Fix file descriptor leak in Linux LD library path parsing

The library_paths method was using readlines which could leave file
descriptors open due to Ruby's garbage collection behavior. When
processing many packages during 'brew upgrade' or 'brew linkage',
this caused "Too many open files" errors on Linux systems.

Changes:
- Replace readlines with explicit file.open block to ensure proper closure
- Add caching to avoid repeatedly reading /etc/ld.so.conf during a session
- Cache included files as well to optimize recursive include processing

Fixes: #19866, #20302, #19177, #20223

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: MikeMcQuaid

Library/Homebrew/os/linux/ld.rb

@@ -50,29 +50,37 @@ def self.library_paths(conf_path = Pathname(sysconfdir)/"ld.so.conf")
         conf_file = Pathname(conf_path)
         return [] unless conf_file.exist?
 
+        @library_paths_cache ||= T.let({}, T.nilable(T::Hash[String, T::Array[String]]))
+        cache_key = conf_file.to_s
+        if (cached_library_path_contents = @library_paths_cache[cache_key])
+          return cached_library_path_contents
+        end
+
         paths = Set.new
         directory = conf_file.realpath.dirname
 
-        conf_file.readlines.each do |line|
-          # Remove comments and leading/trailing whitespace
-          line.strip!
-          line.sub!(/\s*#.*$/, "")
-
-          if line.start_with?(/\s*include\s+/)
-            include_path = Pathname(line.sub(/^\s*include\s+/, "")).expand_path
-            wildcard = include_path.absolute? ? include_path : directory/include_path
-
-            Dir.glob(wildcard.to_s).each do |include_file|
-              paths += library_paths(include_file)
+        conf_file.open("r") do |file|
+          file.each_line do |line|
+            # Remove comments and leading/trailing whitespace
+            line.strip!
+            line.sub!(/\s*#.*$/, "")
+
+            if line.start_with?(/\s*include\s+/)
+              include_path = Pathname(line.sub(/^\s*include\s+/, "")).expand_path
+              wildcard = include_path.absolute? ? include_path : directory/include_path
+
+              Dir.glob(wildcard.to_s).each do |include_file|
+                paths += library_paths(include_file)
+              end
+            elsif line.empty?
+              next
+            else
+              paths << line
             end
-          elsif line.empty?
-            next
-          else
-            paths << line
           end
         end
 
-        paths.to_a
+        @library_paths_cache[cache_key] = paths.to_a
       end
     end
   end