[wip] capture output from run to use elsewhere

[coryb]

This is my current thinking, adding a capture option::run builtin to collect the stdout from the call to allow aliasing. In this example, we have capture as now where I capture the output from the date command. Then it is used later in an echo command.

fs _runDate() {
	image "busybox"
	run "date" with option {
		ignoreCache
		capture as now
	}
}

fs default() {
	image "busybox"
	run "echo" now with ignoreCache
}

The concept seems to at least work:

#1 compiling [default]
#1 ...

#2 docker-image://docker.io/library/busybox:latest
#2 resolve docker.io/library/busybox:latest 2.0s done
#2 DONE 2.0s

#2 docker-image://docker.io/library/busybox:latest
#2 CACHED

#3 /bin/sh -c date
#3 0.103 Mon May  4 07:22:31 UTC 2020
#3 DONE 0.1s

#1 compiling [default]
#1 DONE 2.3s

#2 docker-image://docker.io/library/busybox:latest
#2 resolve docker.io/library/busybox:latest 1.9s done
#2 DONE 1.9s

#2 docker-image://docker.io/library/busybox:latest
#2 CACHED

#4 echo 'Mon May  4 07:22:31 UTC 2020'
#4 0.101 Mon May  4 07:22:31 UTC 2020
#4 0.101
#4 DONE 0.1s

Looking for feedback, there is probably a more elegant way to do this, or some complications I have missing.

[coryb]

1. Previously we defined aliases as inheriting the parent function's signature. How do we know that capture has an alias type of string? I think we need a design issue that answers those type of questions, otherwise the language won't have consistency.

Yeah, this is a general problem with aliases. I agree it is very unclear that aliasing an option::run might return either an fs (ie alias mount) or string (ie alias capture). I don't really have any ideas on how to resolve this.

2. This captures vertex logs and I think we should understand what the caveats that come with this. There are probably differences between the actual stdout from the process and the vertex logs we get as a client.

I was able to resolve this by recording the vertex digest that the capture statement was applied to. Now when running:

fs _runDate() {
	image "busybox"
	run "echo '==> DO NOT CAPTURE <=='"
	run "date" with option {
		ignoreCache
		capture as now
	}
}

we no longer capture the output from the previous echo statement.