Add more detailed CHANGELOG status logic

use dynamic PR number from Github context

reorder conditionals

fix wrong-section logic

.github/workflows/check-components-changelog.yml

@@ -26,12 +26,66 @@ jobs:
                   fetch-depth: ${{ env.PR_COMMIT_COUNT }}
             - name: 'Fetch relevant history from origin'
               run: git fetch origin ${{ github.event.pull_request.base.ref }}
-            - name: Run git diff
+            - name: Check CHANGELOG status
+              env:
+                  PR_NUMBER: ${{ github.event.number }}
               run: |
                   changelog_path="packages/components/CHANGELOG.md"
+                  optional_check_notice="This isn't a required check, so if you think your changes are small enough that they don't warrant a CHANGELOG entry, please go ahead and merge without one."
+
+                  # Fail if the PR doesn't touch the changelog
                   if git diff --quiet ${{ github.event.pull_request.base.sha }} HEAD -- "$changelog_path"; then
                     echo "Please add a CHANGELOG entry to $changelog_path"
                     echo
-                    echo "This isn't a required check, so if you think your changes are small enough that they don't warrant a CHANGELOG entry, please go ahead and merge without one."
+                    echo "${optional_check_notice}"
                     exit 1
                   fi
+
+                  changed_hunks=()
+                  while IFS= read -r range; do
+                    changed_hunks+=("${range}")
+                  done < <( \
+                    # Get CHANGELOG file changes
+                    git diff --unified=0 ${{ github.event.pull_request.base.sha }} HEAD "${changelog_path}" | \
+                    # Remove unwanted headers, meta, and specific change data
+                    grep -v -e '^[+-]' -e '^index' -e '^diff' | \
+                    # sed breakdown (statements are separated by `;`)
+                    # 1. Remove the @@ delimiters from all hunks of changes
+                    # 2. Remove the ` -` from the start of each line
+                    # 3. Remove the number of lines shown in each hunk
+                    # 4. Replace the ` +` with a `-` to show the range of lines changed
+                    sed 's/.*@@\(.*\)@@.*/\1/g; s/^ -//g; s/,[0-9]*//g; s/\(^[0-9]*\) +/\1-/g;' \
+                    )
+                    # Convert the ranges of changed lines to an array of individual line numbers
+                    changed_lines=()
+                    for range in "${changed_hunks[@]}"; do
+                      for (( i=${range%-*}; i<=${range#*-}; i++ )); do changed_lines+=("$i"); done
+                    done
+
+                    # Find important header line numbers
+                    unreleased_header_line_no=$(grep -n -e '^## Unreleased' "${changelog_path}" | head -1 | sed -r 's/^([0-9]*):## Unreleased/\1/')
+                    release_header_pattern='## [0-9]*\.[0-9]*\.[0-9]* \([0-9]{4}-[0-9]{2}-[0-9]{2}\)'
+                    latest_release_header_line_no=$(grep -nE -e "^${release_header_pattern}$" "${changelog_path}" | head -1 | sed -r "s/^([0-9]*):${release_header_pattern}/\1/")
+                    # Find the line number of the current PR's entry in the changelog
+                    pr_link_pattern="\(\[#${PR_NUMBER}\]\(https://github\.com/WordPress/gutenberg/pull/${PR_NUMBER}\)\)"
+                    pr_link_grep_pattern="(\[#${PR_NUMBER}\](https://github\.com/WordPress/gutenberg/pull/${PR_NUMBER}))"
+                    current_pr_entry_line_no=$(grep -n -e "${pr_link_grep_pattern}" "${changelog_path}" | sed -r "s,^([0-9]*):-.*${pr_link_pattern}.*,\1,")
+
+                    # Confirm that there is an 'Unreleased' section and that the relevant entry is in that section
+                    if ! grep -nq -e '^## Unreleased' "${changelog_path}" || \
+                      { [[ -n "${current_pr_entry_line_no}" ]] && ! [[ \
+                        ${current_pr_entry_line_no} -gt ${unreleased_header_line_no} && \
+                        ${current_pr_entry_line_no} -lt ${latest_release_header_line_no} \
+                      ]]; }; then
+                        echo "Please make sure your CHANGELOG entry is in the \`## Unreleased\` section"
+                      exit 1
+                    fi
+
+                    # Confirm that there is an entry with the right PR link it's part of the current changes
+                    if ! ( grep -nq -e "${pr_link_grep_pattern}" "${changelog_path}" \
+                      && echo "${changed_lines[@]}" | grep -qw "$current_pr_entry_line_no" ); then
+                        echo "Please make sure your CHANGELOG entry has a link to the current PR."
+                        echo
+                        echo "${optional_check_notice}"
+                        exit 1
+                    fi