-s/--setting x y gets merged into datasette.yml, refs #2143, #2156

This change updates the `-s/--setting` option to `datasette serve` to allow it to be used to set arbitrarily complex nested settings in a way that is compatible with the new `-c datasette.yml` work happening in:
- #2143

It will enable things like this:
```
datasette data.db --setting plugins.datasette-ripgrep.path "/home/simon/code"
```
For the moment though it just affects [settings](https://docs.datasette.io/en/1.0a4/settings.html) - so you can do this:
```
datasette data.db --setting settings.sql_time_limit_ms 3500
```
I've also implemented a backwards compatibility mechanism, so if you use it this way (the old way):
```
datasette data.db --setting sql_time_limit_ms 3500
```
It will notice that the setting you passed is one of Datasette's core settings, and will treat that as if you said `settings.sql_time_limit_ms` instead.

datasette/cli.py

@@ -31,6 +31,7 @@ from .utils import (
     ConnectionProblem,
     SpatialiteConnectionProblem,
     initial_path_for_datasette,
+    pairs_to_nested_config,
     temporary_docker_directory,
     value_as_boolean,
     SpatialiteNotFound,
@@ -56,35 +57,27 @@ class Setting(CompositeParamType):
 
     def convert(self, config, param, ctx):
         name, value = config
-        if name not in DEFAULT_SETTINGS:
-            msg = (
-                OBSOLETE_SETTINGS.get(name)
-                or f"{name} is not a valid option (--help-settings to see all)"
-            )
-            self.fail(
-                msg,
-                param,
-                ctx,
-            )
-            return
-        # Type checking
-        default = DEFAULT_SETTINGS[name]
-        if isinstance(default, bool):
-            try:
-                return name, value_as_boolean(value)
-            except ValueAsBooleanError:
-                self.fail(f'"{name}" should be on/off/true/false/1/0', param, ctx)
-                return
-        elif isinstance(default, int):
-            if not value.isdigit():
-                self.fail(f'"{name}" should be an integer', param, ctx)
-                return
-            return name, int(value)
-        elif isinstance(default, str):
-            return name, value
-        else:
-            # Should never happen:
-            self.fail("Invalid option")
+        if name in DEFAULT_SETTINGS:
+            # For backwards compatibility with how this worked prior to
+            # Datasette 1.0, we turn bare setting names into setting.name
+            # Type checking for those older settings
+            default = DEFAULT_SETTINGS[name]
+            name = "settings.{}".format(name)
+            if isinstance(default, bool):
+                try:
+                    return name, "true" if value_as_boolean(value) else "false"
+                except ValueAsBooleanError:
+                    self.fail(f'"{name}" should be on/off/true/false/1/0', param, ctx)
+            elif isinstance(default, int):
+                if not value.isdigit():
+                    self.fail(f'"{name}" should be an integer', param, ctx)
+                return name, value
+            elif isinstance(default, str):
+                return name, value
+            else:
+                # Should never happen:
+                self.fail("Invalid option")
+        return name, value
 
 
 def sqlite_extensions(fn):
@@ -425,7 +418,7 @@ def uninstall(packages, yes):
     "--setting",
     "settings",
     type=Setting(),
-    help="Setting, see docs.datasette.io/en/stable/settings.html",
+    help="nested.key, value setting to use in Datasette configuration",
     multiple=True,
 )
 @click.option(
@@ -547,6 +540,13 @@ def serve(
     if config:
         config_data = parse_metadata(config.read())
 
+    config_data = config_data or {}
+
+    # Merge in settings from -s/--setting
+    if settings:
+        settings_updates = pairs_to_nested_config(settings)
+        config_data.update(settings_updates)
+
     kwargs = dict(
         immutables=immutable,
         cache_headers=not reload,
@@ -558,7 +558,7 @@ def serve(
         template_dir=template_dir,
         plugins_dir=plugins_dir,
         static_mounts=static,
-        settings=dict(settings),
+        settings=None,  # These are passed in config= now
         memory=memory,
         secret=secret,
         version_note=version_note,