Behavior of copy and ARGS

[danielschlegel]

Does existing documentation exist?

https://docs.earthbuild.dev/docs/earthfile#copy

Request

Earthbuild docs as far as i understand them say that COPY from a target is behaving like BUILD. But this is not the case if it comes to ARG's. COPY does automatically pass args and BUILD does not.

What is not clear from the docs:

• BUILD needs --pass-args to populate ARG to child target but COPY does automatically do that
• We can call a parent target and pass ARG's which are only defined in child target even without passing them to COPY
• If we call earth cli with ARG which is not defined in parent target the ARG is still available in child target but not in parent(which is correct but not clear).

Earthfile

VERSION 0.8

FROM alpine:latest

deps:
  ARG MY_ARG=default
  RUN echo "${MY_ARG}" > /my_arg.txt
  RUN echo "MY_ARG from deps '$(cat /my_arg.txt)'" 
  SAVE ARTIFACT /my_arg.txt
  
build:
  BUILD +deps
  RUN echo "Build arg '${MY_ARG}'"

copy:
  COPY +deps/my_arg.txt ./my_arg.txt
  RUN echo "Build arg '${MY_ARG}'"
  RUN echo "Copied arg from deps '$(cat /my_arg.txt)'"

Different outputs with and without passing ARG.

earth --no-cache +build
 Init 🚀
————————————————————————————————————————————————————————————————————————————————
...
+deps | MY_ARG from deps 'default'
+build | Build arg ''
...
========================== 🌍 earth Build  ✅ SUCCESS ==========================

earth --no-cache +build-pass-arg
 Init 🚀
————————————————————————————————————————————————————————————————————————————————
...
+deps | MY_ARG from deps 'passed_arg'
+build-pass-arg | Build arg ''
...
========================== 🌍 earth Build  ✅ SUCCESS ==========================

earth --no-cache +copy
 Init 🚀
————————————————————————————————————————————————————————————————————————————————
...
+deps | MY_ARG from deps 'default'
+copy | Copied arg from deps 'default'
+copy | Build arg ''
...
========================== 🌍 earth Build  ✅ SUCCESS ==========================

earth --no-cache +build --MY_ARG=hugo
 Init 🚀
————————————————————————————————————————————————————————————————————————————————
...
+deps | MY_ARG from deps 'hugo' 
+build | Build arg ''

========================== 🌍 earth Build  ✅ SUCCESS ==========================

earth --no-cache +build-pass-arg --MY_ARG=hugo
 Init 🚀
————————————————————————————————————————————————————————————————————————————————
...
+deps | MY_ARG from deps 'passed_arg'
+build-pass-arg | Build arg ''
...
========================== 🌍 earth Build  ✅ SUCCESS ==========================

earth --no-cache +copy --MY_ARG=hugo
 Init 🚀
————————————————————————————————————————————————————————————————————————————————
...
+deps | MY_ARG from deps 'hugo'
+copy | Build arg ''
+copy | Copied arg from deps 'hugo'
...
========================== 🌍 earth Build  ✅ SUCCESS ==========================

[kmannislands]

I agree that this is counter-intuitive. I think that we should make COPY require explicit arguments.

This will however be quite a subtle breaking change so we should probably ship this behind a flag like VERSION 0.8 --explicit-copy and plan a breaking change to make this the default in the next minor version, v0.9.x