pulumi.DependsOn should allow dependencies on Output

[irl-segfault]

Hi all.

Go currently does not support dynamic providers, which means I cannot implement custom resources. I have a use case which is described by the following pseudo-code:

resource1 := pkg.NewResource(...)

output2 := resource1.output1.ApplyT(func(s) bool { 
    // make API calls to external service
    return true
  })
resource2 := pkg.NewResource(..., pulumi.DependsOn(output2))
                                                                                                          ^ arg requires pulumi.Resource

In this case, I need to make API calls to some external service, and I want those calls to be a pre-requisite of creating any subsequent resources. In order to do this, I can return a BoolOutput from the ApplyT call, but I have no way of injecting this Output into the dependency graph.

If Go had support for dynamic providers, it's possible I could wrap this in a custom resource.

In general, I feel like Pulumi users should be able to express dependencies on any awaitable value or asynchronous task, instead of only resources managed by Pulumi. To me, this is a killer feature of Pulumi vs. something like Terraform.

[lukehoban]

This is tracked in #2545 and #3972. Will close this out to continue tracking there. Feel free to add additional notes on use cases there if not already covered in that issue.