uploads: generate thumbnails in parallel.

Make uploads faster by generating and saving thumbnails in parallel.

We generate each thumbnail in parallel, then send each thumbnail to the
backend image servers in parallel.

Most images have 5 variants: 'preview' (150x150), 180x180, 360x360,
720x720, and 'sample' (850px width). Plus the original file, that's 6
files we have to save. In production we have 2 image servers, so we have
to save each file twice, to 2 remote servers. Doing all this in parallel
should make uploads significantly faster.

app/logical/storage_manager.rb

@@ -30,7 +30,7 @@ class StorageManager
   # written, or an error is raised and the original file is left unchanged. The
   # file should never be in a partially written state.
   #
-  # @param io [IO] a file (or a readable IO object)
+  # @param src_file [File] the file to store
   # @param path [String] the remote path where the file should be stored
   def store(io, path)
     raise NotImplementedError, "store not implemented"

app/logical/storage_manager/local.rb

@@ -12,13 +12,12 @@ class StorageManager::Local < StorageManager
     super(**options)
   end
 
-  def store(io, dest_path)
+  def store(src_file, dest_path)
     temp_path = full_path(dest_path) + "-" + SecureRandom.uuid + ".tmp"
 
     FileUtils.mkdir_p(File.dirname(temp_path))
-    io.rewind
-    bytes_copied = IO.copy_stream(io, temp_path)
-    raise Error, "store failed: #{bytes_copied}/#{io.size} bytes copied" if bytes_copied != io.size
+    bytes_copied = IO.copy_stream(src_file.path, temp_path)
+    raise Error, "store failed: #{bytes_copied}/#{src_file.size} bytes copied" if bytes_copied != src_file.size
 
     FileUtils.chmod(DEFAULT_PERMISSIONS, temp_path)
     File.rename(temp_path, full_path(dest_path))

app/logical/storage_manager/mirror.rb

@@ -9,14 +9,14 @@ class StorageManager::Mirror < StorageManager
     super(**options)
   end
 
-  def store(io, dest_path)
-    services.each do |service|
-      service.store(io, dest_path)
+  def store(src_file, dest_path)
+    Parallel.each(services, in_threads: Etc.nprocessors) do |service|
+      service.store(src_file, dest_path)
     end
   end
 
   def delete(path)
-    services.each do |service|
+    Parallel.each(services, in_threads: Etc.nprocessors) do |service|
       service.delete(path)
     end
   end

app/logical/storage_manager/null.rb

@@ -7,7 +7,7 @@ class StorageManager::Null < StorageManager
     super(base_url: nil)
   end
 
-  def store(io, path)
+  def store(src_file, path)
     # no-op
   end
 

app/models/media_asset.rb

@@ -271,7 +271,7 @@ class MediaAsset < ApplicationRecord
     end
 
     def distribute_files!(media_file)
-      variants.each do |variant|
+      Parallel.each(variants, in_threads: Etc.nprocessors) do |variant|
         variant.store_file!(media_file)
       end
     end