Move dev DB creation to a separate process

Context
-------

At startup (when you run `make dev`) Via's Gunicorn processes attempt to
create the DB tables if they don't already exist. This is done in dev
only, not in production:

    # via/db.py

    def includeme(config):
        ...

        if config.registry.settings.get("dev"):
            # Create the database tables if they don't already exist.
            Base.metadata.create_all(engine)

Problem
-------

Via actually has four Gunicorn workers in dev (see the `workers = 4` in
`conf/gunicorn/dev.conf.py`). All 4 try to create the DB tables at the
same time and 3 of them crash, printing out error messages.

The issue is kind of difficult to reproduce:

1. You need to actually have a model, so you won't be able to produce
   the issue on the `db` branch without changes. See
   #1072 and
   #1093 for branches on which you
   can reproduce the issue.

2. The model's table needs to actually be missing from the DB table. You
   can do that by running this command:

       make services args=down && make services && make dev

3. Since the issue is a race condition you won't be able to reproduce it
   reliably, but if you run the above command several times you should
   see it.

Solution
--------

This commit moves the `sqlalchemy.create_all()` call out of the Gunicorn
worker process and instead adds a separate `make_db` process to the list
of processes that `supervisord-dev.conf` runs when you run `make dev`.
This new `make_db` process starts up, calls `create_all()`, and then
exits, and since it's a singular process there's no collision.

.cookiecutter/includes/conf/supervisord-dev.conf.json

@@ -1,5 +1,9 @@
 {
   "programs": {
+    "make_db": {
+      "command": "bin/make_db",
+      "startsecs": "0"
+    },
     "web": {
       "command": "gunicorn -c conf/gunicorn/dev.conf.py --paste conf/development.ini"
     },

bin/make_db

@@ -1,5 +1,35 @@
-"""Initialize the dev environment's DB."""
 #!/usr/bin/env python3
-import sys
+"""Initialize the dev environment's DB."""
+import alembic.command
+import alembic.config
 from pyramid.paster import bootstrap
-bootstrap("conf/development.ini")
+from sqlalchemy import text
+from sqlalchemy.exc import ProgrammingError
+
+from via.db import Base, create_engine
+
+
+def is_stamped(engine) -> bool:
+    """Return True if the DB is stamped with an Alembic revision ID."""
+    with engine.connect() as connection:
+        try:
+            if connection.execute(text("select * from alembic_version")).first():
+                return True
+        except ProgrammingError:
+            pass
+
+    return False
+
+
+def main():
+    with bootstrap("conf/development.ini") as env:
+        settings = env["registry"].settings
+        engine = create_engine(settings)
+
+        if not is_stamped(engine):
+            Base.metadata.create_all(engine)
+            alembic.command.stamp(alembic.config.Config("conf/alembic.ini"), "head")
+
+
+if __name__ == "__main__":
+    main()

conf/supervisord-dev.conf

@@ -2,6 +2,14 @@
 nodaemon=true
 silent=true
 
+[program:make_db]
+command=bin/make_db
+stdout_events_enabled=true
+stderr_events_enabled=true
+stopsignal=KILL
+stopasgroup=true
+startsecs=0
+
 [program:web]
 command=gunicorn -c conf/gunicorn/dev.conf.py --paste conf/development.ini
 stdout_events_enabled=true

via/db.py

@@ -1,8 +1,6 @@
-import alembic.command
-import alembic.config
+import sqlalchemy
 import zope.sqlalchemy
-from sqlalchemy import MetaData, create_engine, text
-from sqlalchemy.exc import ProgrammingError
+from sqlalchemy import MetaData
 from sqlalchemy.orm import DeclarativeBase, MappedAsDataclass, Session
 
 
@@ -18,23 +16,13 @@ class Base(MappedAsDataclass, DeclarativeBase):
     )
 
 
-def stamp(engine):  # pragma: no cover
-    """If the DB isn't already stamped then stamp it with the latest revision."""
-    stamped = False
-
-    with engine.connect() as connection:
-        try:
-            if connection.execute(text("select * from alembic_version")).first():
-                stamped = True
-        except ProgrammingError:
-            pass
-
-    if not stamped:
-        alembic.command.stamp(alembic.config.Config("conf/alembic.ini"), "head")
+def create_engine(settings):  # pragma: no cover
+    """Return a SQLAlchemy engine."""
+    return sqlalchemy.create_engine(settings["database_url"])
 
 
 def includeme(config):  # pragma: no cover
-    engine = create_engine(config.registry.settings["database_url"])
+    engine = create_engine(config.registry.settings)
 
     def db_session(request):
         """Return the SQLAlchemy session for the given request."""
@@ -47,11 +35,6 @@ def db_session(request):
 
         return session
 
-    if config.registry.settings.get("dev"):
-        # Create the database tables if they don't already exist.
-        Base.metadata.create_all(engine)
-        stamp(engine)
-
     # Make the SQLAlchemy session available as `request.db`.
     # `reify=True` means it'll create only one session per request.
     config.add_request_method(db_session, name="db", reify=True)