] ColorTypes v0.11.5
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Info Packages marked with ⌃ and ⌅ have new versions available. Those with ⌃ may be upgradable, but those with ⌅ are restricted by compatibility constraints from upgrading. To see why use `status --outdated -m`

Full output

In case of an error, please paste the full error message and stack trace.

[Alexander-Barth]

Thank you for your complete issue report! I agree that the warning is annoying (it should be displayed only once with this commit JuliaIO/DiskArrays.jl@324867c ).

But is the DiskArrays.setindex_disk! function not part of the private API of DiskArrays ?

Maybe a change in DiskArrays would be more appropriate. For example, if the user sets the fallback_element_size via Preferences.jl then DiskArrays would consider that as a signal that the users knows they are doing? This would "solve" the issue not only for NCDatasets but for all users of DiskArrays. A user could still override per-script DiskArrays.fallback_element_size[].

What you do think Fabian @meggart ?