this is now working, so I would appreciate some feedback on the design.

The basic design is the same as what I outlined earlier in this PR: there are two new functions that take a dict[path, GroupMetadata | ArrayMetadata] like {'a': GroupMetadata(zarr_format=3), 'a/b': ArrayMetadata(...)} and concurrently persist those metadata documents to storage, resulting in a hierarchy on disk that looks like the dict.

approach

basically the same as concurrent group members listing, except we don't need any recursion. I'm scheduling writes and using as_completed to yield Arrays / Groups when they are available.

new functions

• create_nodes is low-level and doesn't do any checking of its input, so it will happily create invalid hierarchies, e.g. nesting groups inside arrays, or mixing v2 and v3 metadata, and it won't create intermediate groups, either.

• create_hierarchy is higher level, it parses the input, checking it for invalid hierarchies, and inserting implicit groups as needed.

• Group.create_hierarchy is a new method on the Group / AsyncGroup classes that takes a hierarchy dict and creates the nodes specified in that dict at locations relative to the path of the group instance. the return value is dict[str, AsyncGroup | AsyncArray], but I guess it also doesn't have tor return anything, or it could be an async iterator, so that you can interact with the nodes as they are formed. This is flexible right now, but I think the iterator idea is nice.

• _from_flat (names welcome) is a new function that creates a group entirely from a hierarchy dict + a store. that dict must specify a root group, otherwise an exception is raised. We could revise this to create a root group if one is not specified. Open to suggestions here.

Implicit groups

Partial hierarchies like {'a': GroupMetadata(), 'a/b/c': ArrayMetadata(...)} implicitly denote a group at a/b. When creating such a hierarchy, if we find an existing group at a/b, then we don't need to create a new one. So in the context of modeling a hierarchy, implicit groups are a little special -- by not specifying the properties of the group, the user / application is tolerant of any group being there. So I introduced a subclass of GroupMetadata called _ImplicitGroupMetadata, which can be inserted into a hierarchy dict to explicitly denote groups that don't need to be written if one already exists. _ImplicitGroupMetadata is just like GroupMetadata except it errors if you try to set any parameter except zarr_format.

streaming v2 vs v3 node creation

creating v3 arrays / groups requires writing 1 metadata document, but v2 requires 2. To get the most concurrency I await the write of each metadata document separately, which means that foo/.zattrs might resolve before foo/.zarray. So in the v2 case I only yield up an array / group when both documents were written.

Overlap with metadata consolidation logic

there's a lot of similarity between the stuff in this PR and routines used for consolidated metadata. it would be great to find ways to factor out some of the overlap areas

still to do:

• write some more tests (checking that implicit groups don't get written if a group already exists)
• handle overwriting. I think the plan here is, if overwrite is False, then we do a check before any writing to ensure that there are no conflicts between the proposed hierarchy and the stuff that actually exists in storage. this check will involve more IO.

IMO they should be relative to the path of the group

That sounds fine as it's clear that the group.create_hierarchy API is already looking at one specific group.

• removed the path and allow_root keyword arguments to create_hierarchy. the keys are now paths relative to the root of the store. for the Group.create_hierarchy method, the keys are relative to the path to the group, which ensures that only sub-arrays and sub-groups of that group can be created.
• created a new core/sync_group.py module, which contains the synchronous wrappers around async core/group.py functions. this allows us to have two functions named create_nodes, where one is async, and the other is sync, but neither is public. We will eventually want a more structured layout but that's for a later PR.
• usage examples added to docstrings for create_hierarchy
• examples added to docs, in both the quickstart and group pages

in the interest of a narrow scope, I've limited the public api to just create_hierarchy.

test failure is unrelated to this PR (looks like an fsspec thing)