Fix folding non-ASCII header

lib/mail/fields/unstructured_field.rb

@@ -102,6 +102,11 @@ def fold(prepend = 0) # :nodoc:
       encoding       = normalized_encoding
       decoded_string = decoded.to_s
       should_encode  = !decoded_string.ascii_only?
+      # If a header with only ASCII characters exceeds 998 characters without whitespace,
+      # it can be safely folded by encoding it.
+      if !should_encode && decoded_string.split(/[ \t]/).any? { |word| word.length > 900 }
+        should_encode = true
+      end
       if should_encode
         first = true
         words = decoded_string.split(/[ \t]/).map do |word|
@@ -110,11 +115,9 @@ def fold(prepend = 0) # :nodoc:
           else
             word = " #{word}"
           end
-          if !word.ascii_only?
-            word
-          else
-            word.scan(/.{7}|.+$/)
-          end
+          # A UTF-8 character can take up to 4 bytes, which becomes 12 bytes when encoded with QP.
+          # 70 characters is safe for the 998-byte limit.
+          word.scan(/.{1,70}/)
         end.flatten
       else
         words = decoded_string.split(/[ \t]/)
@@ -140,9 +143,6 @@ def fold(prepend = 0) # :nodoc:
           word = encode_crlf(word)
           # Skip to next line if we're going to go past the limit
           # Unless this is the first word, in which case we're going to add it anyway
-          # Note: This means that a word that's longer than 998 characters is going to break the spec. Please fix if this is a problem for you.
-          # (The fix, it seems, would be to use encoded-word encoding on it, because that way you can break it across multiple lines and
-          # the linebreak will be ignored)
           break if !line.empty? && (line.length + word.length + 1 > limit)
           # Remove the word from the queue ...
           words.shift

spec/mail/encodings_spec.rb

@@ -325,6 +325,15 @@
       expect(unwrapped.gsub("Subject: ", "")).to eq original
     end
 
+    it "should round trip ASCII-only word with more than 998 characters and no white space" do
+      original = "ThisIsASubjectHeaderMessageThatIsGoingToBeMoreThan998CharactersLong." * 20
+      mail = Mail.new
+      mail.subject = original
+      wrapped = mail[:subject].wrapped_value
+      unwrapped = Mail::Encodings.value_decode(wrapped)
+      expect(unwrapped.gsub("Subject: ", "")).to eq original
+    end
+
     it "should decode a blank string" do
       expect(Mail::Encodings.value_decode("=?utf-8?Q??=")).to eq ""
     end

spec/mail/fields/unstructured_field_spec.rb

@@ -157,11 +157,32 @@
       string = %|{"unique_args": {"mailing_id":147,"account_id":2}, "to": ["larspind@gmail.com"], "category": "mailing", "filters": {"domainkeys": {"settings": {"domain":1,"enable":1}}}, "sub": {"{{open_image_url}}": ["http://betaling.larspind.local/O/token/147/Mailing::FakeRecipient"], "{{name}}": ["[FIRST NAME]"], "{{signup_reminder}}": ["(her kommer til at stå hvornår folk har skrevet sig op ...)"], "{{unsubscribe_url}}": ["http://betaling.larspind.local/U/token/147/Mailing::FakeRecipient"], "{{email}}": ["larspind@gmail.com"], "{{link:308}}": ["http://betaling.larspind.local/L/308/0/Mailing::FakeRecipient"], "{{confirm_url}}": [""], "{{ref}}": ["[REF]"]}}|
       @field = Mail::UnstructuredField.new("X-SMTPAPI", string)
       string = string.dup.force_encoding('UTF-8')
-      result = "X-SMTPAPI: =?UTF-8?Q?{=22unique=5Fargs=22:_{=22mailing=5Fid=22:147,=22a?=\r\n =?UTF-8?Q?ccount=5Fid=22:2},_=22to=22:_[=22larspind@gmail.com=22],_=22categ?=\r\n =?UTF-8?Q?ory=22:_=22mailing=22,_=22filters=22:_{=22domainkeys=22:_{=22sett?=\r\n =?UTF-8?Q?ings=22:_{=22domain=22:1,=22enable=22:1}}},_=22sub=22:_{=22{{op?=\r\n =?UTF-8?Q?en=5Fimage=5Furl}}=22:_[=22http://betaling.larspind.local/O?=\r\n =?UTF-8?Q?/token/147/Mailing::FakeRecipient=22],_=22{{name}}=22:_[=22[FIRST?=\r\n =?UTF-8?Q?_NAME]=22],_=22{{signup=5Freminder}}=22:_[=22=28her_kommer_til_at?=\r\n =?UTF-8?Q?_st=C3=A5_hvorn=C3=A5r_folk_har_skrevet_sig_op_...=29=22],?=\r\n =?UTF-8?Q?_=22{{unsubscribe=5Furl}}=22:_[=22http://betaling.larspind.?=\r\n =?UTF-8?Q?local/U/token/147/Mailing::FakeRecipient=22],_=22{{email}}=22:?=\r\n =?UTF-8?Q?_[=22larspind@gmail.com=22],_=22{{link:308}}=22:_[=22http://beta?=\r\n =?UTF-8?Q?ling.larspind.local/L/308/0/Mailing::FakeRecipient=22],_=22{{con?=\r\n =?UTF-8?Q?firm=5Furl}}=22:_[=22=22],_=22{{ref}}=22:_[=22[REF]=22]}}?=\r\n"
+      result = "X-SMTPAPI: =?UTF-8?Q?{=22unique=5Fargs=22:?=\r\n =?UTF-8?Q?_{=22mailing=5Fid=22:147,=22account=5Fid=22:2},_=22to=22:?=\r\n =?UTF-8?Q?_[=22larspind@gmail.com=22],_=22category=22:_=22mailing=22,?=\r\n =?UTF-8?Q?_=22filters=22:_{=22domainkeys=22:_{=22settings=22:?=\r\n =?UTF-8?Q?_{=22domain=22:1,=22enable=22:1}}},_=22sub=22:?=\r\n =?UTF-8?Q?_{=22{{open=5Fimage=5Furl}}=22:?=\r\n =?UTF-8?Q?_[=22http://betaling.larspind.local/O/token/147/Mailing::FakeRecipient=22]?=\r\n =?UTF-8?Q?,_=22{{name}}=22:_[=22[FIRST_NAME]=22],?=\r\n =?UTF-8?Q?_=22{{signup=5Freminder}}=22:_[=22=28her_kommer_til_at_st=C3=A5?=\r\n =?UTF-8?Q?_hvorn=C3=A5r_folk_har_skrevet_sig_op_...=29=22],?=\r\n =?UTF-8?Q?_=22{{unsubscribe=5Furl}}=22:?=\r\n =?UTF-8?Q?_[=22http://betaling.larspind.local/U/token/147/Mailing::FakeRecipient=22]?=\r\n =?UTF-8?Q?,_=22{{email}}=22:_[=22larspind@gmail.com=22],?=\r\n =?UTF-8?Q?_=22{{link:308}}=22:?=\r\n =?UTF-8?Q?_[=22http://betaling.larspind.local/L/308/0/Mailing::FakeRecipient=22],?=\r\n =?UTF-8?Q?_=22{{confirm=5Furl}}=22:_[=22=22],_=22{{ref}}=22:?=\r\n =?UTF-8?Q?_[=22[REF]=22]}}?=\r\n"
       expect(@field.encoded).to eq result
       expect(@field.decoded).to eq string
     end
 
+    it "should fold an ASCII-only subject with more than 998 characters and no white space" do
+      value = "ThisIsASubjectHeaderMessageThatIsGoingToBeMoreThan998CharactersLong." * 20
+      @field = Mail::UnstructuredField.new("Subject", value)
+      lines = @field.encoded.split("\r\n\s")
+      lines.each { |line| expect(line.length).to be < 998 }
+    end
+
+    it "should fold a Japanese subject with more than 998 characters long and no white space" do
+      value = "これは非常に長い日本語のSubjectです。空白はありません。" * 5
+      @field = Mail::UnstructuredField.new("Subject", value)
+      lines = @field.encoded.split("\r\n\s")
+      lines.each { |line| expect(line.length).to be < 998 }
+    end
+
+    it "should fold full of emoji subject that is going to be more than 998 bytes unfolded" do
+      value = "😄" * 90
+      @field = Mail::UnstructuredField.new("Subject", value)
+      lines = @field.encoded.split("\r\n\s")
+      lines.each { |line| expect(line.length).to be < 998 }
+    end
+
     it "should fold properly with continuous spaces around the linebreak" do
       @field = Mail::UnstructuredField.new("Subject", "This is a header that has continuous spaces around line break point,     which should be folded properly")
       result = "Subject: This is a header that has continuous spaces around line break point,\s\r\n\s\s\s\swhich should be folded properly\r\n"