Make derivation build durations available

[lf-]

Is your feature request related to a problem? Please describe.
I am trying to determine whether my friend is having an exceptionally slow build of man-cache, but I can't get the data to compare, since Nix doesn't store the build times.

Describe the solution you'd like
I would like to be able to query the Nix database to find out how long some build of some derivation took, similar to how nix log can get the logs.

Describe alternatives you've considered
Run hydra locally? not practical.

Additional context

Priorities

Add 👍 to issues you find important.

[tomberek]

The Nix DB has registration time, though i don't believe it is used anywhere. While possible to put this into the DB, it does not seem to be the right place for it to go.

Workarounds

A bit hacky, but these have worked for me to discover the time.

function buildTime(){
    drvPath="$1"
    drv=${drvPath##"/nix/store/"}
    echo $(stat "/nix/var/log/nix/drvs/${drv:0:2}/${drv:2}.bz2" -t | awk '{print $14 - $15}' ) ; }

You can also ask Hydra with this:

curl --silent -H "Accept: application/json" https://hydra.nixos.org/build/400143 | jq '.stoptime - .starttime'

[lf-]

oh, is that how this works? interesting. we should add a command line thing to do this as well

[Kha]

I didn't know that either, would be an interesting option for https://github.com/Kha/nixprof