Introduce flask-less CLI interface (e.g. for programmatic DB access) (#1909)

Resolves #1690.
(Successor of tiny-pilot/tinypilot-pro#1602.)

This PR introduces a new, internal Flask-less CLI entrypoint to our app,
for programmatic access of certain features (mainly DB-related).

As a first use-case, it migrates the `streaming-mode` CLI command,
mainly to demonstrate how this is intended to be used. We’ll soon [add
another command for retrieving the serial
settings](tiny-pilot/tinypilot-pro#1332 (comment)).
In a subsequent step, we can also migrate [the last remaining command
for activating a
license](https://github.com/tiny-pilot/tinypilot-pro/blob/b93dc93f4f2581118325ffe9eaa30d7f5fefae26/app/flask_cli.py#L13-L21)
and thereby sunset the Flask CLI entrypoint altogether. (I’d create a
separate ticket for that.)

## Notes

- Although the footprint of the CLI functionality is still quite small
(and is unlikely to grow substantially in the near-to-mid future), I
thought that a separate Python module (directory) was warranted
(`app/cli`). That way, we hopefully have a clearer separation from the
web app, and we are also a bit more flexible to structure the code
nicely.
- I introduced a `@command` annotation to make it convenient to register
commands. My thinking is not to go overboard with this, though – e.g.,
I’d not expand this mechanism to include arg-parsing etc., as it would
then probably make more sense to rather keep the Flask CLI or introduce
another framework. The main intention of `@command` is to have all
command-related code closely together, as opposed to having to define
the function in one place and then “register” it in a second. Since the
whole CLI is more of an internal helper utility, I think it’s fair to
keep the whole structure basic.

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---------

Co-authored-by: Jan Heuermann <[email protected]>

Author: jotaen4tinypilot

app/cli.py

@@ -0,0 +1,9 @@
+import db.settings
+from cli.registry import command
+
+
+@command('streaming-mode')
+def streaming_mode(_args):
+    """Prints the currently applicable streaming mode."""
+    mode = db.settings.Settings().get_streaming_mode().value
+    print(mode)

app/cli/__init__.py

@@ -0,0 +1,32 @@
+"""Internal entrypoint for accessing certain app features from the CLI.
+
+This package is an internal utility that we use to programmatically access
+certain features of our app without needing to initialize a full Flask
+application context. (The latter would be quite cumbersome and slow.)
+
+The structure of the CLI interface is deliberately minimal, as we don’t expose
+it to end-users directly, but if at all only via dedicated wrapper scripts.
+"""
+
+import sys
+
+import cli.commands  # noqa: F401
+import cli.registry
+
+
+def main():
+    if len(sys.argv) < 2:
+        print('Missing required subcommand!', file=sys.stderr)
+        sys.exit(1)
+
+    try:
+        command = cli.registry.COMMANDS[sys.argv[1]]
+    except KeyError:
+        print('No such subcommand!', file=sys.stderr)
+        sys.exit(1)
+
+    command(sys.argv[1:])
+
+
+if __name__ == '__main__':
+    main()

app/cli/commands.py

@@ -0,0 +1,10 @@
+COMMANDS = {}
+
+
+def command(command_name):
+
+    def decorator(func):
+        COMMANDS[command_name] = func
+        return func
+
+    return decorator

app/cli/main.py

@@ -7,25 +7,42 @@
 
 
 def get():
-    """Returns a database connection (sqlite3.dbapi2.connection)."""
-    # Keep in mind that Flask only caches the connection object on a per-request
-    # basis, and not throughout the entire runtime of the server.
-    connection = _get_flask_db()
+    """Returns a connection to the SQlite database.
+
+    Within a Flask app context, this function applies a caching mechanism to
+    avoid redundant initialization overhead. Outside a Flask app context, it
+    creates a fresh DB connection on every invocation.
+
+    Returns:
+        sqlite3.dbapi2.connection
+    """
+    if not flask.has_app_context():
+        return db.store.create_or_open(_DB_PATH)
+
+    connection = _get_cached_flask_db()
     if connection is None:
         connection = db.store.create_or_open(_DB_PATH)
-        _set_flask_db(connection)
+        _cache_flask_db(connection)
     return connection
 
 
 def close():
-    connection = _get_flask_db()
+    """Closes a potentially cached DB connection.
+
+    Noop if no DB connection was cached or if invocation context is not within
+    Flask.
+    """
+    if not flask.has_app_context():
+        return
+
+    connection = _get_cached_flask_db()
     if connection is not None:
         connection.close()
 
 
-def _get_flask_db():
+def _get_cached_flask_db():
     return getattr(flask.g, '_database', None)
 
 
-def _set_flask_db(flask_db):
+def _cache_flask_db(flask_db):
     flask.g._database = flask_db  # pylint: disable=protected-access

app/cli/registry.py

@@ -11,7 +11,6 @@
 # app-wide logger class before any other module loads it.
 import log
 import api
-import cli
 import json_response
 import license_notice
 import secret_key
@@ -49,7 +48,6 @@
 app.register_blueprint(api.api_blueprint)
 app.register_blueprint(license_notice.blueprint)
 app.register_blueprint(views.views_blueprint)
-app.register_blueprint(cli.cli_blueprint)
 
 
 @app.errorhandler(flask_wtf.csrf.CSRFError)

app/db_connection.py

@@ -166,7 +166,7 @@ print_info "Checking for voltage issues..."
 print_info "Checking TinyPilot streaming mode..."
 {
   echo "Streaming mode"
-  echo "Selected mode: $(runuser tinypilot --command '/opt/tinypilot/scripts/streaming-mode')"
+  echo "Selected mode: $(runuser tinypilot --command '/opt/tinypilot/scripts/cli streaming-mode')"
   printf "Current mode: "
   # H264 mode is considered active when the last Janus video log line contains
   # "Memsink opened; reading frames".

app/main.py

@@ -1,6 +1,11 @@
 #!/bin/bash
 
-# Prints TinyPilot's preferred video streaming mode, either H264 or MJPEG.
+# Helper script to invoke the CLI entrypoint of the TinyPilot app.
+# See `app/cli/main.py` for more info, including the available subcommands.
+#
+# Example of usage:
+#     ./scripts/cli streaming-mode
+
 
 # Exit on first failure.
 set -e
@@ -18,7 +23,6 @@ fi
 SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )"
 readonly SCRIPT_DIR
 cd "${SCRIPT_DIR}/.."
-. ./venv/bin/activate
 
-export FLASK_APP='app/main.py'
-flask cli streaming-mode
+. venv/bin/activate
+python -m app.cli.main "${@:1}"