Add rules for branches

1. User can set the "primary" branch to live at page root.
This is `publish` by default.
2. User can set a list of aliases (symbolic links).
The default setting maps `draft` to `main`, and `book` to `publish`.
3. User can set a list of branch to be deployed.
By default: deploy all branches (indicated by '*').

NOTE: If user wants no branches deployed, or no aliases, they can set
the corresponding variables to ' '.

.github/workflows/deploy-book-ghpages.yml

@@ -16,6 +16,20 @@ on:
     # - some-subfolder/**
   workflow_dispatch:
 
+# Inherit configuration variables in the environment, or assign default values.
+# Configuration variables may be set as explained here:
+# https://docs.github.com/en/actions/learn-github-actions/variables#creating-configuration-variables-for-a-repository
+env:
+  # `publish` as primary by default
+  PRIMARY_BRANCH: ${{ vars.PRIMARY_BRANCH != '' && vars.PRIMARY_BRANCH || 'publish' }}
+  # Space-separated list of alias-rules, e.g. 'draft:main alias:really-long-branch-name'
+  # By default, `draft` links to `main`.
+  # If no aliases are wanted, BRANCH_ALIASES may be set to ' ' (space).
+  BRANCH_ALIASES: ${{ vars.BRANCH_ALIASES != '' && vars.BRANCH_ALIASES || 'draft:main book:publish' }}
+  # Space-separated list of branch names, e.g. 'main second third'.
+  # By default, deploy all branches. This is indicated by '*'.
+  BRANCHES_TO_DEPLOY: ${{ vars.BRANCHES_TO_DEPLOY != '' && vars.BRANCHES_TO_DEPLOY || '*' }}
+
 jobs:
   get-branches:
     runs-on: ubuntu-latest
@@ -27,15 +41,21 @@ jobs:
     - uses: actions/checkout@v4
       with:
         fetch-depth: 0
+
     - id: set-branches
-      name: Collect all remote branch names for the matrix
+      name: Set branches
       run: |
-        branches=$(git branch -r | sed 's,\s*origin/,,g' | jq -Rn '[inputs]')
+        if [ "$BRANCHES_TO_DEPLOY" == '*' ]; then
+          branches=$(git branch -r | sed 's,\s*origin/,,g' | jq -Rn '[inputs]')
+        else
+          branches=$(echo $BRANCHES_TO_DEPLOY | tr ' ' '\n' | jq -Rn '[inputs]')
+        fi
         echo "branches=$(echo $branches)" >> $GITHUB_OUTPUT
 
   build-books:
     runs-on: ubuntu-latest
     needs: get-branches
+    if: ${{ needs.get-branches.outputs.branches != '[]' }}
     permissions:
       pages: write
       id-token: write
@@ -115,9 +135,16 @@ jobs:
         fi
         ls -a final/
 
-    - name: Load root page to redirect to `book/`
+    - name: Symlink branch aliases
       run: |
-        cp redirect.html final/index.html
+        echo $BRANCH_ALIASES | tr ' ' '\n' | { grep -E '\S' | :; } |
+        while IFS=':' read -r key value; do
+          # If the target branch is to be deployed, make symlink to it.
+          if [ "$BRANCHES_TO_DEPLOY" == "*" ] || echo $BRANCHES_TO_DEPLOY | tr ' ' '\n' | grep "^$value$"; then
+            echo link $key -> $value
+            ln -s $value final/$key
+          fi
+        done
 
     - name: Upload final Pages artifact
       uses: actions/upload-pages-artifact@v3