When Azure DevOps deploys .NET core application to Azure App Service it automatically sets value of WEBSITE_RUN_FROM_PACKAGE to 1. This setting is causing website root directory become readonly and stdout logging from .NET core process would fail within that directory. So if you web.config looks like below it will stop working as a result.
<?xml version="1.0" encoding="utf-8"?>
<configuration>
<system.webServer>
<modules>
<remove name="WebDAVModule" />
</modules>
<handlers>
<add name="aspNetCore" path="*" verb="*" modules="AspNetCoreModuleV2" resourceType="Unspecified" />
</handlers>
<aspNetCore processPath="dotnet" arguments=".\binary.dll" stdoutLogEnabled="true" stdoutLogFile=".\log" forwardWindowsAuthToken="false"/>
</system.webServer>
</configuration>
With WEBSITE_RUN_FROM_PACKAGE even though wwwroot is locked, home directory is not, which is d:\home\LogFiles, so web.config shall be transformed during deployment, so value of <aspNetCore processPath="dotnet" arguments=".\binary.dll" stdoutLogEnabled="true" stdoutLogFile=".\log" forwardWindowsAuthToken="false"/> shall be transformed into <aspNetCore processPath="dotnet" arguments=".\binary.dll" stdoutLogEnabled="true" stdoutLogFile="d:\home\LogFiles" forwardWindowsAuthToken="false"/>
For this to happen web.config transformation shall be applied during release pipeline. Transformation happens based on file which will be named based on environment where this setting shall be applied to. For example if you have environment which is called qa-blue then transformation file shall be called web.qa-blue.config and have following content which instructs transform engine to replace value of aspNetCore node.
<?xml version="1.0" encoding="utf-8"?>
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
<system.webServer>
<modules>
<remove name="WebDAVModule" />
</modules>
<handlers>
<add name="aspNetCore" path="*" verb="*" modules="AspNetCoreModuleV2" resourceType="Unspecified"/>
</handlers>
<aspNetCore processPath="dotnet" arguments=".\binary.dll" stdoutLogEnabled="true" stdoutLogFile="d:\home\LogFiles" forwardWindowsAuthToken="false" xdt:Transform="Replace" />
</system.webServer>
</configuration>
Release pipeline would need to indicate that XML transformation happen during AppService deployment to QA-blue environment
You would be able to see in output transformation taking place
022-09-14T15:54:41.7194241Z Start tranformation to 'D:\a\_temp\temp_web_package_9382618325923051\web.config'.
2022-09-14T15:54:41.7195325Z Source file: 'D:\a\_temp\temp_web_package_9382618325923051\web.config'.
2022-09-14T15:54:41.7196147Z Transform file: 'D:\a\_temp\temp_web_package_9382618325923051\web.QA-blue.config'.
2022-09-14T15:54:41.7197018Z Transformation task is using encoding 'System.Text.UTF8Encoding'. Change encoding in source file, or use the 'encoding' parameter if you want to change encoding.
2022-09-14T15:54:41.7197772Z Executing Replace (transform line 16, 153)
2022-09-14T15:54:41.7198321Z on /configuration/system.webServer/aspNetCore
2022-09-14T15:54:41.7198896Z Applying to 'aspNetCore' element (no source line info)
2022-09-14T15:54:41.7199412Z Replaced 'aspNetCore' element
2022-09-14T15:54:41.7199887Z Done executing Replace
2022-09-14T15:54:41.7200481Z XML Transformations applied successfully
Various Tech notes 