flake.nix: start synapse automatically, add space usage warning (#15613)

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

changelog.d/15613.doc

+Warn users that at least 3.75GB of space is needed for the nix Synapse development environment.

flake.nix

-# A nix flake that sets up a complete Synapse development environment. Dependencies
+# A Nix flake that sets up a complete Synapse development environment. Dependencies
 # for the SyTest (https://github.com/matrix-org/sytest) and Complement
 # (https://github.com/matrix-org/complement) Matrix homeserver test suites are also
 # installed automatically.
 #
-# You must have already installed nix (https://nixos.org) on your system to use this.
-# nix can be installed on Linux or MacOS; NixOS is not required. Windows is not
-# directly supported, but nix can be installed inside of WSL2 or even Docker
+# You must have already installed Nix (https://nixos.org) on your system to use this.
+# Nix can be installed on Linux or MacOS; NixOS is not required. Windows is not
+# directly supported, but Nix can be installed inside of WSL2 or even Docker
 # containers. Please refer to https://nixos.org/download for details.
 #
 # You must also enable support for flakes in Nix. See the following for how to
 # do so permanently: https://nixos.wiki/wiki/Flakes#Enable_flakes
 #
+# Be warned: you'll need over 3.75 GB of free space to download all the dependencies.
+#
 # Usage:
 #
-# With nix installed, navigate to the directory containing this flake and run
+# With Nix installed, navigate to the directory containing this flake and run
 # `nix develop --impure`. The `--impure` is necessary in order to store state
 # locally from "services", such as PostgreSQL and Redis.
 #
 # You should now be dropped into a new shell with all programs and dependencies
 # availabile to you!
 #
-# You can start up pre-configured, local PostgreSQL and Redis instances by
+# You can start up pre-configured local Synapse, PostgreSQL and Redis instances by
 # running: `devenv up`. To stop them, use Ctrl-C.
 #
-# A PostgreSQL database called 'synapse' will be set up for you, along with
-# a PostgreSQL user named 'synapse_user'.
-# The 'host' can be found by running `echo $PGHOST` with the development
-# shell activated. Use these values to configure your Synapse to connect
-# to the local PostgreSQL database. You do not need to specify a password.
-# https://matrix-org.github.io/synapse/latest/postgres
-#
 # All state (the venv, postgres and redis data and config) are stored in
 # .devenv/state. Deleting a file from here and then re-entering the shell
 # will recreate these files from scratch.
@@ -66,7 +61,7 @@
         let
           pkgs = nixpkgs.legacyPackages.${system};
         in {
-          # Everything is configured via devenv - a nix module for creating declarative
+          # Everything is configured via devenv - a Nix module for creating declarative
           # developer environments. See https://devenv.sh/reference/options/ for a list
           # of all possible options.
           default = devenv.lib.mkShell {
@@ -153,11 +148,39 @@
                 # Redis is needed in order to run Synapse in worker mode.
                 services.redis.enable = true;
 
+                # Configure and start Synapse. Before starting Synapse, this shell code:
+                #  * generates a default homeserver.yaml config file if one does not exist, and
+                #  * ensures a directory containing two additional homeserver config files exists;
+                #    one to configure using the development environment's PostgreSQL as the
+                #    database backend and another for enabling Redis support.
+                process.before = ''
+                  python -m synapse.app.homeserver -c homeserver.yaml --generate-config --server-name=synapse.dev --report-stats=no
+                  mkdir -p homeserver-config-overrides.d
+                  cat > homeserver-config-overrides.d/database.yaml << EOF
+                  ## Do not edit this file. This file is generated by flake.nix
+                  database:
+                    name: psycopg2
+                    args:
+                      user: synapse_user
+                      database: synapse
+                      host: $PGHOST
+                      cp_min: 5
+                      cp_max: 10
+                  EOF
+                  cat > homeserver-config-overrides.d/redis.yaml << EOF
+                  ## Do not edit this file. This file is generated by flake.nix
+                  redis:
+                    enabled: true
+                  EOF
+                '';
+                # Start synapse when `devenv up` is run.
+                processes.synapse.exec = "poetry run python -m synapse.app.homeserver -c homeserver.yaml --config-directory homeserver-config-overrides.d";
+
                 # Define the perl modules we require to run SyTest.
                 #
                 # This list was compiled by cross-referencing https://metacpan.org/
                 # with the modules defined in './cpanfile' and then finding the
-                # corresponding nix packages on https://search.nixos.org/packages.
+                # corresponding Nix packages on https://search.nixos.org/packages.
                 #
                 # This was done until `./install-deps.pl --dryrun` produced no output.
                 env.PERL5LIB = "${with pkgs.perl536Packages; makePerlPath [