Merge pull request #330: bound attachment preview work

Reviewed and merged by GPT on behalf of DHH. Keep both boost-cache and preview-loading regressions.

lib/rails_ext/time_limited_video_previewer.rb

@@ -0,0 +1,27 @@
+# Rails waits for ffmpeg however long it takes, and a video's preview is drawn inside the request that posts it.
+# This previewer gives ffmpeg a wall-clock limit, kills it past that, and reports a failed preview, so the message
+# is posted without one.
+class TimeLimitedVideoPreviewer < ActiveStorage::Previewer::VideoPreviewer
+  TIME_LIMIT = 10 # seconds
+
+  private
+    def capture(*argv, to:)
+      to.binmode
+
+      open_tempfile do |err|
+        IO.popen(argv, in: IO::NULL, err: err) do |out|
+          Timeout.timeout(TIME_LIMIT) { IO.copy_stream(out, to) }
+        rescue Timeout::Error
+          Process.kill :KILL, out.pid
+          raise ActiveStorage::PreviewError, "#{argv.first} took longer than #{TIME_LIMIT} seconds"
+        end
+        err.rewind
+
+        unless $?.success?
+          raise ActiveStorage::PreviewError, "#{argv.first} failed (status #{$?.exitstatus}): #{err.read.to_s.chomp}"
+        end
+      end
+
+      to.rewind
+    end
+end