Create new blosc2_schunk_decompress_chunk_ctx()

[FrancescAlted]

The existing blosc2_schunk_decompress_chunk() function uses the internal context associated with the superchunk for decompressing the specified chunk. However, reusing the same context is intrinsically thread unsafe.

It would be nice to add another blosc2_schunk_decompress_chunk_ctx() function where the user can pass a different context per thread, so that it can be thread safe.

[cf-natali]

Just to be clear, this means that it's not possible to use blosc concurrently from multiple threads.
I'm not talking about the threads managed by blosc itself, but by user threads.

For example if one creates a b2nd_array_t there is no way to concurrently perform read-only operations on it (e.g. slicing without holding a lock?

[FrancescAlted]

Well, there are two different kinds of functions. The ones that create a new b2nd_array_t (e.g. b2nd_get_slice), they accept a context as param, and should be thread-safe. However, the ones that do not create a new array (e.g. b2nd_get_slice_cbuffer) they should be not. So in case you want thread-safety, you can use the former.

[cf-natali]

Hm, reading the code I can't convince myself that event b2nd_get_slice is thread-safe:
Under the hood it calls b2nd_get_slice_cbuffer with the source array, which in turn calls get_set_slice, which in turns calls blosc2_compress_ctx on the source array, with the source context.

Generally I think that any function which uses the source array context is not thread-safe, and should actually use a user-provided context?

[FrancescAlted]

Ah, that maybe. Would you be up to send a PR with a _ctx counterpart of the functions you need?