2017-10-1 Photos app stuck at 'Updating Library 100% Complete' for going on 5 hours, after High Sierra upgrade. Option to Quit app is greyed out. Cannot use or access any app user interface or features. Just stuck on the last slide of the 'Have Fun With Photos' tour. The easy and the established approach to viewing all your photos again in Photo Library is to employ a professional Photo repair software that is supported in Mac. Alternatively, you can also try Apple’s inbuilt Photo Library Repair tool but that may not fix your severely corrupt photos.
Follow these steps to use the Photos library repair tool:
You might be asked to type in your user account password to authorize the library repair.
The repair tool analyzes the the library's database and repairs any inconsistencies it detects. Depending on the size of your library, this might take some time. When the process is completed, Photos opens the library.
If you repair a library that's automatically updated with iCloud Photos, the entire contents of the library re-updates with iCloud after the repair process completes.