Following up on API baseline check and documentation (dotnet#30221)

* Update codecheck logic for verifying api baselines

* Update baseline documentation

Co-authored-by: Doug Bunting <6431421+dougbu@users.noreply.github.com>

Author: John Luo

docs/APIBaselines.md

@@ -1,21 +1,25 @@
 # API Baselines
 
-This document contains information regarding API baseline files and how to work with them.
+This document contains information regarding API baseline files and how to work with them. For additional details on how these files works, consult:
+-	https://github.com/dotnet/roslyn-analyzers/blob/master/src/PublicApiAnalyzers/PublicApiAnalyzers.Help.md
+-	https://github.com/dotnet/roslyn-analyzers/blob/master/src/PublicApiAnalyzers/Microsoft.CodeAnalysis.PublicApiAnalyzers.md
 
 ## Add baseline files for new projects
 
 When creating a new implementation (i.e. src) project, it's necessary to manually add API baseline files since API baseline are enabled by default. If the project is a non-shipping or test only project, add `<AddPublicApiAnalyzers>false</AddPublicApiAnalyzers>` to the project to disable these checks. To add baseline files to the project:
 
-1. Copy eng\PublicAPI.empty.txt to the project directory and rename it to PublicAPI.Shipped.txt
-1. Copy eng\PublicAPI.empty.txt to the project directory and rename it to PublicAPI.Unshipped.txt
+1. `cp .\eng\PublicAPI.empty.txt {new folder}\PublicAPI.Shipped.txt`
+1. `cp .\eng\PublicAPI.empty.txt {new folder}\PublicAPI.Unshipped.txt`
+
+See [Steps for adding and updating APIs](#steps-for-adding-and-updating-apis) for steps on how to add APIs to the Unshipped.txt files
 
 ## PublicAPI.Shipped.txt
 
 This file contains APIs that were released in the last major version. This file should only be modified after a major release by the build team and should never be modified otherwise.
 
 ## PublicAPI.Unshipped.txt
 
-This file contains new APIs since the last major version.
+This file contains new APIs since the last major version. Steps for working with changes in APIs and updating this file are found in [Steps for adding and updating APIs](#steps-for-adding-and-updating-apis).
 
 ### New APIs
 
@@ -45,6 +49,21 @@ Two new entry needs to be added to the PublicAPI.Unshipped.txt file for an updat
 Microsoft.AspNetCore.DataProtection.Infrastructure.IApplicationDiscriminator.Discriminator.get -> string?
 ```
 
+### Steps for adding and updating APIs
+
+1. Update AspNetCore.sln and relevant `*.slnf` file to include the new project if needed
+1. `{directory containing relevant *.slnf}\startvs.cmd`
+1. F6 *or whatever your favourite build gesture is*
+1. Click on a RS0016 (or whatever) error
+1. Right click in editor on underscored symbol or go straight to the “quick fix” icon to its left. Control-. also works.
+1. Choose “Add Blah to public API” / “Fix all occurrences in … Solution”
+1. Click Apply
+1. F6 *again to see if the fixer missed anything or if other RS00xx errors show up (not uncommon)*
+1. Suppress or fix other problems as needed but please suppress (if suppressing) using attributes and not globally or with `#pragma`s because attributes make the justification obvious e.g. for common errors that can't be fixed
+    `[SuppressMessage("ApiDesign", "RS0026:Do not add multiple public overloads with optional parameters", Justification = "Required to maintain compatibility")]`
+    or
+    `[SuppressMessage("ApiDesign", "RS0027:Public API with optional parameter(s) should have the most parameters amongst its public overloads.", Justification = "Required to maintain compatibility")]`
+
 ## Updating baselines after major releases
 
-TODO
+This will be performed by the build team using scripts at https://github.com/dotnet/roslyn/tree/master/scripts/PublicApi (or an Arcade successor). The process moves the content of PublicAPI.Unshipped.txt into PublicAPI.Shipped.txt

eng/scripts/CodeCheck.ps1

@@ -191,18 +191,27 @@ try {
         }
     }
 
-    Write-Host "Checking for changes to API baseline files"
+    $targetBranch = $env:SYSTEM_PULLREQUEST_TARGETBRANCH
+    if ($targetBranch.StartsWith('refs/heads/')) {
+        $targetBranch = $targetBranch.Replace('refs/heads/','')
+    }
 
     # Retrieve the set of changed files compared to main
-    $commitSha = git rev-parse HEAD
-    $changedFilesFromMain = git --no-pager diff origin/main...$commitSha --ignore-space-change --name-only --diff-filter=ar
+    Write-Host "Checking for changes to API baseline files $targetBranch"
+
+    $changedFilesFromTarget = git --no-pager diff origin/$targetBranch --ignore-space-change --name-only --diff-filter=ar
     $changedAPIBaselines = [System.Collections.Generic.List[string]]::new()
 
-    if ($changedFilesFromMain) {
-        foreach ($file in $changedFilesFromMain) {
+    if ($changedFilesFromTarget) {
+        foreach ($file in $changedFilesFromTarget) {
+            # Check for changes in Shipped in all branches
             if ($file -like '*PublicAPI.Shipped.txt') {
                 $changedAPIBaselines.Add($file)
             }
+            # Check for changes in Unshipped in servicing branches
+            if ($targetBranch -like 'release*' -and $file -like '*PublicAPI.Unshipped.txt') {
+                $changedAPIBaselines.Add($file)
+            }
         }
     }