Validate content length on broadcast API

The maximum content count of a broadcast varies depending on its
encoding, so we can’t simply validate it against a schema. This commit
moves to using the validation from `notifications-utils`, and raising a
custom error response.

tests/app/v2/broadcast/sample_cap_xml_documents.py

@@ -198,3 +198,40 @@ CANCEL = """
       </info>
     </alert>
 """
+
+WITH_PLACEHOLDER_FOR_CONTENT = """
+    <alert xmlns="urn:oasis:names:tc:emergency:cap:1.2">
+        <identifier>50385fcb0ab7aa447bbd46d848ce8466E</identifier>
+        <sender>www.gov.uk/environment-agency</sender>
+        <sent>2020-02-16T23:01:13-00:00</sent>
+        <status>Actual</status>
+        <msgType>Alert</msgType>
+        <source>Flood warning service</source>
+        <scope>Public</scope>
+        <references>www.gov.uk/environment-agency,4f6d28b10ab7aa447bbd46d85f1e9effE,2020-02-16T19:20:03+00:00</references>
+        <info>
+            <language>en-GB</language>
+            <category>Met</category>
+            <event>053/055 Issue Severe Flood Warning EA</event>
+            <urgency>Immediate</urgency>
+            <severity>Severe</severity>
+            <certainty>Likely</certainty>
+            <expires>2020-02-26T23:01:14-00:00</expires>
+            <senderName>Environment Agency</senderName>
+            <description>{}</description>
+            <web>https://flood-warning-information.service.gov.uk</web>
+            <contact>0345 988 1188</contact>
+            <area>
+                <areaDesc>River Steeping in Wainfleet All Saints</areaDesc>
+                <polygon>53.10569,0.24453 53.10593,0.24430 53.10601,0.24375 53.10615,0.24349 53.10629,0.24356 53.10656,0.24336 53.10697,0.24354 53.10684,0.24298 53.10694,0.24264 53.10721,0.24302 53.10752,0.24310 53.10777,0.24308 53.10805,0.24320 53.10803,0.24187 53.10776,0.24085 53.10774,0.24062 53.10702,0.24056 53.10679,0.24088 53.10658,0.24071 53.10651,0.24049 53.10656,0.24022 53.10642,0.24022 53.10632,0.24052 53.10629,0.24082 53.10612,0.24093 53.10583,0.24133 53.10564,0.24178 53.10541,0.24282 53.10569,0.24453</polygon>
+                <geocode>
+                    <valueName>TargetAreaCode</valueName>
+                    <value>053FWFSTEEP4</value>
+                </geocode>
+            </area>
+        </info>
+    </alert>
+"""
+
+LONG_GSM7 = WITH_PLACEHOLDER_FOR_CONTENT.format('a' * 1396)
+LONG_UCS2 = WITH_PLACEHOLDER_FOR_CONTENT.format('ŵ' * 616)

tests/app/v2/broadcast/test_post_broadcast.py

@@ -191,3 +191,34 @@ def test_unsupported_message_types_400(
     } in (
         json.loads(response.get_data(as_text=True))['errors']
     )
+
+
+@pytest.mark.parametrize('xml_document, expected_error', (
+    (sample_cap_xml_documents.LONG_UCS2, (
+        'description must be 615 characters or fewer (because it '
+        'could not be GSM7 encoded)'
+    )),
+    (sample_cap_xml_documents.LONG_GSM7, (
+        'description must be 1,395 characters or fewer'
+    )),
+))
+def test_content_too_long_returns_400(
+    client,
+    sample_broadcast_service,
+    xml_document,
+    expected_error,
+):
+    auth_header = create_authorization_header(service_id=sample_broadcast_service.id)
+    response = client.post(
+        path='/v2/broadcast',
+        data=xml_document,
+        headers=[('Content-Type', 'application/cap+xml'), auth_header],
+    )
+
+    assert json.loads(response.get_data(as_text=True)) == {
+        'errors': [{
+            'error': 'ValidationError',
+            'message': expected_error,
+        }],
+        'status_code': 400,
+    }