Cannot open .realm file on exFAT and FAT32 drives

[gangsports]

Goals

Open a saved .realm file on an exFAT drive in Realm Studio

Expected Results

The file is opened as expected

Actual Results

Realm Studio reports an error

fcntl() inside prealloc() failed allocating 1184 bytes, new_size=1184, cur_size=0, allocated_size=0, event: Operation not supported

Steps to Reproduce

1. Create a sample CSV file has the following content and save as sample.csv on an APFS drive (In my case, it was my Macintosh HD on my MBP)

T1,T2,T3
one,two,three

2. Use Realm Studio (Version 3.11.0 (3.11.0.719)) > Create Realm from > CSV and select 'sample.csv' and save as sample.realm

3. Open sample.realm on APFS and it works as expected

4. Move sample.realm to an external exFAT drive and use Realm Studio to open it again

5. Ream Studio reports the following error

fcntl() inside prealloc() failed allocating 1184 bytes, new_size=1184, cur_size=0, allocated_size=0, event: Operation not supported

Code Sample

Above replication does not require a sample code, but the same issue happens to our OS X application too when it tries to read a saved Realm file on exFAT or FAT32 drives.

Version of Realm and Tooling

Realm framework version: 4.4.1

Realm Object Server version: ?

Xcode version: 11.3.1

iOS/OSX version: ?

Dependency manager + version: ?

[tgoyne]

@ironage This sounds like File::prealloc() is failing to fall back to manual prealloc when the filesystem doesn't support it. I assigned you since I think you were the one to write that bit of code, but just let me know if you won't be able to get this.

[gangsports]

Thanks for checking this!
When we say filesystem doesn't support it, did the it refer to File::prealloc() or RealmCocoa?

Appreciate your time.

[tgoyne]

Prealloc. We use a APFS feature to expand the Realm file when needed, and just aren't properly falling back to the non-APFS case.

[gangsports]

Thanks for your reply. So, is RealmCocoa supposed to support non-APFS? For example, exFAT?
This is quite important for us to understand the limitation of our current project.

Really looking forward to your reply, and thanks.

[tgoyne]

It is supposed to work and the fact that it doesn't in a bug which should be simple to fix.

[gangsports]

This is awesome!
I will forward the information back to my team. Looking forward to the fix!

Thank you very much for the quick and helpful replies!