re-opening `ZipStore` with a new mode

[d-v-b]

In main it's not possible to make a read-only copy of a ZipStore instance. Is there any particular reason why this is not allowed, or is it something we could add? Naively, it seems like a read-only copy of any store class should always be safe to generate.

[TomAugspurger]

IIRC, ZipStore has some instance level state that makes it hard to re-open. The file needs to be closed and the ZIP footer written before anyone else can open the file (going off memory, so I might have some details wrong).

[d-v-b]

that means that ZipStore always errors if it reaches this conditional in make_store_path:

zarr-python/src/zarr/storage/common.py

Lines 224 to 227 in 4c3081c

	elif isinstance(store_like, Store):
	if mode is not None and mode != store_like.mode.str:
	store_like = store_like.with_mode(mode)
	await store_like._ensure_open()

If we want to keep these store semantics (I'm not sure that we do...) we should add a zipstore-specific exception here that instructs users how to open their zipstored data with a different mode. Otherwise, invoking something like open(store=store, path="doesnotexist", mode="r", zarr_format=zarr_format) raises NotImplementedError with no guidance.

[d-v-b]

would it be terrible if ZipStore.with_mode(self, mode) called self.close(), effectively invalidating the original store instance?

[TomAugspurger]

we should add a zipstore-specific exception here that instructs users how to open their zipstored data with a different mode

I wonder how you'd do that... Looking more closely, I see my earlier comment about ZipStore sharing some state wasn't 100% right. Really all we share is a Path, we don't try to share a particular instance of zipfile.ZipFile or any locks. We just need to ensure that there's a valid Zip file at that path before we try to open it, which leads to...

would it be terrible if ZipStore.with_mode(self, mode) called self.close(), effectively invalidating the original store instance?

Is it possible to have both, by having our write / read calls automatically call open if needed? If so, then maybe with_mode could call close (which writes the footer, making the ZipFile readable), without breaking the original store since any subsequent writes would automatically re-open the ZipFile.

I'm not sure what the downsides of this kind of just-in-time reopening would be.

[d-v-b]

upon calling _sync_open, ZipStore instances acquire a lock and a file handle:

zarr-python/src/zarr/storage/zip.py

Lines 86 to 93 in 4c3081c

	self._lock = threading.RLock()
	self._zf = zipfile.ZipFile(
	self.path,
	mode=self._zmode,
	compression=self.compression,
	allowZip64=self.allowZip64,
	)

, so I think it's about as stateful as you can get.