notification email: Send followup_day2 email two days later.

This changes the followup_day2 emails delay from one day later to two days
later if it is getting delivered on any working days(i.e. Mon - Fri).
For Thursday it is compromised to next day as it would be too late to
postponed to Monday and for Friday it should be Monday.
At last actually, emails should send one hour before the above calculated so
that user can catch them when they are dealing with these kinds of stuff.
Fixes: #7078.

zerver/lib/notifications.py

@@ -23,7 +23,7 @@ from zerver.models import (
     Realm,
 )
 
-import datetime
+from datetime import timedelta, datetime
 from email.utils import formataddr
 import lxml.html
 import re
@@ -31,6 +31,7 @@ import subprocess
 import ujson
 import urllib
 from collections import defaultdict
+import pytz
 
 def one_click_unsubscribe_link(user_profile: UserProfile, email_type: str) -> str:
     """
@@ -464,6 +465,25 @@ def log_digest_event(msg: Text) -> None:
     logging.basicConfig(filename=settings.DIGEST_LOG_PATH, level=logging.INFO)
     logging.info(msg)
 
+def followup_day2_email_delay(user: UserProfile) -> timedelta:
+    days_to_delay = 2
+    user_tz = user.timezone
+    if user_tz == '':
+        user_tz = 'UTC'
+    signup_day = user.date_joined.astimezone(pytz.timezone(user_tz)).isoweekday()
+    if signup_day == 5:
+        # If the day is Friday then delay should be till Monday
+        days_to_delay = 3
+    elif signup_day == 4:
+        # If the day is Thursday then delay should be till Friday
+        days_to_delay = 1
+
+    # The delay should be 1 hour before the above calculated delay as
+    # our goal is to maximize the chance that this email is near the top
+    # of the user's inbox when the user sits down to deal with their inbox,
+    # or comes in while they are dealing with their inbox.
+    return timedelta(days=days_to_delay, hours=-1)
+
 def enqueue_welcome_emails(user: UserProfile) -> None:
     from zerver.context_processors import common_context
     if settings.WELCOME_EMAIL_SENDER is not None:
@@ -487,7 +507,7 @@ def enqueue_welcome_emails(user: UserProfile) -> None:
         from_address=from_address, context=context)
     send_future_email(
         "zerver/emails/followup_day2", user.realm, to_user_id=user.id, from_name=from_name,
-        from_address=from_address, context=context, delay=datetime.timedelta(days=1))
+        from_address=from_address, context=context, delay=followup_day2_email_delay(user))
 
 def convert_html_to_markdown(html: Text) -> Text:
     # On Linux, the tool installs as html2markdown, and there's a command called

zerver/tests/test_signup.py

@@ -47,7 +47,7 @@ from zerver.lib.actions import (
 from zerver.lib.mobile_auth_otp import xor_hex_strings, ascii_to_hex, \
     otp_encrypt_api_key, is_valid_otp, hex_to_ascii, otp_decrypt_api_key
 from zerver.lib.notifications import enqueue_welcome_emails, \
-    one_click_unsubscribe_link
+    one_click_unsubscribe_link, followup_day2_email_delay
 from zerver.lib.subdomains import is_root_domain_available
 from zerver.lib.test_helpers import find_key_by_email, queries_captured, \
     HostRequestMock, load_subdomain_token
@@ -67,6 +67,7 @@ from typing import Any, Dict, List, Optional, Set, Text
 
 import urllib
 import os
+import pytz
 
 class RedirectAndLogIntoSubdomainTestCase(ZulipTestCase):
     def test_cookie_data(self) -> None:
@@ -2598,3 +2599,25 @@ class LoginOrAskForRegistrationTestCase(ZulipTestCase):
         self.assertEqual(user_id, user_profile.id)
         self.assertEqual(response.status_code, 302)
         self.assertIn('http://zulip.testserver', response.url)
+
+class FollowupEmailTest(ZulipTestCase):
+    def test_followup_day2_email(self) -> None:
+        user_profile = self.example_user('hamlet')
+        # Test date_joined == Sunday
+        user_profile.date_joined = datetime.datetime(2018, 1, 7, 1, 0, 0, 0, pytz.UTC)
+        self.assertEqual(followup_day2_email_delay(user_profile), datetime.timedelta(days=2, hours=-1))
+        # Test date_joined == Tuesday
+        user_profile.date_joined = datetime.datetime(2018, 1, 2, 1, 0, 0, 0, pytz.UTC)
+        self.assertEqual(followup_day2_email_delay(user_profile), datetime.timedelta(days=2, hours=-1))
+        # Test date_joined == Thursday
+        user_profile.date_joined = datetime.datetime(2018, 1, 4, 1, 0, 0, 0, pytz.UTC)
+        self.assertEqual(followup_day2_email_delay(user_profile), datetime.timedelta(days=1, hours=-1))
+        # Test date_joined == Friday
+        user_profile.date_joined = datetime.datetime(2018, 1, 5, 1, 0, 0, 0, pytz.UTC)
+        self.assertEqual(followup_day2_email_delay(user_profile), datetime.timedelta(days=3, hours=-1))
+
+        # Time offset of America/Phoenix is -07:00
+        user_profile.timezone = 'America/Phoenix'
+        # Test date_joined == Friday in UTC, but Thursday in the user's timezone
+        user_profile.date_joined = datetime.datetime(2018, 1, 5, 1, 0, 0, 0, pytz.UTC)
+        self.assertEqual(followup_day2_email_delay(user_profile), datetime.timedelta(days=1, hours=-1))