FEATURE: Upload to s3 in parallel to speed up backup restores (#13391)

Uploading lots of small files can be made significantly faster by parallelizing the `s3.put_object` calls. In testing, an UPLOAD_CONCURRENCY of 10 made a large restore 10x faster. An UPLOAD_CONCURRENCY of 20 made the same restore 18x faster.

This commit is careful to parallelize as little as possible, to reduce the chance of concurrency issues. In the worker threads, no database transactions are performed. All modification of shared objects is controlled with a mutex.

Unfortunately we do not have any existing tests for the `ToS3Migration` class. This change has been tested with a large site backup (120k uploads totalling 45GB)

lib/file_store/to_s3_migration.rb

@@ -7,6 +7,7 @@ module FileStore
 
   class ToS3Migration
     MISSING_UPLOADS_RAKE_TASK_NAME ||= 'posts:missing_uploads'
+    UPLOAD_CONCURRENCY ||= 20
 
     def initialize(s3_options:, dry_run: false, migrate_to_multisite: false, skip_etag_verify: false)
 
@@ -197,9 +198,25 @@ module FileStore
       log " => #{s3_objects.size} files"
       log " - Syncing files to S3"
 
+      queue = Queue.new
       synced = 0
       failed = []
 
+      lock = Mutex.new
+      upload_threads = UPLOAD_CONCURRENCY.times.map do
+        Thread.new do
+          while obj = queue.pop
+            if s3.put_object(obj[:options]).etag[obj[:etag]]
+              putc "."
+              lock.synchronize { synced += 1 }
+            else
+              putc "X"
+              lock.synchronize { failed << obj[:path] }
+            end
+          end
+        end
+      end
+
       local_files.each do |file|
         path = File.join(public_directory, file)
         name = File.basename(path)
@@ -244,15 +261,14 @@ module FileStore
         if @dry_run
           log "#{file} => #{options[:key]}"
           synced += 1
-        elsif s3.put_object(options).etag[etag]
-          putc "."
-          synced += 1
         else
-          putc "X"
-          failed << path
+          queue << { path: path, options: options, etag: etag }
         end
       end
 
+      queue.close
+      upload_threads.each(&:join)
+
       puts
 
       failure_message = "S3 migration failed for db '#{@current_db}'."