XEP-0004: fix: prevent multiple <values> for 'text-single' field

According to XEP-0004:
- if there is no "type" attribute on a <field />, we should assume it is
  "text-single";
- "text-single" MUST NOT contain morethan one <value />.

Before this patch, not specifying a field type and passing a multi-line
string would result in an illegal stanza.

While it would be cleaner to log a warning or even raise an exception if
set_value() is called with an incompatible type, this breaks a lot of
tests and backward-compatibility, so we introduce some heuristic in
FormField.set_value() to infer the field type based on the provided
value instead.

I also changed FormField.get_value() so that it returns a list by
default for 'text-multi' fields. This is a breaking change, but I have
not found the justification for the previous behaviour.

slixmpp/plugins/xep_0004/stanza/field.py

@@ -1,8 +1,9 @@
-
 # Slixmpp: The Slick XMPP Library
 # Copyright (C) 2011 Nathanael C. Fritz, Lance J.T. Stout
 # This file is part of Slixmpp.
 # See the file LICENSE for copying permission.
+import logging
+
 from slixmpp.xmlstream import ElementBase, ET
 
 
@@ -78,7 +79,14 @@ class FormField(ElementBase):
         reqXML = self.xml.find('{%s}required' % self.namespace)
         return reqXML is not None
 
-    def get_value(self, convert=True):
+    def get_value(self, convert=True, convert_list=False):
+        """
+        Gets the value for this field
+
+        :param convert: Convert truthy values to boolean
+        :param convert_list: Convert text-multi fields to a string with
+            \n as separator for values
+        """
         valsXML = self.xml.findall('{%s}value' % self.namespace)
         if len(valsXML) == 0:
             return None
@@ -92,7 +100,7 @@ class FormField(ElementBase):
                 if valXML.text is None:
                     valXML.text = ''
                 values.append(valXML.text)
-            if self._type == 'text-multi' and convert:
+            if self._type == 'text-multi' and convert_list:
                 values = "\n".join(values)
             return values
         else:
@@ -127,6 +135,17 @@ class FormField(ElementBase):
         del self['value']
         valXMLName = '{%s}value' % self.namespace
 
+        if not self._type:
+            if isinstance(value, bool):
+                log.debug("Passed a 'boolean' as value of an untyped field, assuming it is a 'boolean'")
+                self._type = "boolean"
+            elif isinstance(value, str):
+                log.debug("Passed a 'str' as value of an untyped field, assuming it is a 'text-single'")
+                self._type = "text-single"
+            elif isinstance(value, (list, tuple)):
+                log.debug("Passed a %s as value of an untyped field, assuming it is a 'text-multi'")
+                self._type = "text-multi"
+
         if self._type == 'boolean':
             if value in self.true_values:
                 valXML = ET.Element(valXMLName)
@@ -180,3 +199,6 @@ FormField.setOptions = FormField.set_options
 FormField.setRequired = FormField.set_required
 FormField.setTrue = FormField.set_true
 FormField.setValue = FormField.set_value
+
+
+log = logging.getLogger(__name__)

tests/test_stanza_xep_0004.py

@@ -95,6 +95,21 @@ class TestDataForms(SlixTest):
           </message>
         """)
 
+    def testMultiLineField(self):
+        msg = self.Message()
+        form = msg['form']
+        form.addField(var='f1',
+                      value='Some text\non several\n\nlines')
+        self.check(msg, """
+          <message>
+            <x xmlns="jabber:x:data" type="form">
+              <field var="f1">
+                <value>Some text\non several\n\nlines</value>
+              </field>
+            </x>
+          </message>
+        """)
+
     def testSetValues(self):
         """Testing setting form values"""
 
@@ -117,7 +132,7 @@ class TestDataForms(SlixTest):
                 <value>b</value>
               </field>
             </x>
-          </message>""")
+          </message>""", use_values=False)
 
     def testSubmitType(self):
         """Test that setting type to 'submit' clears extra details"""