Remove use of `tempname()` from `setindex!` of an `AttributeDict`

[nhz2]

Currently setindex! uses tempname() to get a random unique name for a temporary attribute. However, tempname() is quite slow and requires reading the file system to check if the names it generates are unique. In this PR I am replacing tempname() with "-><7;*<_NY" * bytes2hex(rand(Random.RandomDevice(), UInt8, 16))

julia> randname1() = "-><7;*<_NY" * bytes2hex(rand(Random.RandomDevice(), UInt8, 16))
randname1 (generic function with 1 method)

julia> randname2() = tempname()
randname2 (generic function with 1 method)

julia> @btime randname1()
  256.557 ns (4 allocations: 264 bytes)
"-><7;*<_NY2390f39a585aec3b0caac0f127a0a83a"

julia> @btime randname2()
  2.085 μs (9 allocations: 1.22 KiB)
"/tmp/jl_yjyjMTO1uh"

[mkitti]

Do we actually need a random name?

[mkitti]

Could we use some predictable sequence of names or perhaps some variant of the old the name?

[nhz2]

In terms of needing the temperary name, from what I can tell the HDF5 C library doesn't have a way to change the datatype, dataspace, and value of an attribute at the same time. h5py is doing something similar https://github.com/h5py/h5py/blob/fcaca1d1b81d25c0d83b11d5bdf497469b5980e9/h5py/_hl/attrs.py#L191-L219 using a uuid4. I think the API.h5a_write can change the value of an attribute but cannot change the type and space, so maybe there should be a check to see if the type and space need to be changed and if not just update the value?

[mkitti]

uuid4 or other uuids seem like a good idea. Why don't we do that? Is that slow?

Also, why not make the name related to the old name in case there is an error and we never complete the operation?

julia> using UUIDs

julia> name = "helloworld"
"helloworld"

julia> _name = name * "_hdf5jl_" * string(uuid4())
"helloworld_hdf5jl_452f6a32-c1db-4756-bbfa-42b2fd7d0a29"

[mkitti]

On my rather slow computer:

julia> randname1() = "-><7;*<_NY" * bytes2hex(rand(Random.RandomDevice(), UInt8, 16))
randname1 (generic function with 1 method)

julia> randname2() = tempname()
randname2 (generic function with 1 method)

julia> randname3(name) = name * "_hdf5jl_" * string(uuid4())
randname3 (generic function with 1 method)

julia> @btime randname1()
  631.651 ns (4 allocations: 264 bytes)
"-><7;*<_NY3ac27142c20bbb1c3129af9503747718"

julia> @btime randname2()
  6.007 μs (9 allocations: 1.22 KiB)
"/tmp/jl_jEhEJkVQre"

julia> @btime randname3($name)
  608.423 ns (3 allocations: 192 bytes)
"helloworld_hdf5jl_7ca25e59-4c2e-406e-90e0-80332425c05e"

[mkitti]

@simonbyrne want to take a quick look?