Permit opening a blob in a custom tempdir

activestorage/app/models/active_storage/blob.rb

@@ -167,8 +167,8 @@ class ActiveStorage::Blob < ActiveRecord::Base
   end
 
   # Downloads the blob to a tempfile on disk. Yields the tempfile.
-  def open(&block)
-    ActiveStorage::Downloader.new(self).download_blob_to_tempfile(&block)
+  def open(tempdir: nil, &block)
+    ActiveStorage::Downloader.new(self, tempdir: tempdir).download_blob_to_tempfile(&block)
   end
 
 

activestorage/lib/active_storage/analyzer.rb

@@ -24,11 +24,15 @@ module ActiveStorage
     private
       # Downloads the blob to a tempfile on disk. Yields the tempfile.
       def download_blob_to_tempfile(&block) #:doc:
-        blob.open(&block)
+        blob.open tempdir: tempdir, &block
       end
 
       def logger #:doc:
         ActiveStorage.logger
       end
+
+      def tempdir #:doc:
+        Dir.tmpdir
+      end
   end
 end

activestorage/lib/active_storage/downloader.rb

@@ -1,9 +1,10 @@
 # frozen_string_literal: true
 
 module ActiveStorage
-  class Downloader
-    def initialize(blob)
-      @blob = blob
+  class Downloader #:nodoc:
+    def initialize(blob, tempdir: nil)
+      @blob    = blob
+      @tempdir = tempdir
     end
 
     def download_blob_to_tempfile
@@ -14,10 +15,10 @@ module ActiveStorage
     end
 
     private
-      attr_reader :blob
+      attr_reader :blob, :tempdir
 
       def open_tempfile
-        file = Tempfile.open([ "ActiveStorage", tempfile_extension_with_delimiter ])
+        file = Tempfile.open([ "ActiveStorage", tempfile_extension_with_delimiter ], tempdir)
 
         begin
           yield file

activestorage/lib/active_storage/previewer.rb

@@ -26,7 +26,7 @@ module ActiveStorage
     private
       # Downloads the blob to a tempfile on disk. Yields the tempfile.
       def download_blob_to_tempfile(&block) #:doc:
-        blob.open(&block)
+        blob.open tempdir: tempdir, &block
       end
 
       # Executes a system command, capturing its binary output in a tempfile. Yields the tempfile.

activestorage/test/models/blob_test.rb

@@ -93,6 +93,18 @@ class ActiveStorage::BlobTest < ActiveSupport::TestCase
     end
   end
 
+  test "open in a custom tempdir" do
+    tempdir = Dir.mktmpdir
+
+    create_file_blob(filename: "racecar.jpg").open(tempdir: tempdir) do |file|
+      assert file.binmode?
+      assert_equal 0, file.pos
+      assert_match(/\.jpg\z/, file.path)
+      assert file.path.starts_with?(tempdir)
+      assert_equal file_fixture("racecar.jpg").binread, file.read, "Expected downloaded file to match fixture file"
+    end
+  end
+
   test "urls expiring in 5 minutes" do
     blob = create_blob