From d11fd2cbaa6b31933b1319f81b5d1520726cb0b6 Mon Sep 17 00:00:00 2001
From: Simon Willison <swillison@gmail.com>
Date: Mon, 25 Nov 2019 20:57:52 -0800
Subject: [PATCH] Experimental load_metadata() hook, refs #357

---
 datasette/cli.py              | 10 +++-------
 datasette/hookspecs.py        |  5 +++++
 datasette/plugins.py          |  1 +
 docs/datasette-serve-help.txt |  3 ++-
 docs/plugins.rst              |  9 +++++++++
 5 files changed, 20 insertions(+), 8 deletions(-)

diff --git a/datasette/cli.py b/datasette/cli.py
index c2c03251..d8278d5d 100644
--- a/datasette/cli.py
+++ b/datasette/cli.py
@@ -265,8 +265,8 @@ def package(
 @click.option(
     "-m",
     "--metadata",
-    type=click.File(mode="r"),
-    help="Path to JSON file containing license/source metadata",
+    type=str,
+    help="Path to JSON file containing license/source metadata, or input to a metadata plugin",
 )
 @click.option(
     "--template-dir",
@@ -329,16 +329,12 @@ def serve(
 
         reloader = hupper.start_reloader("datasette.cli.serve")
         reloader.watch_files(files)
-        if metadata:
-            reloader.watch_files([metadata.name])
 
     inspect_data = None
     if inspect_file:
         inspect_data = json.load(open(inspect_file))
 
-    metadata_data = None
-    if metadata:
-        metadata_data = json.loads(metadata.read())
+    metadata_data = pm.hook.load_metadata(metadata_value=metadata)
 
     click.echo(
         "Serve! files={} (immutables={}) on port {}".format(files, immutable, port)
diff --git a/datasette/hookspecs.py b/datasette/hookspecs.py
index 3c6726b7..c16ab4f9 100644
--- a/datasette/hookspecs.py
+++ b/datasette/hookspecs.py
@@ -58,3 +58,8 @@ def register_output_renderer(datasette):
 @hookspec
 def register_facet_classes():
     "Register Facet subclasses"
+
+
+@hookspec(firstresult=True)
+def load_metadata(metadata_value):  # , datasette):
+    "Return metadata dictionary loaded from metadata_value"
diff --git a/datasette/plugins.py b/datasette/plugins.py
index bf3735dc..de87cb4c 100644
--- a/datasette/plugins.py
+++ b/datasette/plugins.py
@@ -8,6 +8,7 @@ DEFAULT_PLUGINS = (
     "datasette.publish.now",
     "datasette.publish.cloudrun",
     "datasette.facets",
+    "datasette.default_metadata",
 )
 
 pm = pluggy.PluginManager("datasette")
diff --git a/docs/datasette-serve-help.txt b/docs/datasette-serve-help.txt
index e71e177d..30eba5aa 100644
--- a/docs/datasette-serve-help.txt
+++ b/docs/datasette-serve-help.txt
@@ -17,7 +17,8 @@ Options:
   --cors                    Enable CORS by serving Access-Control-Allow-Origin: *
   --load-extension PATH     Path to a SQLite extension to load
   --inspect-file TEXT       Path to JSON file created using "datasette inspect"
-  -m, --metadata FILENAME   Path to JSON file containing license/source metadata
+  -m, --metadata TEXT       Path to JSON file containing license/source metadata, or
+                            input to a metadata plugin
   --template-dir DIRECTORY  Path to directory containing custom templates
   --plugins-dir DIRECTORY   Path to directory containing custom plugins
   --static MOUNT:DIRECTORY  Serve static files from this directory at /MOUNT/...
diff --git a/docs/plugins.rst b/docs/plugins.rst
index 9bceb961..c2795870 100644
--- a/docs/plugins.rst
+++ b/docs/plugins.rst
@@ -813,3 +813,12 @@ This example plugin adds a ``x-databases`` HTTP header listing the currently att
                 await app(scope, recieve, wrapped_send)
             return add_x_databases_header
         return wrap_with_databases_header
+
+.. _plugin_load_metadata:
+
+load_metadata(metadata_value)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Return a Python dictionary of metadata configuration values. The ``metadata_value`` argument will be the value passed to the ``-m`` or ``--metadata`` argument to ``datasette serve``.
+
+Only one plugin can implement this at a time. The first registered plugin will get to provide the metadata dictionary, while all other plugins will be ignored.