Matthew Kenigsberg cole-h already got a tentative response:

We're tentatively looking at setting the GID of our group (nixbld) to 331 and the UID of our first build user (_nixbld1) to 331. Default installs create 32 build users, but Nix users can customize this at install time. Since Nix needs one build user for every concurrent build, we're trying to balance between the risk of low UIDs clashing with new macOS users in future updates, and still starting low enough that Nix users with Max/Ultra CPUs configure enough users to put that hardware to good use :)

We had a bit of a discussion, and this is probably the most expedient but the number being tossed around was 350.

I’m getting access to the bug again to see if anyone had any better advice.

Matthew Kenigsberg cole-h already got a tentative response:

We're tentatively looking at setting the GID of our group (nixbld) to 331 and the UID of our first build user (_nixbld1) to 331. Default installs create 32 build users, but Nix users can customize this at install time. Since Nix needs one build user for every concurrent build, we're trying to balance between the risk of low UIDs clashing with new macOS users in future updates, and still starting low enough that Nix users with Max/Ultra CPUs configure enough users to put that hardware to good use :)

We had a bit of a discussion, and this is probably the most expedient but the number being tossed around was 350.

I’m getting access to the bug again to see if anyone had any better advice.

Matthew Kenigsberg cole-h already got a tentative response:

Abathur: We're tentatively looking at setting the GID of our group (nixbld) to 331 and the UID of our first build user (_nixbld1) to 331. Default installs create 32 build users, but Nix users can customize this at install time. Since Nix needs one build user for every concurrent build, we're trying to balance between the risk of low UIDs clashing with new macOS users in future updates, and still starting low enough that Nix users with Max/Ultra CPUs configure enough users to put that hardware to good use :)

Devrel:
We had a bit of a discussion, and this is probably the most expedient but the number being tossed around was 350.

I’m getting access to the bug again to see if anyone had any better advice.

Matthew Kenigsberg cole-h already got a tentative response:

**Abathur: **
We're tentatively looking at setting the GID of our group (nixbld) to 331 and the UID of our first build user (_nixbld1) to 331. Default installs create 32 build users, but Nix users can customize this at install time. Since Nix needs one build user for every concurrent build, we're trying to balance between the risk of low UIDs clashing with new macOS users in future updates, and still starting low enough that Nix users with Max/Ultra CPUs configure enough users to put that hardware to good use :)

Devrel:
We had a bit of a discussion, and this is probably the most expedient but the number being tossed around was 350.

I’m getting access to the bug again to see if anyone had any better advice.

Matthew Kenigsberg cole-h already got a tentative response:

Abathur:

We're tentatively looking at setting the GID of our group (nixbld) to 331 and the UID of our first build user (_nixbld1) to 331. Default installs create 32 build users, but Nix users can customize this at install time. Since Nix needs one build user for every concurrent build, we're trying to balance between the risk of low UIDs clashing with new macOS users in future updates, and still starting low enough that Nix users with Max/Ultra CPUs configure enough users to put that hardware to good use :)

Devrel:
We had a bit of a discussion, and this is probably the most expedient but the number being tossed around was 350.

I’m getting access to the bug again to see if anyone had any better advice.

Matthew Kenigsberg cole-h already got a tentative response:

Abathur:

We're tentatively looking at setting the GID of our group (nixbld) to 331 and the UID of our first build user (_nixbld1) to 331. Default installs create 32 build users, but Nix users can customize this at install time. Since Nix needs one build user for every concurrent build, we're trying to balance between the risk of low UIDs clashing with new macOS users in future updates, and still starting low enough that Nix users with Max/Ultra CPUs configure enough users to put that hardware to good use :)

Devrel:
We had a bit of a discussion, and this is probably the most expedient but the number being tossed around was 350.

I’m getting access to the bug again to see if anyone had any better advice.

Matthew Kenigsberg cole-h already got a tentative response:

Abathur:
We're tentatively looking at setting the GID of our group (nixbld) to 331 and the UID of our first build user (_nixbld1) to 331. Default installs create 32 build users, but Nix users can customize this at install time. Since Nix needs one build user for every concurrent build, we're trying to balance between the risk of low UIDs clashing with new macOS users in future updates, and still starting low enough that Nix users with Max/Ultra CPUs configure enough users to put that hardware to good use :)

Devrel:
We had a bit of a discussion, and this is probably the most expedient but the number being tossed around was 350.

I’m getting access to the bug again to see if anyone had any better advice.

cole-hMatthew Kenigsberg after y'all had to drop @tomberek had one more thought that may be worth weighing WRT migration

The most-immediate cause of the _nixbldN errors may be the nixbld group holding on to GroupMembership entries for the deleted users.

I think he ran something like dscl . delete /Groups/nixbld GroupMembership _nixbld1 _nixbld2 _nixbld3 _nixbld4 to clean them up, which should mean the daemon won't try assigning builds to the clobbered users.

I'm still chewing on whether that's useful...

• It does give us something like a one-liner to get affected Nix installs working (as long as they had at least 5 build users)
• If something was built into Nix-nix that could fully fix up the users, the above oneliner could be chained with something to update Nix
• But without that feature, I do worry a little that publicizing it as a workaround may also lead people to avoid dealing with the UIDs of the remaining users and leave themselves open to a repeat of the same problem on near-future macOS updates?

I also looked at the 2nd install experience and (with the asterisk that this was on the pre-catalina macbook) after installing 2.18.5 once, both the 2.18.5 and 2.24.2 installers failed before the user stage on the check for existing shell profile backups.

That has me leaning towards not shimming the migration script into the installer, or at least saving that for a followup. Even if I shimmed it in before that check, I don't think it sends the right impression if an install that ultimately fails early does indeed still mutate these users (whether or not we're telling those users to fix it that way or not).