Fix media processing getting stuck on too much stdin/stderr (#16136)

* Fix media processing getting stuck on too much stdin/stderr

See thoughtbot/terrapin#5

* Remove dependency on paperclip-av-transcoder gem

* Remove dependency on streamio-ffmpeg gem

* Disable stdin on ffmpeg process

lib/terrapin/multi_pipe_extensions.rb

@@ -0,0 +1,63 @@
+# frozen_string_literal: false
+# Fix adapted from https://github.com/thoughtbot/terrapin/pull/5
+
+module Terrapin
+  module MultiPipeExtensions
+    def read
+      read_streams(@stdout_in, @stderr_in)
+    end
+
+    def close_read
+      begin
+        @stdout_in.close
+      rescue IOError
+        # Do nothing
+      end
+
+      begin
+        @stderr_in.close
+      rescue IOError
+        # Do nothing
+      end
+    end
+
+    def read_streams(output, error)
+      @stdout_output = ''
+      @stderr_output = ''
+
+      read_fds = [output, error]
+
+      until read_fds.empty?
+        to_read, = IO.select(read_fds)
+
+        if to_read.include?(output)
+          @stdout_output << read_stream(output)
+          read_fds.delete(output) if output.closed?
+        end
+
+        if to_read.include?(error)
+          @stderr_output << read_stream(error)
+          read_fds.delete(error) if error.closed?
+        end
+      end
+    end
+
+    def read_stream(io)
+      result = ''
+
+      begin
+        while (partial_result = io.read_nonblock(8192))
+          result << partial_result
+        end
+      rescue EOFError, Errno::EPIPE
+        io.close
+      rescue Errno::EINTR, Errno::EWOULDBLOCK, Errno::EAGAIN
+        # Do nothing
+      end
+
+      result
+    end
+  end
+end
+
+Terrapin::CommandLine::MultiPipe.prepend(Terrapin::MultiPipeExtensions)