✨(backend) implement 9-digit room PIN codes for SIP access

Enable users to join rooms via SIP telephony by:
- Dialing the SIP trunk number
- Entering the room's PIN followed by '#'

The PIN code needs to be generated before the LiveKit room is created,
allowing the owner to send invites to participants in advance.

With 9-digit PINs (10^9 combinations) and a large number of rooms
(e.g., 1M), collisions are inevitable. However, a retry mechanism reduces
the likelihood of multiple consecutive collisions.

Discussion points:
- The `while` loop should be reviewed. Should we add rate limiting
  for failed attempts?
- A systematic existence check before `INSERT` is more costly for a rare
  event and doesn't prevent race conditions, whereas retrying on integrity
  errors is more efficient overall.
- Should we add logging or monitoring to track and analyze collisions?

I tried balances performance and simplicity while ensuring the
robustness of the PIN generation process.

src/backend/core/migrations/0011_room_pin_code.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.1.4 on 2025-01-12 22:34
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0010_alter_resourceaccess_options_alter_user_options'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='room',
+            name='pin_code',
+            field=models.CharField(blank=True, help_text="Unique n-digit code that identifies this room. Automatically generated on creation. Displayed with '#' suffix.", max_length=100, null=True, unique=True, verbose_name='Room PIN code'),
+        ),
+    ]

src/backend/core/models.py

@@ -11,13 +11,15 @@ from django.contrib.auth import models as auth_models
 from django.contrib.auth.base_user import AbstractBaseUser
 from django.core import mail, validators
 from django.core.exceptions import PermissionDenied, ValidationError
-from django.db import models
+from django.db import IntegrityError, models
 from django.utils.functional import lazy
 from django.utils.text import capfirst, slugify
 from django.utils.translation import gettext_lazy as _
 
 from timezone_field import TimeZoneField
 
+from core import utils
+
 logger = getLogger(__name__)
 
 
@@ -361,13 +363,23 @@ class Room(Resource):
         primary_key=True,
     )
     slug = models.SlugField(max_length=100, blank=True, null=True, unique=True)
-
     configuration = models.JSONField(
         blank=True,
         default=dict,
         verbose_name=_("Visio room configuration"),
         help_text=_("Values for Visio parameters to configure the room."),
     )
+    pin_code = models.CharField(
+        max_length=100,
+        unique=True,
+        blank=True,
+        null=True,
+        verbose_name=_("Room PIN code"),
+        help_text=_(
+            "Unique n-digit code that identifies this room. Automatically generated on creation."
+            " Displayed with '#' suffix."
+        ),
+    )
 
     class Meta:
         db_table = "meet_room"
@@ -378,6 +390,23 @@ class Room(Resource):
     def __str__(self):
         return capfirst(self.name)
 
+    def save(self, *args, **kwargs):
+        """Generate a unique n-digit pin code for new rooms.
+
+        This uses the DB to ensure uniqueness of the code. Better than checking separately
+        with the DB and then saving, which introduces a race condition.
+        """
+
+        if self.pk or self.pin_code:
+            return super().save(*args, **kwargs)
+
+        while True:
+            try:
+                self.pin_code = utils.generate_pin_code(n=settings.ROOM_PIN_CODE_LENGTH)
+                return super().save(*args, **kwargs)
+            except IntegrityError:
+                continue
+
     def clean_fields(self, exclude=None):
         """
         Automatically generate the slug from the name and make sure it does not look like a UUID.

src/backend/core/utils.py

@@ -15,6 +15,11 @@ from django.conf import settings
 from livekit.api import AccessToken, VideoGrants
 
 
+def generate_pin_code(n: int) -> str:
+    """Generate a n-digit pin code"""
+    return f"{''.join([str(random.randint(0, 9)) for _ in range(n)])}"
+
+
 def generate_color(identity: str) -> str:
     """Generates a consistent HSL color based on a given identity string.
 

src/backend/meet/settings.py

@@ -483,6 +483,18 @@ class Base(Configuration):
     )
     BREVO_API_CONTACT_ATTRIBUTES = values.DictValue({"VISIO_USER": True})
 
+    # SIP Telephony
+    ROOM_PIN_CODE_LENGTH = values.PositiveIntegerValue(
+        9,  # this value cannot exceed 100 digits due to database constraints
+        environ_name="ROOM_PIN_CODE_LENGTH",
+        environ_prefix=None,
+    )
+    ROOM_PIN_CODE_GENERATION_MAX_RETRY = values.PositiveIntegerValue(
+        3,
+        environ_name="ROOM_PIN_CODE_GENERATION_MAX_RETRY",
+        environ_prefix=None,
+    )
+
     # pylint: disable=invalid-name
     @property
     def ENVIRONMENT(self):