libcamera: Introduce YamlParser as a helper to parse yaml files

Introduce YamlParser as a helper to convert contents of a yaml file to
a tree based structure for easier reading, and to avoid writing parser
with raw yaml tokens. The class is based on libyaml, and only support
reading but not writing a yaml file.

The interface is inspired by Json::Value class from jsoncpp:
http://jsoncpp.sourceforge.net/class_json_1_1_value.html

Signed-off-by: Han-Lin Chen <hanlinchen@chromium.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

2 files changed, 684 insertions, 0 deletions

diff --git a/src/libcamera/meson.build b/src/libcamera/meson.build
index 26912ca1..f8e18e03 100644
--- a/src/libcamera/meson.build
+++ b/src/libcamera/meson.build
@@ -46,6 +46,7 @@ libcamera_sources = files([
     'v4l2_pixelformat.cpp',
     'v4l2_subdevice.cpp',
     'v4l2_videodevice.cpp',
+    'yaml_parser.cpp',
 ])
 
 libcamera_sources += libcamera_public_headers
@@ -66,6 +67,7 @@ subdir('proxy')
 libdl = cc.find_library('dl')
 libgnutls = cc.find_library('gnutls', required : true)
 libudev = dependency('libudev', required : false)
+libyaml = dependency('yaml-0.1', required : true)
 
 if libgnutls.found()
     config_h.set('HAVE_GNUTLS', 1)
@@ -126,6 +128,7 @@ libcamera_deps = [
     libgnutls,
     liblttng,
     libudev,
+    libyaml,
 ]
 
 # We add '/' to the build_rpath as a 'safe' path to act as a boolean flag.
diff --git a/src/libcamera/yaml_parser.cpp b/src/libcamera/yaml_parser.cpp
new file mode 100644
index 00000000..92fedaeb
--- /dev/null
+++ b/src/libcamera/yaml_parser.cpp
@@ -0,0 +1,681 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2022, Google Inc.
+ *
+ * yaml_parser.cpp - libcamera YAML parsing helper
+ */
+
+#include "libcamera/internal/yaml_parser.h"
+
+#include <cstdlib>
+#include <errno.h>
+#include <functional>
+
+#include <libcamera/base/log.h>
+
+#include <yaml.h>
+
+/**
+ * \file libcamera/internal/yaml_parser.h
+ * \brief A YAML parser helper
+ */
+
+namespace libcamera {
+
+LOG_DEFINE_CATEGORY(YamlParser)
+
+namespace {
+
+/* Empty static YamlObject as a safe result for invalid operations */
+static const YamlObject empty;
+
+void setOk(bool *ok, bool result)
+{
+	if (ok)
+		*ok = result;
+}
+
+} /* namespace */
+
+/**
+ * \class YamlObject
+ * \brief A class representing the tree structure of the YAML content
+ *
+ * The YamlObject class represents the tree structure of YAML content. A
+ * YamlObject can be a dictionary or list of YamlObjects or a value if a tree
+ * leaf.
+ */
+
+YamlObject::YamlObject()
+	: type_(Value)
+{
+}
+
+YamlObject::~YamlObject() = default;
+
+/**
+ * \fn YamlObject::isValue()
+ * \brief Return whether the YamlObject is a value
+ *
+ * \return True if the YamlObject is a value, false otherwise
+ */
+
+/**
+ * \fn YamlObject::isList()
+ * \brief Return whether the YamlObject is a list
+ *
+ * \return True if the YamlObject is a list, false otherwise
+ */
+
+/**
+ * \fn YamlObject::isDictionary()
+ * \brief Return whether the YamlObject is a dictionary
+ *
+ * \return True if the YamlObject is a dictionary, false otherwise
+ */
+
+/**
+ * \fn template<typename T> YamlObject::get<T>(
+ *	const T &defaultValue, bool *ok) const
+ * \brief Parse the YamlObject as a \a T value
+ * \param[in] defaultValue The default value when failing to parse
+ * \param[out] ok The result of whether the parse succeeded
+ *
+ * This function parses the value of the YamlObject as a \a T object, and
+ * returns the value. If parsing fails (usually because the YamlObject doesn't
+ * store a \a T value), the \a defaultValue is returned, and \a ok is set to
+ * false. Otherwise, the YamlObject value is returned, and \a ok is set to true.
+ *
+ * The \a ok pointer is optional and can be a nullptr if the caller doesn't
+ * need to know if parsing succeeded.
+ *
+ * \return Value as a bool type
+ */
+
+#ifndef __DOXYGEN__
+
+template<>
+bool YamlObject::get(const bool &defaultValue, bool *ok) const
+{
+	setOk(ok, false);
+
+	if (type_ != Value)
+		return defaultValue;
+
+	if (value_ == "true") {
+		setOk(ok, true);
+		return true;
+	} else if (value_ == "false") {
+		setOk(ok, true);
+		return false;
+	}
+
+	return defaultValue;
+}
+
+template<>
+int32_t YamlObject::get(const int32_t &defaultValue, bool *ok) const
+{
+	setOk(ok, false);
+
+	if (type_ != Value)
+		return defaultValue;
+
+	if (value_ == "")
+		return defaultValue;
+
+	char *end;
+
+	errno = 0;
+	int32_t value = std::strtol(value_.c_str(), &end, 10);
+
+	if ('\0' != *end || errno == ERANGE)
+		return defaultValue;
+
+	setOk(ok, true);
+	return value;
+}
+
+template<>
+uint32_t YamlObject::get(const uint32_t &defaultValue, bool *ok) const
+{
+	setOk(ok, false);
+
+	if (type_ != Value)
+		return defaultValue;
+
+	if (value_ == "")
+		return defaultValue;
+
+	/*
+	 * libyaml parses all scalar values as strings. When a string has
+	 * leading spaces before a minus sign, for example "  -10", strtoul
+	 * skips leading spaces, accepts the leading minus sign, and the
+	 * calculated digits are negated as if by unary minus. Rule it out in
+	 * case the user gets a large number when the value is negative.
+	 */
+	std::size_t found = value_.find_first_not_of(" \t");
+	if (found != std::string::npos && value_[found] == '-')
+		return defaultValue;
+
+	char *end;
+
+	errno = 0;
+	uint32_t value = std::strtoul(value_.c_str(), &end, 10);
+
+	if ('\0' != *end || errno == ERANGE)
+		return defaultValue;
+
+	setOk(ok, true);
+	return value;
+}
+
+template<>
+double YamlObject::get(const double &defaultValue, bool *ok) const
+{
+	setOk(ok, false);
+
+	if (type_ != Value)
+		return defaultValue;
+
+	if (value_ == "")
+		return defaultValue;
+
+	char *end;
+
+	errno = 0;
+	double value = std::strtod(value_.c_str(), &end);
+
+	if ('\0' != *end || errno == ERANGE)
+		return defaultValue;
+
+	setOk(ok, true);
+	return value;
+}
+
+template<>
+std::string YamlObject::get(const std::string &defaultValue, bool *ok) const
+{
+	setOk(ok, false);
+
+	if (type_ != Value)
+		return defaultValue;
+
+	setOk(ok, true);
+	return value_;
+}
+
+template<>
+Size YamlObject::get(const Size &defaultValue, bool *ok) const
+{
+	setOk(ok, false);
+
+	if (type_ != List)
+		return defaultValue;
+
+	if (list_.size() != 2)
+		return defaultValue;
+
+	/*
+	 * Add a local variable to validate each dimension in case
+	 * that ok == nullptr.
+	 */
+	bool valid;
+	uint32_t width = list_[0]->get<uint32_t>(0, &valid);
+	if (!valid)
+		return defaultValue;
+
+	uint32_t height = list_[1]->get<uint32_t>(0, &valid);
+	if (!valid)
+		return defaultValue;
+
+	setOk(ok, true);
+	return Size(width, height);
+}
+
+#endif /* __DOXYGEN__ */
+
+/**
+ * \fn YamlObject::size()
+ * \brief Retrieve the number of elements in a list YamlObject
+ *
+ * This function retrieves the size of the YamlObject, defined as the number of
+ * child elements it contains. Only YamlObject instances of List type have a
+ * size, calling this function on other types of instances is invalid and
+ * results in undefined behaviour.
+ *
+ * \return The size of the YamlObject
+ */
+std::size_t YamlObject::size() const
+{
+	if (type_ != List)
+		return 0;
+
+	return list_.size();
+}
+
+/**
+ * \fn YamlObject::operator[](std::size_t index) const
+ * \brief Retrieve the element from list YamlObject by index
+ *
+ * This function retrieves an element of the YamlObject. Only YamlObject
+ * instances of List type associate elements with index, calling this function
+ * on other types of instances is invalid and results in undefined behaviour.
+ *
+ * \return The YamlObject as an element of the list
+ */
+const YamlObject &YamlObject::operator[](std::size_t index) const
+{
+	if (type_ != List || index >= size())
+		return empty;
+
+	return *list_[index];
+}
+
+/**
+ * \fn YamlObject::contains()
+ * \brief Check if an element of a dictionary exists
+ *
+ * This function check if the YamlObject contains an element. Only YamlObject
+ * instances of Dictionary type associate elements with names, calling this
+ * function on other types of instances is invalid and results in undefined
+ * behaviour.
+ *
+ * \return True if an element exists, false otherwise
+ */
+bool YamlObject::contains(const std::string &key) const
+{
+	if (dictionary_.find(key) == dictionary_.end())
+		return false;
+
+	return true;
+}
+
+/**
+ * \fn YamlObject::memberNames()
+ * \brief Retrieve all member names of the dictionary
+ *
+ * This function retrieve member names of a YamlObject. Only YamlObject
+ * instances of Dictionary type associate elements with names, calling this
+ * function on other types of instances is invalid and results in undefined
+ * behaviour.
+ *
+ * \todo Replace this function with an iterator-based API
+ *
+ * \return A vector of string as the member names
+ */
+std::vector<std::string> YamlObject::memberNames() const
+{
+	std::vector<std::string> memberNames;
+	for (auto &[key, _] : dictionary_)
+		memberNames.push_back(key);
+
+	return memberNames;
+}
+
+/**
+ * \fn YamlObject::operator[](const std::string &key) const
+ * \brief Retrieve a member by name from the dictionary
+ *
+ * This function retrieve a member of a YamlObject by name. Only YamlObject
+ * instances of Dictionary type associate elements with names, calling this
+ * function on other types of instances is invalid and results in undefined
+ * behaviour.
+ *
+ * \return The YamlObject corresponding to the \a key member
+ */
+const YamlObject &YamlObject::operator[](const std::string &key) const
+{
+	if (type_ != Dictionary || !contains(key))
+		return empty;
+
+	auto iter = dictionary_.find(key);
+	return *iter->second;
+}
+
+#ifndef __DOXYGEN__
+
+class YamlParserContext
+{
+public:
+	YamlParserContext();
+	~YamlParserContext();
+
+	int init(std::FILE *fh);
+	int parseContent(YamlObject &yamlObject);
+
+private:
+	struct EventDeleter {
+		void operator()(yaml_event_t *event) const
+		{
+			yaml_event_delete(event);
+			delete event;
+		}
+	};
+	using EventPtr = std::unique_ptr<yaml_event_t, EventDeleter>;
+
+	EventPtr nextEvent();
+
+	void readValue(std::string &value, EventPtr event);
+	int parseDictionaryOrList(YamlObject::Type type,
+				  const std::function<int(EventPtr event)> &parseItem);
+	int parseNextYamlObject(YamlObject &yamlObject, EventPtr event);
+
+	bool parserValid_;
+	yaml_parser_t parser_;
+};
+
+/**
+ * \class YamlParserContext
+ * \brief Class for YamlParser parsing and context data
+ *
+ * The YamlParserContext class stores the internal yaml_parser_t and provides
+ * helper functions to do event-based parsing for YAML files.
+ */
+YamlParserContext::YamlParserContext()
+	: parserValid_(false)
+{
+}
+
+/**
+ * \class YamlParserContext
+ * \brief Destructor of YamlParserContext
+ */
+YamlParserContext::~YamlParserContext()
+{
+	if (parserValid_) {
+		yaml_parser_delete(&parser_);
+		parserValid_ = false;
+	}
+}
+
+/**
+ * \fn YamlParserContext::init()
+ * \brief Initialize a parser with an opened file for parsing
+ * \param[in] fh The YAML file to parse
+ *
+ * Prior to parsing the YAML content, the YamlParserContext must be initialized
+ * with an opened FILE to create an internal parser. The FILE needs to stay
+ * valid during the process.
+ *
+ * \return 0 on success or a negative error code otherwise
+ * \retval -EINVAL The parser has failed to initialize
+ */
+int YamlParserContext::init(std::FILE *fh)
+{
+	/* yaml_parser_initialize returns 1 when it succeededs */
+	if (!yaml_parser_initialize(&parser_)) {
+		LOG(YamlParser, Error) << "Failed to initialize YAML parser";
+		return -EINVAL;
+	}
+	parserValid_ = true;
+	yaml_parser_set_input_file(&parser_, fh);
+
+	return 0;
+}
+
+/**
+ * \fn YamlParserContext::nextEvent()
+ * \brief Get the next event
+ *
+ * Get the next event in the current YAML event stream, and return nullptr when
+ * there is no more event.
+ *
+ * \return The next event on success or nullptr otherwise
+ */
+YamlParserContext::EventPtr YamlParserContext::nextEvent()
+{
+	EventPtr event(new yaml_event_t);
+
+	/* yaml_parser_parse returns 1 when it succeeds */
+	if (!yaml_parser_parse(&parser_, event.get()))
+		return nullptr;
+
+	return event;
+}
+
+/**
+ * \fn YamlParserContext::parseContent()
+ * \brief Parse the content of a YAML document
+ * \param[in] yamlObject The result of YamlObject
+ *
+ * Check YAML start and end events of a YAML document, and parse the root object
+ * of the YAML document into a YamlObject.
+ *
+ * \return 0 on success or a negative error code otherwise
+ * \retval -EINVAL The parser has failed to validate end of a YAML file
+ */
+int YamlParserContext::parseContent(YamlObject &yamlObject)
+{
+	/* Check start of the YAML file. */
+	EventPtr event = nextEvent();
+	if (!event || event->type != YAML_STREAM_START_EVENT)
+		return -EINVAL;
+
+	event = nextEvent();
+	if (!event || event->type != YAML_DOCUMENT_START_EVENT)
+		return -EINVAL;
+
+	/* Parse the root object. */
+	event = nextEvent();
+	if (parseNextYamlObject(yamlObject, std::move(event)))
+		return -EINVAL;
+
+	/* Check end of the YAML file. */
+	event = nextEvent();
+	if (!event || event->type != YAML_DOCUMENT_END_EVENT)
+		return -EINVAL;
+
+	event = nextEvent();
+	if (!event || event->type != YAML_STREAM_END_EVENT)
+		return -EINVAL;
+
+	return 0;
+}
+
+/**
+ * \fn YamlParserContext::readValue()
+ * \brief Parse event scalar and fill its content into a string
+ * \param[in] value The string reference to fill value
+ *
+ * A helper function to parse a scalar event as string. The caller needs to
+ * guarantee the event is of scaler type.
+ */
+void YamlParserContext::readValue(std::string &value, EventPtr event)
+{
+	value.assign(reinterpret_cast<char *>(event->data.scalar.value),
+		     event->data.scalar.length);
+}
+
+/**
+ * \fn YamlParserContext::parseDictionaryOrList()
+ * \brief A helper function to abstract the common part of parsing dictionary or list
+ *
+ * \param[in] isDictionary True for parsing a dictionary, and false for a list
+ * \param[in] parseItem The callback to handle an item
+ *
+ * A helper function to abstract parsing an item from a dictionary or a list.
+ * The differences of them in a YAML event stream are:
+ *
+ * 1. The start and end event types are different
+ * 2. There is a leading scalar string as key in the items of a dictionary
+ *
+ * The caller should handle the leading key string in its callback parseItem
+ * when it's a dictionary.
+ *
+ * \return 0 on success or a negative error code otherwise
+ * \retval -EINVAL The parser is failed to initialize
+ */
+int YamlParserContext::parseDictionaryOrList(YamlObject::Type type,
+					     const std::function<int(EventPtr event)> &parseItem)
+{
+	yaml_event_type_t endEventType = YAML_SEQUENCE_END_EVENT;
+	if (type == YamlObject::Dictionary)
+		endEventType = YAML_MAPPING_END_EVENT;
+
+	/*
+	 * Add a safety counter to make sure we don't loop indefinitely in case
+	 * the YAML file is malformed.
+	 */
+	for (unsigned int sentinel = 1000; sentinel; sentinel--) {
+		auto evt = nextEvent();
+		if (!evt)
+			return -EINVAL;
+
+		if (evt->type == endEventType)
+			return 0;
+
+		int ret = parseItem(std::move(evt));
+		if (ret)
+			return ret;
+	}
+
+	LOG(YamlParser, Error) << "The YAML file contains a List or Dictionary"
+				  " whose size exceeds the parser's limit (1000)";
+
+	return -EINVAL;
+}
+
+/**
+ * \fn YamlParserContext::parseNextYamlObject()
+ * \brief Parse next YAML event and read it as a YamlObject
+ * \param[in] yamlObject The result of YamlObject
+ * \param[in] event The leading event of the object
+ *
+ * Parse next YAML object separately as a value, list or dictionary.
+ *
+ * \return 0 on success or a negative error code otherwise
+ * \retval -EINVAL Fail to parse the YAML file.
+ */
+int YamlParserContext::parseNextYamlObject(YamlObject &yamlObject, EventPtr event)
+{
+	if (!event)
+		return -EINVAL;
+
+	switch (event->type) {
+	case YAML_SCALAR_EVENT:
+		yamlObject.type_ = YamlObject::Value;
+		readValue(yamlObject.value_, std::move(event));
+		return 0;
+
+	case YAML_SEQUENCE_START_EVENT: {
+		yamlObject.type_ = YamlObject::List;
+		auto &list = yamlObject.list_;
+		auto handler = [this, &list](EventPtr evt) {
+			list.emplace_back(new YamlObject());
+			return parseNextYamlObject(*list.back(), std::move(evt));
+		};
+		return parseDictionaryOrList(YamlObject::List, handler);
+	}
+
+	case YAML_MAPPING_START_EVENT: {
+		yamlObject.type_ = YamlObject::Dictionary;
+		auto &dictionary = yamlObject.dictionary_;
+		auto handler = [this, &dictionary](EventPtr evtKey) {
+			/* Parse key */
+			if (evtKey->type != YAML_SCALAR_EVENT) {
+				LOG(YamlParser, Error) << "Expect key at line: "
+						       << evtKey->start_mark.line
+						       << " column: "
+						       << evtKey->start_mark.column;
+				return -EINVAL;
+			}
+
+			std::string key;
+			readValue(key, std::move(evtKey));
+
+			/* Parse value */
+			EventPtr evtValue = nextEvent();
+			if (!evtValue)
+				return -EINVAL;
+
+			auto elem = dictionary.emplace(key, std::make_unique<YamlObject>());
+			return parseNextYamlObject(*elem.first->second.get(), std::move(evtValue));
+		};
+		return parseDictionaryOrList(YamlObject::Dictionary, handler);
+	}
+
+	default:
+		LOG(YamlParser, Error) << "Invalid YAML file";
+		return -EINVAL;
+	}
+}
+
+#endif /* __DOXYGEN__ */
+
+/**
+ * \class YamlParser
+ * \brief A helper class for parsing a YAML file
+ *
+ * The YamlParser class provides an easy interface to parse the contents of a
+ * YAML file into a tree of YamlObject instances.
+ *
+ * Example usage:
+ *
+ * \code{.unparsed}
+ *
+ * name:
+ * 	"John"
+ * numbers:
+ * 	- 1
+ * 	- 2
+ *
+ * \endcode
+ *
+ * The following code illustrates how to parse the above YAML file:
+ *
+ * \code{.cpp}
+ *
+ * std::unique_ptr<YamlObject> root = YamlParser::parse(fh);
+ * if (!root)
+ *   return;
+ *
+ * if (!root->isDictionary())
+ *   return;
+ *
+ * const YamlObject &name = (*root)["name"];
+ * std::cout << name.get<std::string>("") << std::endl;
+ *
+ * const YamlObject &numbers = (*root)["numbers"];
+ * if (!numbers.isList())
+ *   return;
+ *
+ * for (std::size_t i = 0; i < numbers.size(); i++)
+ *   std::cout << numbers[i].get<int32_t>(0) << std::endl;
+ *
+ * \endcode
+ *
+ * The YamlParser::parse() function takes an open FILE, parses its contents, and
+ * returns a pointer to a YamlObject corresponding to the root node of the YAML
+ * document.
+ */
+
+/**
+ * \fn YamlParser::parse()
+ * \brief Parse a YAML file as a YamlObject
+ * \param[in] fh The YAML file to parse
+ *
+ * The YamlParser::parse() function takes an open FILE, parses its contents, and
+ * returns a pointer to a YamlObject corresponding to the root node of the YAML
+ * document. The caller is responsible for closing the file.
+ *
+ * \return Pointer to result YamlObject on success or nullptr otherwise
+ */
+std::unique_ptr<YamlObject> YamlParser::parse(std::FILE *fh)
+{
+	YamlParserContext context;
+
+	if (context.init(fh))
+		return nullptr;
+
+	std::unique_ptr<YamlObject> root(new YamlObject());
+
+	if (context.parseContent(*root)) {
+		LOG(YamlParser, Error) << "Failed to parse YAML content";
+		return nullptr;
+	}
+
+	return root;
+}
+
+} /* namespace libcamera */