When parsing UTF-7, do not raise exceptions (except for B encodings)

lib/mail/utilities.rb

@@ -306,10 +306,10 @@ def encode(string, charset)
     end
 
     class BestEffortCharsetEncoder
-      def encode(string, charset)
+      def encode(string, charset, raise_utf7_exceptions = false)
         case charset
         when /utf-?7/i
-          Mail::Utilities.decode_utf7(string)
+          Mail::Utilities.decode_utf7(string, raise_utf7_exceptions)
         else
           string.force_encoding(pick_encoding(charset))
         end
@@ -396,10 +396,20 @@ def Utilities.encode_utf7(string)
       end.force_encoding(Encoding::ASCII_8BIT)
     end
 
-    def Utilities.decode_utf7(utf7)
+    # Note: UTF-7 is an obscure encoding, and it's not clear that it's
+    # even valid for email bodies (the case in which we need
+    # raise_utf7_exceptions=false.)  If we remove support for UTF-7 in
+    # email bodies, we can also remove the argument here.
+    def Utilities.decode_utf7(utf7, raise_utf7_exceptions)
       utf7.gsub(/&([^-]+)?-/n) do
         if $1
-          ($1.tr(",", "/") + "===").unpack1("m").encode(Encoding::UTF_8, Encoding::UTF_16BE)
+          utf_16be = ($1.tr(",", "/") + "===").unpack("m")[0].force_encoding(Encoding::UTF_16BE)
+          if raise_utf7_exceptions
+            # special case - in B encoding, raise exceptions on conversion errors
+            utf_16be.encode(Encoding::UTF_8)
+          else
+            transcode_to_scrubbed_utf8(utf_16be)
+          end
         else
           "&"
         end
@@ -416,7 +426,7 @@ def Utilities.b_value_decode(str)
       if match
         charset = match[1]
         str = Utilities.decode_base64(match[2])
-        str = charset_encoder.encode(str, charset)
+        str = charset_encoder.encode(str, charset, true)
       end
       transcode_to_scrubbed_utf8(str)
     rescue Encoding::UndefinedConversionError, ArgumentError, Encoding::ConverterNotFoundError, Encoding::InvalidByteSequenceError

spec/fixtures/emails/multi_charset/quoted-printable.eml

@@ -0,0 +1,4 @@
+Content-Type: text/plain; charset="utf-8"
+Content-Transfer-Encoding: quoted-printable
+
+Valid: =E6=98=AF Invalid: =E6

spec/fixtures/emails/multi_charset/utf7.eml

@@ -0,0 +1,3 @@
+Content-Type: text/plain; charset="utf-7"
+
+Valid: &Zi8- Invalid: &5g-

spec/mail/message_spec.rb

@@ -401,6 +401,16 @@ def create_mail_with_splat_args
       expect(raw_message.encoding).to eq original_encoding if raw_message.respond_to?(:encoding)
     end
 
+    it "should parse utf-7 email without raising exceptions" do
+      mail = read_fixture('emails', 'multi_charset', 'utf7.eml')
+      expect(mail.decoded.chomp).to eq "Valid: 是 Invalid: �"
+    end
+
+    it "should parse quoted-printable email without raising exceptions" do
+      mail = read_fixture('emails', 'multi_charset', 'quoted-printable.eml')
+      expect(mail.decoded.chomp).to eq "Valid: 是 Invalid: �"
+    end
+
     if '1.9+'.respond_to?(:encoding)
       it "should be able to normalize CRLFs on non-UTF8 encodings" do
         File.open(fixture_path('emails', 'multi_charset', 'japanese_shift_jis.eml'), 'rb') do |io|