I reported this as a bug (FB18614667), but also wanted to ask here in case this is actually just me doing something wrong, or maybe I'm misunderstanding the entire use case of metadataRead. (My understanding is that metadataRead is basically read but it checks a cache that the kernel manages before trying to read the physical resource, and in the case of a cache miss it would just go to the physical resource and then add the bytes to the cache. Is that right?)
I’m encountering an issue in an FSKit file system extension where (for example)
read(into: buf, startingAt: 0, length: Int(physicalBlockSize))
works, but
metadataRead(into: buf, startingAt: 0, length: Int(physicalBlockSize))
throws an EIO error (Input/output error) no matter what I do.
(Note: physicalBlockSize is 512 in this example.)
The documentation (https://developer.apple.com/documentation/fskit/fsblockdeviceresource/metadataread(into:startingat:length:)) indicates that the restrictions on metadataRead are that the operations must be sector-addressed (which is the case here, especially as regular read has the same restriction and succeeds) and that partial reading of metadata is not supported. (I don’t think that applies here?)
In a sample project I was able to replicate this behavior where the module only ever reads the block device in its enumerateDirectory implementation, and so trying to list the contents of a directory leads to an "Input/output error" when e.g. running ls on the volume.
The enumerateDirectory sample implementation is like so:
func enumerateDirectory(_ directory: FSItem, startingAt cookie: FSDirectoryCookie, verifier: FSDirectoryVerifier, attributes: FSItem.GetAttributesRequest?, packer: FSDirectoryEntryPacker) async throws -> FSDirectoryVerifier {
let buf = UnsafeMutableRawBufferPointer.allocate(byteCount: Int(blockDevice.physicalBlockSize), alignment: 1)
defer {
buf.deallocate()
}
// metadataRead will throw...
try blockDevice.metadataRead(into: buf, startingAt: 0, length: Int(blockDevice.physicalBlockSize))
// but read will work.
// try await blockDevice.read(into: buf, startingAt: 0, length: Int(blockDevice.physicalBlockSize))
// ... return dummy file here (won't reach this point because metadataRead throws)
}
I'm observing this behavior on both macOS 15.5 (24F74) and macOS 15.6 beta 3 (24G5074c).
Has anyone been able to get metadataRead to work? I see it used in Apple's msdos FSKit implementation so it seems like it has to work at some level.
