Use Ruby to decode uuencoded attachments

Ruby can natively decode uuencoded text with String#unpack. [1] This
avoids the uuencode program dependency and avoids writing tempfiles.

The actual implementation is taken from mail [2]. The UnixToUnix module
is not available in our bundled version of mail.

This commit includes a spec to illustrate the failure of uuencode(1) to
decode the particular attachment in incoming-request-bad-uuencoding-2.
email. Parsing with uuencode returns the following error:

    External Command: Error from command "uudecode -o /dev/stdout
	/tmp/foiuu20150530-14811-u6j936":
	uudecode: /tmp/foiuu20150530-14811-u6j936: No `end' line

The file _is_ created and appears to be OK, but the exit code is 1. This
causes AlaveteliExternalCommand to fail and return nil.

See #2508 [3] for the bug report.

[1] http://ruby-doc.org/core-1.8.7/String.html#method-i-unpack
[2] https://github.com/mikel/mail/blob/bc4c9bb9321e9d36a678692f2f562d3146b63f78/lib/mail/encodings/unix_to_unix.rb#L7
[3] https://github.com/mysociety/alaveteli/issues/2508

1 files changed, 1 insertions, 6 deletions

diff --git a/app/models/incoming_message.rb b/app/models/incoming_message.rb
index 546964c95..2e697140c 100644
--- a/app/models/incoming_message.rb
+++ b/app/models/incoming_message.rb
@@ -507,12 +507,7 @@ class IncomingMessage < ActiveRecord::Base
         attachments = []
         uus.each do |uu|
             # Decode the string
-            content = nil
-            tempfile = Tempfile.new('foiuu')
-            tempfile.print uu
-            tempfile.flush
-            content = AlaveteliExternalCommand.run("uudecode", "-o", "/dev/stdout", tempfile.path)
-            tempfile.close
+            content = uu.sub(/\Abegin \d+ [^\n]*\n/, '').unpack('u').first
             # Make attachment type from it, working out filename and mime type
             filename = uu.match(/^begin\s+[0-9]+\s+(.*)$/)[1]
             calc_mime = AlaveteliFileTypes.filename_and_content_to_mimetype(filename, content)