fix(patch): resolve emacs-29 symlink patches to their real URL

Annoyingly, when downloading raw files from GitHub, symlinks do not
return the content of the symlink target, but instead it simply returns
the symlink target path (../emacs-28/fix-window-role.patch).

Hence we have to check the patch file content, and if it contains only a
single line of text, we assume it's a symlink, and resolve it relative
to the original download URL.

build-emacs-for-macos

@@ -717,7 +717,14 @@ class Build
       info "Downloading patch: #{patch[:url]}"
       run_cmd('curl', '-L#', patch[:url], '-o', patch_file)
 
-      apply_patch({ file: patch_file }, target)
+      real_patch_url = detect_github_symlink_patch(patch[:url], patch_file)
+      if real_patch_url
+        FileUtils.rm(patch_file)
+        apply_patch({ url: real_patch_url }, target)
+      else
+        apply_patch({ file: patch_file }, target)
+      end
+
     elsif patch[:replace]
       err 'Patch replace input error' unless patch[:replace].size == 3
 
@@ -735,6 +742,30 @@ class Build
       f.close
     end
   end
+
+  # When downloading raw files from GitHub, if the target file is a symlink, it
+  # will return the actual target path of the symlink instead of the content of
+  # the target file. Hence we have to check if the patch file we have downloaded
+  # contains one and only one line, and if so, assume it's a symlink.
+  def detect_github_symlink_patch(original_url, patch_file)
+    lines = []
+
+    # read first two lines
+    File.open(patch_file) do |f|
+      lines << f.gets
+      lines << f.gets
+    end
+
+    # if the file contains more than one line of text, it's not a symlink.
+    return unless lines[1].nil?
+
+    symlink_target = lines[0].strip
+    # Assume patch file content is something along the lines of
+    # "../emacs-28/fix-window-role.patch", hence we resolve it relative to the
+    # original url.
+    info "patch is symlink to #{symlink_target}"
+    URI.join(original_url, symlink_target).to_s
+  end
 end
 
 class AbstractEmbedder