Fix windows function apps (and slots) on standard plans

[literatesnow]

The settings WEBSITE_CONTENTAZUREFILECONNECTIONSTRING and WEBSITE_CONTENTSHARE are required for windows function apps on Standard plans.

Fixes issue #13135

Related PR #12553 (function app on premium plans)
Related PR #12778 (function app slots on premium plans)

[shibayan]

This condition should not be added. If WEBSITE_CONTENTAZUREFILECONNECTIONSTRING is added to a Function App that is already running, the deployed application will be lost. I have confirmed that when I apply changes using v2.76.0 to resources created using v2.66.0, applications deployed to Premium V2 are lost.

I deployed from Azure Portal for all Azure Functions platform combinations and checked if WEBSITE_CONTENTAZUREFILECONNECTIONSTRING exists. ⭕ means that it existed.

• Windows
  • ⭕ Consumption
  • ⭕ Premium Plan (Elastic Premium)
  • ❌ App Service Plan (Standard / Premium v2, v3)
• Linux
  • ❌ Consumption
  • ⭕ Premium Plan (Elastic Premium)
  • ❌ App Service Plan (Standard / Premium v2, v3)

The documentation states that it only supports Consumption Plan and Premium Plan (not Premium v2/v3) for Windows, but in fact it was also used for Premium Plan for Linux.

https://docs.microsoft.com/en-us/azure/azure-functions/functions-app-settings#website_contentazurefileconnectionstring

[shibayan]

It seems that #12553 and #12778 already contain changes that may cause you to lose existing applications that use Premium v1/v2/v3. Fortunately, the changes will not be reflected unless you change the App Settings, but I would like to recommend that you make the conditions consistent with when you created it from the Azure Portal.

[literatesnow]

You're right, the docs for WEBSITE_CONTENTAZUREFILECONNECTIONSTRING says "where the function app code and configuration are stored in event-driven scaling plans running on Windows" which is key for the next sentence "Only used when deploying to a Premium plan or to a Consumption plan running on Windows".

Therefore, Elastic Premium (EP) requires WEBSITE_CONTENTAZUREFILECONNECTIONSTRING which is different to the dedicated hosting Premium (P) plans that does not require it.

My function apps had WEBSITE_CONTENTAZUREFILECONNECTIONSTRING and when they were moved to the non-elastic Premium P3V1 plan, Terraform was removing the app setting causing them not to load. I unfortunately missed the difference of elastic premium vs non-elastic premium in PR's #12553 / #12778.

Logic should be:

• Add WEBSITE_CONTENTAZUREFILECONNECTIONSTRING for Elastic Premium/Consumption if it doesn't exist (as per table above)
• All other plan types, if WEBSITE_CONTENTAZUREFILECONNECTIONSTRING exists, don't remove it (which will allow apps that move between plan types to still start)

I'll circle back on this PR and fix the logic when I can.

Thanks for taking the time @shibayan.

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