Hi, I have question regarding handling null values in preview. Code like this:

import * as docker from "@pulumi/docker";

const ubuntu = new docker.RemoteImage("ubuntu", {
    name: "ubuntu:precise"
});

const val = ubuntu.build.apply(_ => "ubuntu:precise")

const ubuntu2 = new docker.RemoteImage("ubuntu2", {
    name: val
});

works not as expected. Because build is null, during preview name of the second ubuntu will be always unknown. Are there any tickets regarding making

null

and

notyetcomputed

values different during preview? That code above ends with infinite cycle where there is one value to replace during preview that is untouched during actual run:

Enter your passphrase to unlock config/secrets
Previewing update (dev):
     Type                         Name                Plan        Info
     pulumi:pulumi:Stack          pulumi-ts-test-dev
 +-  └─ docker:index:RemoteImage  ubuntu2             replace     [diff: ~name]
Resources:
    +-1 to replace
    2 unchanged

Do you want to perform this update? yes
Updating (dev):
     Type                 Name                Status
     pulumi:pulumi:Stack  pulumi-ts-test-dev
Resources:
    3 unchanged

Duration: 2s

There seems to be a few misunderstandings about how to use the Docker provider in this code.

RemoteImage.build

is an input, it's used for passing command line parameters to the Docker CLI. Calling

apply

on it is using it as an output: you're looking up what values were passed to the CLI back when it was called. You're then taking the object containing all those parameters, ignoring it, and returning the value "ubuntu:precise". So

val

will always be an Output wrapping "ubuntu:precise". That's essentially hard-coded. What is it that you are hoping to do, and I'll help you write code that does that.

I'm writing support for new language and I can't figure out how Pulumi differentiates between

notyetcomputed

and

null

values during preview (grpc server just seems to return nothing - as in struct field does not exist at all - in both cases). From this examples it seems that official languages also cannot differentiate - during preview all nulls become

notyetcomputed

values. I found example that use only outputs:

import * as docker from "@pulumi/docker";

const container = new docker.Container("cont", {
    image: "sha256:e4c58958181a5925816faa528ce959e487632f4cfd192f8132f71b32df2744b4",
    networkMode: "bridge",
    command: ["sleep", "10000"],
})

const val = container.containerLogs.apply(_ => "ubuntu:precise")

const ubuntu2 = new docker.RemoteImage("ubuntu2", {
    name: val
});

Previewing update (dev):
     Type                         Name                Plan        Info
     pulumi:pulumi:Stack          pulumi-ts-test-dev
 +-  └─ docker:index:RemoteImage  ubuntu2             replace     [diff: ~name]
Resources:
    +-1 to replace
    2 unchanged

Do you want to perform this update? yes
Updating (dev):
     Type                 Name                Status
     pulumi:pulumi:Stack  pulumi-ts-test-dev
Resources:
    3 unchanged

Duration: 2s

Yes, that's fine. Applied outputs in preview are defined to be not known. If you want to tell the difference, you can check if you're in preview from the pulumi.runtime object.