config: Add configuration retrieval helpers

Let's add some helpers to make accessing simple configuration values
simpler.  The helpers are used in the followup patches.

GlobalConfiguration::option ensures that no value is returned rather
than a value of YamlObject::empty.

Signed-off-by: Milan Zamazal <mzamazal@redhat.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

include/libcamera/internal/global_configuration.h

@@ -8,6 +8,11 @@
 #pragma once
 
 #include <filesystem>
+#include <optional>
+#include <string>
+#include <string_view>
+
+#include <libcamera/base/utils.h>
 
 #include "libcamera/internal/yaml_parser.h"
 
@@ -23,6 +28,29 @@ public:
 	unsigned int version() const;
 	Configuration configuration() const;
 
+	template<typename T>
+	std::optional<T> option(
+		const std::initializer_list<std::string_view> confPath) const
+	{
+		const YamlObject *c = &configuration();
+		for (auto part : confPath) {
+			c = &(*c)[part];
+			if (!*c)
+				return {};
+		}
+		return c->get<T>();
+	}
+
+	std::optional<std::vector<std::string>> listOption(
+		const std::initializer_list<std::string_view> confPath) const;
+	std::optional<std::string> envOption(
+		const char *const envVariable,
+		const std::initializer_list<std::string_view> confPath) const;
+	std::optional<std::vector<std::string>> envListOption(
+		const char *const envVariable,
+		const std::initializer_list<std::string_view> confPath,
+		const std::string delimiter = ":") const;
+
 private:
 	bool loadFile(const std::filesystem::path &fileName);
 	void load();