[PATCH v5 05/15] config: Add configuration retrieval helpers

[Milan Zamazal]

They make accessing simple configuration values simpler.

Due to the restrictions of YamlObject class, ugly type casting dance is
needed.  Also, GlobalConfiguration::option must check for the key
presence before retrieving the value using operator[] because if the key
is not present then YamlObject::empty may be returned as a regular value.

Signed-off-by: Milan Zamazal <mzamazal at redhat.com>
---
 .../libcamera/internal/global_configuration.h |  5 ++
 src/libcamera/base/global_configuration.cpp   | 53 ++++++++++++++++++-
 src/libcamera/base/log.cpp                    | 13 ++---
 3 files changed, 61 insertions(+), 10 deletions(-)

diff --git a/include/libcamera/internal/global_configuration.h b/include/libcamera/internal/global_configuration.h
index 8058fd8e2..b1529f392 100644
--- a/include/libcamera/internal/global_configuration.h
+++ b/include/libcamera/internal/global_configuration.h
@@ -8,6 +8,8 @@
 #pragma once
 
 #include <filesystem>
+#include <optional>
+#include <string>
 #include <vector>
 
 #include "libcamera/internal/yaml_parser.h"
@@ -25,6 +27,9 @@ public:
 
 	static unsigned int version();
 	static Configuration configuration();
+	static std::optional<std::string> option(const std::string &confPath);
+	static std::optional<std::string> envOption(const char *const envVariable,
+						    const std::string &confPath);
 
 private:
 	static const std::vector<std::filesystem::path> globalConfigurationFiles;
diff --git a/src/libcamera/base/global_configuration.cpp b/src/libcamera/base/global_configuration.cpp
index c6f4ebd7b..f184d4bab 100644
--- a/src/libcamera/base/global_configuration.cpp
+++ b/src/libcamera/base/global_configuration.cpp
@@ -9,6 +9,7 @@
 
 #include <filesystem>
 #include <memory>
+#include <string>
 #include <sys/types.h>
 
 #include <libcamera/base/file.h>
@@ -60,8 +61,10 @@ void GlobalConfiguration::initialize()
  * configuration files exist then only the first one found is used and no
  * configuration merging is performed.
  *
- * The class is used as a private singleton and the configuration can be
- * accessed using GlobalConfiguration::configuration().
+ * The class is used as a private singleton accessed by the provided
+ * helpers. Namely GlobalConfiguration::option or GlobalConfiguration::envOption
+ * to access individual options or GlobalConfiguration::configuration() to
+ * access the whole configuration.
  */
 
 /**
@@ -145,6 +148,52 @@ GlobalConfiguration::Configuration GlobalConfiguration::get()
 	return *instance_->configuration_;
 }
 
+/**
+ * \brief Return value of the configuration option identified by \a confPath
+ * \param[in] confPath Sequence of the YAML section names (excluding
+ * `configuration') leading to the requested option separated by dots
+ * \return A value if an item corresponding to \a confPath exists in the
+ * configuration file, no value otherwise
+ */
+std::optional<std::string> GlobalConfiguration::option(
+	const std::string &confPath)
+{
+	YamlObject *c = &const_cast<YamlObject &>(configuration());
+	for (auto part : utils::details::StringSplitter(confPath, "."))
+		if (c->contains(part))
+			c = &const_cast<YamlObject &>((*c)[part]);
+		else
+			return std::optional<std::string>();
+	return c->get<std::string>();
+}
+
+/**
+ * \brief Return value of the configuration option from a file or environment
+ * \param[in] envVariable Environment variable to get the value from
+ * \param[in] confPath The same as in GlobalConfiguration::option
+ *
+ * This helper looks first at the given environment variable and if it is
+ * defined then it returns its value (even if it is empty). Otherwise it looks
+ * for \a confPath the same way as in GlobalConfiguration::option. Only string
+ * values are supported.
+ *
+ * \note Support for using environment variables to configure libcamera behavior
+ * is provided here mostly for backward compatibility reasons. Introducing new
+ * configuration environment variables is discouraged.
+ *
+ * \return A value retrieved from the given environment option or configuration
+ * file or no value if not found
+ */
+std::optional<std::string> GlobalConfiguration::envOption(
+	const char *const envVariable,
+	const std::string &confPath)
+{
+	const char *envValue = utils::secure_getenv(envVariable);
+	if (envValue)
+		return std::optional{ std::string{ envValue } };
+	return option(confPath);
+}
+
 /**
  * \brief Return configuration version
  *
diff --git a/src/libcamera/base/log.cpp b/src/libcamera/base/log.cpp
index 0f6767107..891b4ae9d 100644
--- a/src/libcamera/base/log.cpp
+++ b/src/libcamera/base/log.cpp
@@ -628,14 +628,11 @@ void Logger::parseLogFile()
  */
 void Logger::parseLogLevels()
 {
-	const char *debug = utils::secure_getenv("LIBCAMERA_LOG_LEVELS");
-	if (!debug) {
-		const std::optional<std::string> confDebug =
-			GlobalConfiguration::configuration()["log"]["levels"].get<std::string>();
-		if (!confDebug.has_value())
-			return;
-		debug = confDebug.value().c_str();
-	}
+	const std::optional<std::string> confDebug =
+		GlobalConfiguration::envOption("LIBCAMERA_LOG_LEVELS", "log.levels");
+	if (!confDebug.has_value())
+		return;
+	const char *debug = confDebug.value().c_str();
 
 	for (const char *pair = debug; *debug != '\0'; pair = debug) {
 		const char *comma = strchrnul(debug, ',');
-- 
2.44.1