Validate length of broadcast content

Depends on:
- [ ] https://github.com/alphagov/notifications-utils/pull/826/files

Adds error messages for when the content of a broadcast template is too
long.

The error message is explicit when this is cause by non-GSM characters.
We may not want to expose this complexity to our users, but it’s useful
for now while we’re testing things out.

app/main/forms.py

@@ -40,6 +40,7 @@ from wtforms.validators import URL, DataRequired, Length, Optional, Regexp
 
 from app import format_thousands
 from app.main.validators import (
+    BroadcastLength,
     CommonlyUsedPassword,
     CsvFileValidator,
     DoesNotStartWithDoubleZero,
@@ -1324,6 +1325,7 @@ class BroadcastTemplateForm(SMSTemplateForm):
     def validate_template_content(self, field):
         OnlySMSCharacters(template_type='broadcast')(None, field)
         NoPlaceholders()(None, field)
+        BroadcastLength()(None, field)
 
 
 class LetterAddressForm(StripWhitespaceForm):

app/main/validators.py

@@ -7,6 +7,7 @@ from notifications_utils.recipients import (
     validate_email_address,
 )
 from notifications_utils.sanitise_text import SanitiseSMS
+from notifications_utils.template import BroadcastMessageTemplate
 from wtforms import ValidationError
 
 from app.main._commonly_used_passwords import commonly_used_passwords
@@ -120,6 +121,28 @@ class NoPlaceholders:
             raise ValidationError(self.message)
 
 
+class BroadcastLength:
+
+    def __call__(self, form, field):
+        template = BroadcastMessageTemplate({
+            'template_type': 'broadcast',
+            'content': field.data,
+        })
+
+        if template.content_too_long:
+            non_gsm_characters = list(sorted(template.non_gsm_characters))
+            if non_gsm_characters:
+                raise ValidationError(
+                    f'Content must be {template.max_content_count:,.0f} '
+                    f'characters or fewer because it contains '
+                    f'{formatted_list(non_gsm_characters, conjunction="and", before_each="", after_each="")}'
+                )
+            raise ValidationError(
+                f'Content must be {template.max_content_count:,.0f} '
+                f'characters or fewer'
+            )
+
+
 class LettersNumbersFullStopsAndUnderscoresOnly:
 
     regex = re.compile(r'^[a-zA-Z0-9\s\._]+$')