calling `isopen` on a multi-file NCDataset returns `true` even after `close` is called

[fergu]

Describe the bug

If a multi-file dataset is opened, calling close(ds) and then querying the file using isopen(ds) returns true. Additionally, reads of the dataset still succeed even after calling close(ds). This may not be a big issue because (I think) the underlying NetCDF files aren't held open until they need to be accessed, but this is slightly unexpected behavior as compared to the single-file case.

To Reproduce

Please provide a minimal julia code example which reproduces the behavior (bug, performance regression, ...).

julia> using NCDatasets
julia> files = readdir("test/data/"; join=true)
2-element Vector{String}
 "test/data/test_001.nc"
 "test/data/test_002.nc"
julia> ds = NCDataset(files, "r"; aggdim="time")
julia> isopen(ds)
true
julia> close(ds)
julia> isopen(ds)
true
julia> ds["time"][:]
# Expected time vector is returned

As a note, test_xxx.nc are just test NC files that I've created for some unit tests in a package I'm writing. They were also created using NCDatasets.

Expected behavior

isopen(ds) should have returned false, and reads should not have succeeded, similar to the single-file case.

Environment

• operating system: [e.g. Ubuntu XX.YY]: Red Hat Enterprise Linux 8.10
• Julia version: [e.g. julia 1.1.1, official binaries from https://julialang.org/downloads/]: 1.12.5, via juliaup
• Output of the julia command versioninfo():

Julia Version 1.12.5
Commit 5fe89b8ddc1 (2026-02-09 16:05 UTC)
Build Info:
    Official https://julialang.org release
Platform Info:
    OS: Linux (x86_64-linux-gnu)
    CPU: 112 x Intel(R) Xeon(R) Platinum 8180 CPU @ 2.50GHz
    WORD_SIZE: 64
    LLVM: libLLVM-18.1.7 (ORCJIT, skylake-avx512)
    GC: Built with stock GC
Threads: 1 default, 1 interactive, 1 GC (on 112 virtual cores)

• NCDatasets version: v0.14.15
• Output of using Pkg; Pkg.status(mode=PKGMODE_MANIFEST)

Status `~/code/julia/workspaces/NCTest/Manifest.toml`
  [179af706] CFTime v0.2.10
  [1fbeeb36] CommonDataModel v0.4.3
  [187b0558] ConstructionBase v1.6.0
  [864edb3b] DataStructures v0.19.5
  [3c3547ce] DiskArrays v0.4.21
  [692b3bcd] JLLWrappers v1.8.0
  [8ac3fa9e] LRUCache v1.6.2
  [3da0fdf6] MPIPreferences v0.1.12
  [85f8d34a] NCDatasets v0.14.15
  [6fe1bfb0] OffsetArrays v1.17.0
⌅ [bac558e1] OrderedCollections v1.8.2
  [21216c6a] Preferences v1.5.2
  [10745b16] Statistics v1.11.1
  [0b7ba130] Blosc_jll v1.21.7+0
  [6e34b625] Bzip2_jll v1.0.9+0
  [0234f1f7] HDF5_jll v2.1.2+0
  [e33a78d0] Hwloc_jll v2.13.0+1
  [94ce4f54] Libiconv_jll v1.18.0+0
  [5ced341a] Lz4_jll v1.10.1+0
  [b5ada748] MPIABI_jll v0.1.5+0
  [7cb0a576] MPICH_jll v5.0.1+0
  [f1f71cc9] MPItrampoline_jll v5.5.6+0
  [9237b28f] MicrosoftMPI_jll v10.1.4+3
  [7243133f] NetCDF_jll v401.1000.0+0
  [fe0851c0] OpenMPI_jll v5.0.11+0
⌅ [02c8fc9c] XML2_jll v2.13.9+0
  [ffd25f8a] XZ_jll v5.8.3+0
  [a65dc6b1] Xorg_libpciaccess_jll v0.19.0+0
  [3161d3a3] Zstd_jll v1.5.7+1
⌅ [2b3700d1] aws_c_auth_jll v0.9.6+0
  [70f11efc] aws_c_cal_jll v0.9.13+0
  [73048d1d] aws_c_common_jll v0.12.6+0
  [73a04cd5] aws_c_compression_jll v0.3.2+0
  [3254fc65] aws_c_http_jll v0.10.13+0
  [13c41daa] aws_c_io_jll v0.26.3+0
⌅ [bd1f34fb] aws_c_s3_jll v0.11.5+0
  [1282aa60] aws_c_sdkutils_jll v0.2.4+1
  [b2a88e68] aws_checksums_jll v0.2.10+0
  [c4b69c83] dlfcn_win32_jll v1.4.2+0
  [477f73a3] libaec_jll v1.1.7+0
  [337d8026] libzip_jll v1.11.3+0
  [9aeb927a] mpif_jll v0.1.7+0
  [cddc5d3d] s2n_tls_jll v1.7.3+0
  [0dad84c5] ArgTools v1.1.2
  [56f22d72] Artifacts v1.11.0
  [2a0f44e3] Base64 v1.11.0
  [ade2ca70] Dates v1.11.0
  [f43a241f] Downloads v1.7.0
  [7b1f6079] FileWatching v1.11.0
  [ac6e5ff7] JuliaSyntaxHighlighting v1.12.0
  [4af54fe1] LazyArtifacts v1.11.0
  [b27032c2] LibCURL v0.6.4
  [76f85450] LibGit2 v1.11.0
  [8f399da3] Libdl v1.11.0
  [37e2e46d] LinearAlgebra v1.12.0
  [56ddb016] Logging v1.11.0
  [d6f4376e] Markdown v1.11.0
  [a63ad114] Mmap v1.11.0
  [ca575930] NetworkOptions v1.3.0
  [44cfe95a] Pkg v1.12.1
  [de0858da] Printf v1.11.0
  [9a3f8284] Random v1.11.0
  [ea8e919c] SHA v0.7.0
  [f489334b] StyledStrings v1.11.0
  [fa267f1f] TOML v1.0.3
  [a4e569a6] Tar v1.10.0
  [cf7118a7] UUIDs v1.11.0
  [4ec0a83e] Unicode v1.11.0
  [e66e0078] CompilerSupportLibraries_jll v1.3.0+1
  [deac9b47] LibCURL_jll v8.15.0+0
  [e37daf67] LibGit2_jll v1.9.0+0
  [29816b5a] LibSSH2_jll v1.11.3+1
  [14a3606d] MozillaCACerts_jll v2025.11.4
  [4536629a] OpenBLAS_jll v0.3.29+0
  [458c3c95] OpenSSL_jll v3.5.4+0
  [83775a58] Zlib_jll v1.3.1+2
  [8e850b90] libblastrampoline_jll v5.15.0+0
  [8e850ede] nghttp2_jll v1.64.0+1
  [3f19e933] p7zip_jll v17.7.0+0
Info Packages marked with ⌅ have new versions available but compatibility constraints restrict them from upgrading. To see why use `status --outdated -m`