A technique for porting PowerShell based Azure DevOps Extensions to Node so they can be run cross-platform without a complete re-write

calendarDec 28, 2019 · 3 min read · DevOps DevOps  ·
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Background

I've written a good few extensions for Azure DevOps Services. Most of the early ones I wrote were written in PowerShell, but of late I have tended to use Typescript (targeting Node.JS) for the added cross-platform support.

This has led me to consider if it was worth the effort to convert all my legacy extensions to support cross-platform usage?

This is of course assuming the tasks the extension contains are useful on a non-Window platform. There is no point porting a Windows only tool away from PowerShell.

Assuming a conversion is a useful thing, there are two obvious ways to go about it:

  • Completely re-write the task in TypeScript, but I would like to avoid this effort if possible.
  • To use PowerShell Core, this is option I decide to experiment with.

A Solution

You might think the answer is to just alter the task’s manifest to run PSCore as opposed PowerShell3. The problem is that the Azure DevOps Agent does not provide support for PSCore, only Node or PowerShell3 execution of scripts.

However, there is a way around this limitation. You can shell a PSCore session from Node, as is done with the Microsoft PowerShell/PSCore script runner tasks.

I had previously experimented with this technique with my Pester Test Runner. The process I followed was

  1. Alter the PowerShell script to accept all the task parameters it previously got via the SDK calls as script parameters
  2. Alter the task manifest to run a Node script
  3. In the new Node wrapper script get all the Azure DevOps variables and then run the old script via a PSCore shell with the variables passed as parameters

This had worked surprising well, the only negative was that all log messages seem to gain an extra line break, but I can live with that.  Oh, and yes before you ask there is a new cross platform version of the Pester test runner on the way, but it is moving home. More on that soon.

However, when I tried the same technique on another extension, specifically my Build Updating one, I hit a problem.

All the Pester task’s operations are against the file system, there is no communication back to the Azure DevOps Server. This is not true for the Build tasks. They needed to talk to the Azure DevOps API. To do this they have to get the agent’s access token. This was done using the PowerShell Azure DevOps SDK, which in this new way of working is not loaded (the agent previously did it automatically when executing a script via PowerShell3).

After a bit of experimentation trying to load the PowerShell Azure DevOps SDK inside my PowerShell script inside a Node wrapper (a bad idea) I found the best option was to use the Azure DevOps Node SDK to get the token in the wrapper script and pass it into the PowerShell script as an extra parameter (it is then passing into all the functions as needed). This is more of an edit than I wanted but not too much work, far easier than a complete rewrite.

You can see an example of a wrapper here

In Summary

So have now have a mechanism to port extensions for cross platform usage without a complete re-write. Hence adding value to what has already been created. I guess I have found some OSS work for 2020