Mounting FSKit with FSPathURLResource programatically in MacOS 26

I already successfully did it with the "mount" command and I can instantiate my file-system with FSPathURLResource.

What is your file system actually "doing"? What's the path target?

The issue here:

Also, I managed to mount the file-system with DiskArbitration in a SwiftUI app, but I only managed to get it working with FSBlockDeviceResource.

...is that block storage devices are the only reason DiskArbitration exists at all. More specifically, DiskArb exists because the expectation that disk will automatically mount when you plug them in is fairly complicated to implement, which then to its larger role of telling the rest of the system what was going to happen and letting them "vote" on that activity. None of that really applies to other file system types (for example, network file systems or disk images), which basically all work by saying "mount this target using this specific file system". Note, for example, that while DiskArb can "see" network volume mounts (meaning, they appear in its callbacks), it can't actually initiate them. The mount itself is either initiated through the NetFS.framework or one of the mount_* commands.

That leads to here:

Or is "mount" command currently the only option?

In practice, "/sbin/mount" is the system’s primary mounting interface. I think the "mount()" syscall is better understood as the SPI mount commands use to communicate with the kernel, NOT the API apps use to "mount volumes". Notably, the way diskarbitrationd ultimately mounts volumes is by using posix_spawn to run "/sbin/mount". There's nothing particularly "wrong" about doing the same thing by calling "/sbin/mount" yourself.

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Kevin Elliott
DTS Engineer, CoreOS/Hardware

Thanks for the comprehensive answer!

My file system is mounting a video container which contains RAW DNG images, so the volume is actually exposing these DNG files.

I'm glad to say that mounting with "mount" command also works with passed security scope, when enabling "FSRequiresSecurityScopedPathURLResources". I was constantly trying to pass in a security scope bookmark through my client app, but in the end this option did it for me.

If I may ask a follow up question, what do you think would be a preferred way to mount these files that my file system supports (they have ".mcraw" file extension) in Finder? Is having a client app that associates with this file extension okay? Which in turn calls the "mount" command internally.

But that would cause some problems with App Sandbox, since I don't think it's possible to call "mount" with Sandbox enabled. That I think is the current limitation.

Are there any plans to be able to do it with Sandbox in the future?

Thanks for the comprehensive answer! My file system is mounting a video container which contains RAW DNG images, so the volume is actually exposing these DNG files.

Cool! That's exactly the kind of thing FSPathURLResource is intended to support.

If I may ask a follow-up question, what do you think would be a preferred way to mount these files that my file system supports (they have ".mcraw" file extension) in Finder? Is having a client app that associates with this file extension okay?

Yes, that's exactly what I'd do. FYI, this is basically how DiskImage mounting works except that it's preparing the content for the KEXT when it publishes an artificial block device.

But that would cause some problems with App Sandbox, since I don't think it's possible to call "mount" with Sandbox enabled. That I think is the current limitation.

Please file a bug on this, then post the bug number back here. As part of that bug, please note how close you are to shipping and if you want to ship on the Mac App Store.

Are there any plans to be able to do it with Sandbox in the future?

As you might expect, I can't talk about our future plans. What I will say here is that part of FSKit's designs is that it not only replaces the VFS layer itself but that it also helps file systems "fit" comfortably into the larger system. As part of that integration, I do think it's important that we provide a mount API solution that works with systems like the App Sandbox.

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Kevin Elliott
DTS Engineer, CoreOS/Hardware