Implement mail tracker system

* Implement mail tracking

Signed-off-by: Victor "multun" Collod <victor.collod@prologin.org>

* Implement task merging

* Add a mail tracker title format pattern

* Autocomplete task names

* Fix comment display

* Track notification answers

* Add a socket timeout for the mail worker

A mail worker is a long running application. And sometimes, the IMAP server
just hangs for hours for no apparent reason. imaplib doesn't enable setting
a timeout, and setting it globally seems fine.

* Only validate the merge form when submitted

* Redirect to the new form when merging

* Prettier task edit UI

* Make task merging optional

* Test mail tracking

* Update documentation for mail tracking

* Update dependencies

* Add the TODO_COMMENT_CLASSES setting

* Fix dependencies install order

* Remove debug leftovers, improve documentation

* Fail on missing from_address

todo/models.py

@@ -1,13 +1,50 @@
 from __future__ import unicode_literals
 import datetime
+import textwrap
 
 from django.conf import settings
 from django.contrib.auth.models import Group
-from django.db import models
+from django.db import models, DEFAULT_DB_ALIAS
+from django.db.transaction import Atomic, get_connection
 from django.urls import reverse
 from django.utils import timezone
 
 
+class LockedAtomicTransaction(Atomic):
+    """
+    modified from https://stackoverflow.com/a/41831049
+    this is needed for safely merging
+
+    Does a atomic transaction, but also locks the entire table for any transactions, for the duration of this
+    transaction. Although this is the only way to avoid concurrency issues in certain situations, it should be used with
+    caution, since it has impacts on performance, for obvious reasons...
+    """
+
+    def __init__(self, *models, using=None, savepoint=None):
+        if using is None:
+            using = DEFAULT_DB_ALIAS
+        super().__init__(using, savepoint)
+        self.models = models
+
+    def __enter__(self):
+        super(LockedAtomicTransaction, self).__enter__()
+
+        # Make sure not to lock, when sqlite is used, or you'll run into problems while running tests!!!
+        if settings.DATABASES[self.using]["ENGINE"] != "django.db.backends.sqlite3":
+            cursor = None
+            try:
+                cursor = get_connection(self.using).cursor()
+                for model in self.models:
+                    cursor.execute(
+                        "LOCK TABLE {table_name}".format(
+                            table_name=model._meta.db_table
+                        )
+                    )
+            finally:
+                if cursor and not cursor.closed:
+                    cursor.close()
+
+
 class TaskList(models.Model):
     name = models.CharField(max_length=60)
     slug = models.SlugField(default="")
@@ -32,7 +69,10 @@ class Task(models.Model):
     completed = models.BooleanField(default=False)
     completed_date = models.DateField(blank=True, null=True)
     created_by = models.ForeignKey(
-        settings.AUTH_USER_MODEL, related_name="todo_created_by", on_delete=models.CASCADE
+        settings.AUTH_USER_MODEL,
+        null=True,
+        related_name="todo_created_by",
+        on_delete=models.CASCADE,
     )
     assigned_to = models.ForeignKey(
         settings.AUTH_USER_MODEL,
@@ -63,6 +103,17 @@ class Task(models.Model):
             self.completed_date = datetime.datetime.now()
         super(Task, self).save()
 
+    def merge_into(self, merge_target):
+        if merge_target.pk == self.pk:
+            raise ValueError("can't merge a task with self")
+
+        # lock the comments to avoid concurrent additions of comments after the
+        # update request. these comments would be irremediably lost because of
+        # the cascade clause
+        with LockedAtomicTransaction(Comment):
+            Comment.objects.filter(task=self).update(task=merge_target)
+            self.delete()
+
     class Meta:
         ordering = ["priority"]
 
@@ -73,14 +124,35 @@ class Comment(models.Model):
     a comment and change task details at the same time. Rolling our own since it's easy.
     """
 
-    author = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
+    author = models.ForeignKey(
+        settings.AUTH_USER_MODEL, on_delete=models.CASCADE, blank=True, null=True
+    )
     task = models.ForeignKey(Task, on_delete=models.CASCADE)
     date = models.DateTimeField(default=datetime.datetime.now)
+    email_from = models.CharField(max_length=320, blank=True, null=True)
+    email_message_id = models.TextField(blank=True, null=True)
+
     body = models.TextField(blank=True)
 
+    class Meta:
+        # an email should only appear once per task
+        unique_together = ("task", "email_message_id")
+
+    @property
+    def author_text(self):
+        if self.author is not None:
+            return str(self.author)
+
+        assert self.email_message_id is not None
+        return str(self.email_from)
+
+    @property
     def snippet(self):
+        body_snippet = textwrap.shorten(self.body, width=35, placeholder="...")
         # Define here rather than in __str__ so we can use it in the admin list_display
-        return "{author} - {snippet}...".format(author=self.author, snippet=self.body[:35])
+        return "{author} - {snippet}...".format(
+            author=self.author_text, snippet=body_snippet
+        )
 
     def __str__(self):
-        return self.snippet()
+        return self.snippet