Apple - Mac Photos app stuck on "Restoring from iCloud"
Since I ran into the same issue, I had to research this as well. From what I found, this seems to be normal. After a library repair, each photo is being verified against the iCloud library copy.
This Apple support document 1 states:
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 ...
Just like a normal iCloud library upload, this can take a very long time. For my 20.000 photos, it's currently at 72 hours without an indication of being finished any time soon it took about 80 hours. Then, the Photos app showed Updating (only the word Updating without any additional information) for a few hours before now switching to Uploading 19,800 items.
It's re-uploading every single photo in my library. Due to my low upload bandwidth, this is a very slow process and it uploaded about 1,000 photos in the previous 4 days.
So for me the messages that showed up in the status bar were, in order:
Restoring from iCloud(for about 4 days)Updating(for a few hours)Uploading x items(very slow, will possibly take many weeks to finish)
UPDATE Jan/10: I took the laptop to work because we have a very fast upstream connection there. Over the weekend, it uploaded all photos, downloaded the new ones and is now back to its old Updated Just Now status.
My library is back to normal, but the question is still somewhat unanswered. After 12 days of "Restoring from iCloud..." I gave up hope and turned off iCloud Photos (both in the Preferences pane of Photos and in the system preferences under "Apps on this Mac using iCloud". One or the other told me that one unspecified item would be deleted, which in desperation I agreed to. Restarted and turned them both back on.
Photos thought for a while and then said "Uploading 11X,XXX photos". I left it open while it counted down agonizingly slowly for about a week. The first 50k took much longer than the rest, so maybe it does the bigger files (or more recent photos) first. When it finally got to zero, it downloaded the new photos that were missing, and everything has been working fine since.
If I get the "Restoring from iCloud..." message again, I'll probably skip right to turning iCloud off and back on, since it sounds like there's no way to avoid re-uploading every photo anyway. Of course if anyone has a better solution I would appreciate a real answer.
Update and warning:
When writing this answer I went back and turned off iCloud Photos in both places again, hoping to figure out which one triggered the re-upload. I was expecting it to give me a warning. THERE IS NO WARNING. Now my Photos app is back to "Uploading 11X,XXX" photos.