I've gotten to the point where I can use the mount(8) command line tool and the -t option to mount a file system using my FSKit file system extension, in which case I can see a process for my extension launch, probe, and perform the other necessary actions.
However, when plugging in my USB flash drive or trying to mount with diskutil mount, the file system does not mount:
$ diskutil mount disk20s3
Volume on disk20s3 failed to mount
If you think the volume is supported but damaged, try the "readOnly" option
$ diskutil mount readOnly disk20s3
Volume on disk20s3 failed to mount
If you think the volume is supported but damaged, try the "readOnly" option
Initially I thought it would be enough to just implement probeExtension(resource:replyHandler:) and the system would handle the rest, but this doesn't seem to be the case. Even a trivial implementation that always returns .usable doesn't cause the system to use my FSModule, even though I've enabled my extension in System Settings > General > Login Items & Extensions > File System Extensions.
From looking at some of the open source msdos and Disk Arb code, it seems like my app extension needs to list FSMediaTypes to probe. I eventually tried putting this in my Info.plist of the app extension:
<key>FSMediaTypes</key>
<dict>
<key>EBD0A0A2-B9E5-4433-87C0-68B6B72699C7</key>
<dict>
<key>FSMediaProperties</key>
<dict>
<key>Content Hint</key>
<string>EBD0A0A2-B9E5-4433-87C0-68B6B72699C7</string>
<key>Leaf</key>
<true/>
</dict>
</dict>
<key>0FC63DAF-8483-4772-8E79-3D69D8477DE4</key>
<dict>
<key>FSMediaProperties</key>
<dict>
<key>Content Hint</key>
<string>0FC63DAF-8483-4772-8E79-3D69D8477DE4</string>
<key>Leaf</key>
<true/>
</dict>
</dict>
<key>Whole</key>
<dict>
<key>FSMediaProperties</key>
<dict>
<key>Leaf</key>
<true/>
<key>Whole</key>
<true/>
</dict>
</dict>
<key>ext4</key>
<dict>
<key>FSMediaProperties</key>
<dict>
<key>Content Hint</key>
<string>ext4</string>
<key>Leaf</key>
<true/>
</dict>
</dict>
</dict>
</plist>
(For reference, the partition represented by disk20s3 has a Content Hint of 0FC63DAF-8483-4772-8E79-3D69D8477DE4 and Leaf is True which I verified using IORegistryExplorer.app from the Xcode additional tools.)
Looking in Console it does appear now that the system is trying to use my module (ExtendFS_fskit) to probe when I plug in my USB drive, but I never see a process for my extension actually launch when trying to attach to it from Xcode by name (unlike when I use mount(8), where I can do this). However I do see a Can't find the extension for <private> error which I'm not sure is related but does sound like the system can't find the extension for some reason.
The below messages are when filtering for "FSKit":
default 19:14:53.455826-0400 diskarbitrationd probed disk, id = /dev/disk20s3, with ExtendFS_fskit, ongoing.
default 19:14:53.456038-0400 fskitd Incomming connection, entitled 1
default 19:14:53.456064-0400 fskitd [0x7d4172e40] activating connection: mach=false listener=false peer=true name=com.apple.filesystems.fskitd.peer[350].0x7d4172e40
default 19:14:53.456123-0400 fskitd Hello FSClient! entitlement yes
default 19:14:53.455902-0400 diskarbitrationd [0x7461d8dc0] activating connection: mach=true listener=false peer=false name=com.apple.filesystems.fskitd
default 19:14:53.456151-0400 diskarbitrationd Setting remote protocol to all XPC
default 19:14:53.456398-0400 fskitd About to get current agent for 501
default 19:14:53.457185-0400 diskarbitrationd probed disk, id = /dev/disk20s3, with ExtendFS_fskit, failure.
error 19:14:53.456963-0400 fskitd -[fskitdXPCServer applyResource:targetBundle:instanceID:initiatorAuditToken:authorizingAuditToken:isProbe:usingBlock:]: Can't find the extension for <private>
(I only see these messages after plugging my USB drive in. When running diskutil mount, I see no messages in the console when filtering by FSKit, diskarbitrationd, or ExtendFS afterward. It just fails.)
Is there a step I'm missing to get this to work, or would this be an FSKit bug/current limitation?
