Ensure thread-pool shutdown.

Library/Homebrew/cmd/fetch.rb

@@ -301,6 +301,8 @@ module Homebrew
                   # FIXME: Implement cancellation of running downloads.
                 end
 
+                download_queue.cancel
+
                 if previous_pending_line_count.positive?
                   $stdout.print Tty.move_cursor_down(previous_pending_line_count - 1)
                   $stdout.flush
@@ -314,7 +316,7 @@ module Homebrew
             $stdout.flush
           end
         end
-
+      ensure
         download_queue.shutdown
       end
 

Library/Homebrew/download_queue.rb

@@ -28,9 +28,18 @@ module Homebrew
       # rubocop:enable Lint/ShadowingOuterLocalVariable
     end
 
+    sig { void }
+    def cancel
+      # FIXME: Implement graceful cancellaction of running downloads based on
+      #        https://ruby-concurrency.github.io/concurrent-ruby/master/Concurrent/Cancellation.html
+      #        instead of killing the whole thread pool.
+      pool.kill
+    end
+
     sig { void }
     def shutdown
       pool.shutdown
+      pool.wait_for_termination
     end
   end
 end