FinderSync Extension runtime error: The file couldn’t be opened because you don’t have permission to view it

Question

I'm writing a FinderSync extension that places an item in the context menu. When I right-click, I want the title of the menu item to change based on the selected file's contents. However, when I try to read the contents using Data(contentsOf: selectedUrl), it throws the following error:

Error Domain=NSCocoaErrorDomain Code=257 "The file “Some File.png” couldn’t be opened because you don’t have permission to view it." UserInfo={NSFilePath=/Users/Shared/MySyncExtension Documents/Some File.png, NSUnderlyingError=0x61800004cf00 {Error Domain=NSPOSIXErrorDomain Code=1 "Operation not permitted"}}

This does not change if I add or remove sandboxing.

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Prior Research

• The file couldn’t be opened because you don’t have permission to view it
  • The compiler has always been set to the default
• Read and Write access for FinderSync extension in a sandboxed environment
  • The entitlement is enabled. Also, this manifests with sandboxing turned off.
• "File could not open because you do not have permission to view it." | Official Apple Support Communities
  • The file's permissions say "everyone" has "Read only" access

Answer 1

The issue is that the Sandbox is preventing file access to the file, and thus your app cannot open it.

Read and Write access for FinderSync extension in a sandboxed environment

Even though the Finder Sync App Extension is granted "User Selected File" Sandbox File Access, the files accessed by the user via right-click seemingly do not count as being "user-selected".

It may be possible to guide the user to select a root directory via an NSOpenPanel via Powerbox, after which all subdirectories can be accessed via a security-scoped bookmark.

Using a Temporary Exception entitlement is also a possibility, but will cause App Review problems:

Is there any way to give my sandboxed Mac app read only access to files in ~/Library?

Please duplicate my Apple bug report requesting this behavior be allowed:

Finder Sync App Extension selectedItemURLs() should receive "User Selected File" Sandbox file access.

rdar://42874694
https://openradar.appspot.com/radar?id=5063363058991104