docs: Use the new GstPluginsPath.json to have the right plugin path

When inspecting plugins to generate the json cache file. Otherwise
when we are not in the uninstalled env and using `gst-build` plugins
with dependency might fail/throw warning, etc..

docs/gst-plugins-doc-cache-generator.py

@@ -35,6 +35,7 @@ except ImportError:  # python <3.3
 # make it happen. For properties, the best way is to use th
 # GST_PARAM_DOC_SHOW_DEFAULT flag.
 UNSTABLE_VALUE = "unstable-values"
+BUILD_ROOT = "@BUILD_ROOT@"
 
 
 def dict_recursive_update(d, u):
@@ -85,8 +86,26 @@ if __name__ == "__main__":
         cmd.append(plugin_path)
         gst_plugins_paths.append(os.path.dirname(plugin_path))
 
-    if subenv.get('GST_REGISTRY_UPDATE') != 'no' and len(cmd) >= 2:
-        data = subprocess.check_output(cmd, env=subenv)
+    try:
+        with open(os.path.join(BUILD_ROOT, 'GstPluginsPath.json')) as f:
+            plugin_paths = os.pathsep.join(json.load(f))
+    except FileNotFoundError:
+        plugin_paths = ""
+
+    if plugin_paths:
+        subenv['GST_PLUGIN_PATH'] = subenv.get('GST_PLUGIN_PATH', '') + ':' + plugin_paths
+
+    # Hide stderr unless an actual error happens as we have cases where we get g_warnings
+    # and other issues because plugins are being built while `gst_init` is called
+    stderrlogfile = output_filename + '.stderr'
+    with open(stderrlogfile, 'w') as log:
+        try:
+            data = subprocess.check_output(cmd, env=subenv, stderr=log)
+        except subprocess.CalledProcessError as e:
+            log.flush()
+            with open(stderrlogfile, 'r') as f:
+                print(f.read(), file=sys.stderr)
+            raise
     try:
         plugins = json.loads(data.decode(), object_pairs_hook=OrderedDict)
     except json.decoder.JSONDecodeError:

docs/meson.build

@@ -18,11 +18,16 @@ hotdoc_plugin_scanner = executable('gst-hotdoc-plugins-scanner',
   install: true,
 )
 
+conf = configuration_data()
+# We pass the build root so the doc cache generator can try
+# to find GstPluginsPath.json to set GST_PLUGIN_PATH and
+# thus handle plugins dependencies.
+conf.set('BUILD_ROOT', meson.build_root())
 configure_file(
     input: 'gst-plugins-doc-cache-generator.py',
     output: 'gst-plugins-doc-cache-generator',
-    copy: true,
     install_dir: helpers_install_dir,
+    configuration: conf
 )
 
 plugins_cache_generator = find_program(join_paths(meson.current_build_dir(), 'gst-plugins-doc-cache-generator'))