Support parallel BUILD within a FOR loop

[alexcb]

No description provided.

[alexcb]

One thing that's off with this PR is the final output shows some duplicate BUILD +test-executor output at the end of the build. Here's a complete example:

consider the Earthfile:

VERSION 0.8

FROM earthly/dind:alpine

# this sequentially
test-parallel-for:
     BUILD +parallel

parallel:
    FOR num IN $(seq 5)
        BUILD +test-executor --PROJECT=$num
    END

test-executor:
    ARG PROJECT

    WITH DOCKER
        RUN --no-cache for i in 1 2 3 4; do echo "i=$i PROJECT=$PROJECT" && sleep 1; done; echo bye
    END

example run:

$ ./build/linux/amd64/earthly -P ~/test/with-docker-parallel+test-parallel-for 
 Init 🚀
————————————————————————————————————————————————————————————————————————————————

           buildkitd | Found buildkit daemon as docker container (earthly-dev-buildkitd)

Streaming logs to https://cloud.earthly.dev/builds/b8f686f5-2b5f-4321-a962-ed3fa813fd5d

 Build 🔧
————————————————————————————————————————————————————————————————————————————————

              logbus | Setting organization "" and project ""
/h/a/t/with-docker-parallel+test-parallel-for | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true
/h/a/t/with-docker-parallel+test-parallel-for | --> FROM +base
 earthly/dind:alpine | --> Load metadata earthly/dind:alpine linux/amd64
/h/a/t/with-docker-parallel+test-parallel-for | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true
/h/a/t/with-docker-parallel+test-parallel-for | --> BUILD +parallel
/h/a/t/with-docker-parallel+parallel | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true
/h/a/t/with-docker-parallel+parallel | --> FROM +base
/h/a/t/with-docker-parallel+base | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true
/h/a/t/with-docker-parallel+base | --> FROM earthly/dind:alpine
/h/a/t/with-docker-parallel+base | [----------] 100% FROM earthly/dind:alpine
/h/a/t/with-docker-parallel+parallel | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true
/h/a/t/with-docker-parallel+parallel | *cached* --> mkdir /run/buildargs
/h/a/t/with-docker-parallel+parallel | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true
/h/a/t/with-docker-parallel+parallel | *cached* --> FOR $(seq 5)
/h/a/t/with-docker-parallel+test-executor | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true PROJECT=1
/h/a/t/with-docker-parallel+test-executor | --> FROM +base
/h/a/t/with-docker-parallel+parallel | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true
/h/a/t/with-docker-parallel+parallel | --> BUILD +test-executor
/h/a/t/with-docker-parallel+test-executor | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true PROJECT=3
/h/a/t/with-docker-parallel+test-executor | --> FROM +base
/h/a/t/with-docker-parallel+test-executor | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true PROJECT=2
/h/a/t/with-docker-parallel+test-executor | --> FROM +base
/h/a/t/with-docker-parallel+test-executor | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true PROJECT=4
/h/a/t/with-docker-parallel+test-executor | --> FROM +base
/h/a/t/with-docker-parallel+test-executor | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true PROJECT=5
/h/a/t/with-docker-parallel+test-executor | --> FROM +base
/h/a/t/with-docker-parallel+test-executor | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true PROJECT=1
/h/a/t/with-docker-parallel+test-executor | --> WITH DOCKER RUN
/h/a/t/with-docker-parallel+test-executor | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true PROJECT=4
/h/a/t/with-docker-parallel+test-executor | --> WITH DOCKER RUN
/h/a/t/with-docker-parallel+test-executor | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true PROJECT=2
/h/a/t/with-docker-parallel+test-executor | --> WITH DOCKER RUN
/h/a/t/with-docker-parallel+test-executor | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true PROJECT=3
/h/a/t/with-docker-parallel+test-executor | --> WITH DOCKER RUN
/h/a/t/with-docker-parallel+test-executor | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true PROJECT=5
/h/a/t/with-docker-parallel+test-executor | --> WITH DOCKER RUN
/h/a/t/with-docker-parallel+test-executor | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true PROJECT=1
/h/a/t/with-docker-parallel+test-executor | *cached* --> WITH DOCKER (install deps)
/h/a/t/with-docker-parallel+test-executor | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true PROJECT=2
/h/a/t/with-docker-parallel+test-executor | --> WITH DOCKER RUN --privileged --no-cache for i in 1 2 3 4; do echo "i=$i PROJECT=$PROJECT" && sleep 1; done; echo bye
/h/a/t/with-docker-parallel+test-executor | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true PROJECT=1
/h/a/t/with-docker-parallel+test-executor | --> WITH DOCKER RUN --privileged --no-cache for i in 1 2 3 4; do echo "i=$i PROJECT=$PROJECT" && sleep 1; done; echo bye
/h/a/t/with-docker-parallel+test-executor | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true PROJECT=4
/h/a/t/with-docker-parallel+test-executor | --> WITH DOCKER RUN --privileged --no-cache for i in 1 2 3 4; do echo "i=$i PROJECT=$PROJECT" && sleep 1; done; echo bye
/h/a/t/with-docker-parallel+test-executor | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true PROJECT=5
/h/a/t/with-docker-parallel+test-executor | --> WITH DOCKER RUN --privileged --no-cache for i in 1 2 3 4; do echo "i=$i PROJECT=$PROJECT" && sleep 1; done; echo bye
/h/a/t/with-docker-parallel+test-executor | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true PROJECT=3
/h/a/t/with-docker-parallel+test-executor | --> WITH DOCKER RUN --privileged --no-cache for i in 1 2 3 4; do echo "i=$i PROJECT=$PROJECT" && sleep 1; done; echo bye
/h/a/t/with-docker-parallel+test-executor | Starting dockerd with data root /var/earthly/dind/71f91fed70b5fcc883572f7a61778b00c4ea0cb762f6fd2eefaf6e311b855616/tmp.BeNehE
/h/a/t/with-docker-parallel+test-executor | Starting dockerd with data root /var/earthly/dind/00186b06f9bab75d529909a03ad99a1a343aa955300fb563ea3919b283121dbc/tmp.nomckE
/h/a/t/with-docker-parallel+test-executor | Starting dockerd with data root /var/earthly/dind/e17b17d9bce2adbb5776039189bcfb0ec34a7d98d5bffbe4fa7ba1f32b0f5fe7/tmp.mGFhnE
/h/a/t/with-docker-parallel+test-executor | Starting dockerd with data root /var/earthly/dind/f778f165beb24e8f619f110cee7ce43ed9560292abd77f93f96d8539590a605a/tmp.MlajBF
/h/a/t/with-docker-parallel+test-executor | Starting dockerd with data root /var/earthly/dind/2f06b21ab2493864900561183fa9f0eaaf20a9bc8df777d7ca9d5eaa7932e3f8/tmp.iNFIKF
/h/a/t/with-docker-parallel+test-executor | i=1 PROJECT=4
/h/a/t/with-docker-parallel+test-executor | i=1 PROJECT=3
/h/a/t/with-docker-parallel+test-executor | i=1 PROJECT=1
/h/a/t/with-docker-parallel+test-executor | i=1 PROJECT=2
/h/a/t/with-docker-parallel+test-executor | i=1 PROJECT=5
/h/a/t/with-docker-parallel+test-executor | i=2 PROJECT=4
/h/a/t/with-docker-parallel+test-executor | i=2 PROJECT=3
/h/a/t/with-docker-parallel+test-executor | i=2 PROJECT=1
/h/a/t/with-docker-parallel+test-executor | i=2 PROJECT=2
/h/a/t/with-docker-parallel+test-executor | i=2 PROJECT=5
/h/a/t/with-docker-parallel+test-executor | i=3 PROJECT=4
/h/a/t/with-docker-parallel+test-executor | i=3 PROJECT=3
/h/a/t/with-docker-parallel+test-executor | i=3 PROJECT=1
/h/a/t/with-docker-parallel+test-executor | i=3 PROJECT=2
/h/a/t/with-docker-parallel+test-executor | i=3 PROJECT=5
/h/a/t/with-docker-parallel+test-executor | i=4 PROJECT=4
/h/a/t/with-docker-parallel+test-executor | i=4 PROJECT=3
/h/a/t/with-docker-parallel+test-executor | i=4 PROJECT=1
/h/a/t/with-docker-parallel+test-executor | i=4 PROJECT=2
/h/a/t/with-docker-parallel+test-executor | i=4 PROJECT=5
/h/a/t/with-docker-parallel+test-executor | bye
/h/a/t/with-docker-parallel+test-executor | bye
/h/a/t/with-docker-parallel+test-executor | bye
/h/a/t/with-docker-parallel+test-executor | bye
/h/a/t/with-docker-parallel+test-executor | bye
/h/a/t/with-docker-parallel+parallel | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true
/h/a/t/with-docker-parallel+parallel | --> BUILD +test-executor
/h/a/t/with-docker-parallel+parallel | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true
/h/a/t/with-docker-parallel+parallel | --> BUILD +test-executor
/h/a/t/with-docker-parallel+parallel | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true
/h/a/t/with-docker-parallel+parallel | --> BUILD +test-executor
/h/a/t/with-docker-parallel+parallel | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true
/h/a/t/with-docker-parallel+parallel | --> BUILD +test-executor

What's with the final

/h/a/t/with-docker-parallel+parallel | DOCKERHUB_MIRROR_AUTH_FROM_CLOUD_SECRETS=true
/h/a/t/with-docker-parallel+parallel | --> BUILD +test-executor

that's repeated 5 times -- I suspect it's related to the way the for _, async, in range []{true, false} causes a duplicate build.