multi-user-installer: restore copy-to-store idempotence?

[abathur]

Before #5150, this section of the multi-user installer used rsync and was (IIRC) ~idempotent:

https://github.com/NixOS/nix/blob/7e301fd74e929af916116a04d84e130f9e0112de/scripts/install-multi-user.sh#L759-L768

Dropping the rsync dependency to support debian makes sense, but it would be nice to figure out whether we can portably make this step idempotent again--and do it, if it's simple enough.

Shooting from the hip:

• this might be as simple as using cp's -f flag, if it's portable?
• it's probably not as simple as just ignoring the exit code from cp, since there could be other issues
• it's probably not as simple as just diffing the directories, since an existing store will contain more than the store seed

(If we can't do this without, say, wiping the store or restoring the rsync dependency, we should probably figure out how to fail faster until we can find another approach.)

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/small-installer-idempotence-fix/19791/1

[brejoc]

Due to this change I ran into a problem relate to this on macOS today. Because of an update the changes in /etc/zshrc got wiped. In matrix someone suggested to just reinstall. I know that running the installer again worked in the past. This time it failed with an cp: cannot overwrite directory error.

~~> Installing Nix

---- sudo execution ------------------------------------------------------------
I am executing:

    $ sudo cp -RPp ./store/0ky7rhm0ckf4vw7vg2v5cyarrbsblvmj-aws-crt-cpp-0.17.28 ./store/1dcbaii28xnv98zw2w5f0skn2mar1ck8-aws-c-auth-0.6.13 ./store/2033pdgsv4wgl0zx6nng9i8q5f4h18lk-libidn2-2.3.2 ./store/2im4pw4pl5zsr6mjhwrjfcinrl9qljhy-nss-cacert-3.80 ./store/3b8r3yspv7f7p92jfxg548cfxi7q19d7-bash-5.1-p16 ./store/3k2ygb7mk9ylm2r5g0iad7akg1qwas4g-libssh2-1.10.0 ./store/3qcn93slgnm0wz74fq3nsd2942k3vsfm-aws-c-cal-0.5.17 ./store/58ppyba9pbc8yxqiw11gv9aw2adh1lgg-libxml2-2.9.14 ./store/6d17z21w7h8vmpsnmr8dm73sb3mvkpp0-nlohmann_json-3.10.5 ./store/789cl5k5ca0m9sxkiqyl4ackv5f0cmfz-aws-sdk-cpp-1.9.238 ./store/79z2m5vwbqwzrmfsbbv6rvk04z2nzf0x-curl-7.83.1 ./store/7hn69mlypdg285ar3ia71prgcrsx521g-libiconv-50 ./store/7m37v9mnii266p2l21xl97s4666f7d2s-libarchive-3.6.1-lib ./store/7vfyla60szbi8h35q93mrsij6whb3q9q-aws-c-compression-0.2.14 ./store/8yr5vy7jasrizvnvm9d12iynbbvf8r3p-nghttp2-1.47.0-lib ./store/962rka435k9c9p5ng1z2y3gspi4gs7hr-libcxxabi-11.1.0 ./store/9h5pks3l96gkp7653x4jxsfz1ca41fdc-aws-c-io-0.11.0 ./store/9jzvhm113w1xndcc5p4lvbqpqvk01c17-aws-c-mqtt-0.7.10 ./store/9x2z66kf8g8p9y1bxkraspghsy8jb5nj-zstd-1.5.2 ./store/a08j7vsgfcvzx0ahl986v38mjrx3gkm0-brotli-1.0.9-lib ./store/blnw8pnh6srwwr31g23mh6b70g6viaam-libcxx-11.1.0 ./store/cfc8xcp3q1ccx9wrfkbagia95w6rsvsh-apple-framework-Security-11.0.0 ./store/cpl9nndqjgn238p3kdwws845gscavfh4-sqlite-3.38.5 ./store/czwp4nmcn5skjiylhbyhg5k21wv76w9j-libsodium-1.0.18 ./store/d1hxdg09lxql958cmnrjq68alnd73i39-aws-c-s3-0.1.39 ./store/dg4r67wwna6sb2drh7gj38byq91ix2z9-aws-c-common-0.7.0 ./store/dmk5m3nlqp1awaqrp1f06qhhkh3l102n-nix-2.11.1 ./store/gwxhwcf0m9yrhz5wql1181bjfp654m1b-zlib-1.2.12 ./store/h9z5lncphgm9if86wxrfqg7w7fv7khbh-libkrb5-1.19.3 ./store/jgwmpgiin89dp0jjpjlwily6fgj1xs1a-aws-c-sdkutils-0.1.2 ./store/kd48xgjihd6sz5gvf2bvj7kw7m7ybhmf-xz-5.2.5 ./store/n2bygl2nqg7r35mg4ny45cwkarxgx2xb-curl-7.83.1 ./store/nf20p297fz6sfs97m7rl7al9ywchi83n-openssl-1.1.1q ./store/p7755cvgiwvv6lvin1r25jy711p8fn2n-libunistring-1.0 ./store/r6y0rvzx8z9z459ghmj1zrnpi4r7xa5v-aws-checksums-0.1.12 ./store/s5ikc3jlv67j9bm8463rjz3x09lcwm8v-aws-c-http-0.6.15 ./store/v2p3injpwnyf31vfllpz2k172mr58fn9-apple-lib-libDER ./store/vzbnm6afkdcbgb57npl06gk7rsap4sfy-apple-framework-CoreFoundation-11.0.0 ./store/w18g47jdi000hq064cb83j42kmbc4k3s-editline-1.17.1 ./store/w5n081lpdyhc5aa69cq59j9s1h626jgj-apple-framework-IOKit-11.0.0 ./store/xi7bwrkki4fwr6zam7gmcklnqq3f3d5h-aws-c-event-stream-0.2.7 ./store/y9j96x0zkxwwbqknr66r7bwczlxa4kk6-bzip2-1.0.6.0.2 ./store/y9rzzima6wdy0ph29ff09hr8vadfkx16-libobjc-11.0.0 ./store/z1l79b1phxy2yzcwsvn0ar121gcnh0x6-boehm-gc-8.0.6 /nix/store/

to copy the basic Nix files to the new store at /nix/store

cp: cannot overwrite directory /nix/store/cfc8xcp3q1ccx9wrfkbagia95w6rsvsh-apple-framework-Security-11.0.0/Library/Frameworks/Security.framework/Versions/Current with non-directory ./store/cfc8xcp3q1ccx9wrfkbagia95w6rsvsh-apple-framework-Security-11.0.0/Library/Frameworks/Security.framework/Versions/Current
cp: cannot overwrite directory /nix/store/cfc8xcp3q1ccx9wrfkbagia95w6rsvsh-apple-framework-Security-11.0.0/Library/Frameworks/Security.framework/Headers with non-directory ./store/cfc8xcp3q1ccx9wrfkbagia95w6rsvsh-apple-framework-Security-11.0.0/Library/Frameworks/Security.framework/Headers
cp: cannot overwrite directory /nix/store/cfc8xcp3q1ccx9wrfkbagia95w6rsvsh-apple-framework-Security-11.0.0/Library/Frameworks/Security.framework/Modules with non-directory ./store/cfc8xcp3q1ccx9wrfkbagia95w6rsvsh-apple-framework-Security-11.0.0/Library/Frameworks/Security.framework/Modules
cp: cannot overwrite directory /nix/store/h9z5lncphgm9if86wxrfqg7w7fv7khbh-libkrb5-1.19.3/sbin with non-directory ./store/h9z5lncphgm9if86wxrfqg7w7fv7khbh-libkrb5-1.19.3/sbin
cp: cannot overwrite directory /nix/store/vzbnm6afkdcbgb57npl06gk7rsap4sfy-apple-framework-CoreFoundation-11.0.0/Library/Frameworks/CoreFoundation.framework/Versions/Current with non-directory ./store/vzbnm6afkdcbgb57npl06gk7rsap4sfy-apple-framework-CoreFoundation-11.0.0/Library/Frameworks/CoreFoundation.framework/Versions/Current
cp: cannot overwrite directory /nix/store/vzbnm6afkdcbgb57npl06gk7rsap4sfy-apple-framework-CoreFoundation-11.0.0/Library/Frameworks/CoreFoundation.framework/Headers with non-directory ./store/vzbnm6afkdcbgb57npl06gk7rsap4sfy-apple-framework-CoreFoundation-11.0.0/Library/Frameworks/CoreFoundation.framework/Headers
cp: cannot overwrite directory /nix/store/vzbnm6afkdcbgb57npl06gk7rsap4sfy-apple-framework-CoreFoundation-11.0.0/Library/Frameworks/CoreFoundation.framework/Modules with non-directory ./store/vzbnm6afkdcbgb57npl06gk7rsap4sfy-apple-framework-CoreFoundation-11.0.0/Library/Frameworks/CoreFoundation.framework/Modules
cp: cannot overwrite directory /nix/store/w5n081lpdyhc5aa69cq59j9s1h626jgj-apple-framework-IOKit-11.0.0/Library/Frameworks/IOKit.framework/Versions/Current with non-directory ./store/w5n081lpdyhc5aa69cq59j9s1h626jgj-apple-framework-IOKit-11.0.0/Library/Frameworks/IOKit.framework/Versions/Current
cp: cannot overwrite directory /nix/store/w5n081lpdyhc5aa69cq59j9s1h626jgj-apple-framework-IOKit-11.0.0/Library/Frameworks/IOKit.framework/Headers with non-directory ./store/w5n081lpdyhc5aa69cq59j9s1h626jgj-apple-framework-IOKit-11.0.0/Library/Frameworks/IOKit.framework/Headers
cp: cannot overwrite directory /nix/store/w5n081lpdyhc5aa69cq59j9s1h626jgj-apple-framework-IOKit-11.0.0/Library/Frameworks/IOKit.framework/Modules with non-directory ./store/w5n081lpdyhc5aa69cq59j9s1h626jgj-apple-framework-IOKit-11.0.0/Library/Frameworks/IOKit.framework/Modules

---- oh no! --------------------------------------------------------------------
Jeeze, something went wrong. If you can take all the output and open
an issue, we'd love to fix the problem so nobody else has this issue.

:(

PS: For my original problem it's of course it's much easier to add the three lines to /etc/zshrc again.

[abathur]

Yes, restoring the hook is the right approach for now.

This cp case is a regression, but IIRC it isn't the only idempotence problem in the installer (you'd still run into issues with nix already being in /etc/bashrc and the presence of the plist for the nix-daemon LaunchDaemon, I think).

Until it can be made idempotent, it's best to follow the uninstall instructions before reinstalling it: https://nixos.org/manual/nix/stable/installation/installing-binary.html#macos

[brejoc]

@abathur You are right, there are more idempotency problems. But those can quit easily be circumnavigated. This one not.

Thanks for the link to the uninstall instructions. That's quite handy!

[iFreilicht]

7603 should fix this.