feat: run all migrations in a transaction (#10966)

Updates coder/customers#365

This PR updates our migration framework to run all migrations in a single transaction. This is the same behavior we had in v1 and ensures that failed migrations don't bring the whole deployment down. If a migration fails now, it will automatically be rolled back to the previous version, allowing the deployment to continue functioning.

coderd/database/migrations/create_migration.sh

@@ -7,6 +7,34 @@
 
 set -euo pipefail
 
+cat <<EOF
+
+WARNING: Migrations now all run in a single transaction. This makes upgrades
+safer, but means that 'ALTER TYPE resource_type ADD VALUE' cannot be used if the
+enum value needs to be referenced in another migration.
+
+This also means you should not use "BEGIN;" and "COMMIT;" in your migrations, as
+everything is already in a migration.
+
+An example way of the proper way to add an enum value:
+
+CREATE TYPE new_logintype AS ENUM (
+	'password',
+	'github',
+	'oidc',
+	'token' -- this is our new value
+);
+
+ALTER TABLE users
+	ALTER COLUMN login_type DROP DEFAULT, -- if the column has a default, it must be dropped first
+	ALTER COLUMN login_type TYPE new_logintype USING (login_type::text::new_logintype), -- converts the old enum until the new enum using text as an intermediary
+	ALTER COLUMN login_type SET DEFAULT 'password'::new_logintype; -- re-add the default using the new enum
+
+DROP TYPE login_type;
+ALTER TYPE new_logintype RENAME TO login_type;
+
+EOF
+
 SCRIPT_DIR=$(dirname "${BASH_SOURCE[0]}")
 (
 	cd "$SCRIPT_DIR"