[PATCH] libcamera: yaml-parser: Differentiate between empty and empty string

[Stefan Klug]

When accessing a nonexistent key on a dict the YamlObject returns an
empty element. This element can happily be casted to a string which is
unexpected. For example the following statement:
yamlDict["nonexistant"].get<string>("default") is expected to return
"default" but actually returns "". Fix this by introducing a empty type
to distinguish between an empty YamlObject and a YamlObject of type
value containing an empty string. For completeness add an isEmpty()
function to be able to test for that type.

Signed-off-by: Stefan Klug <stefan.klug at ideasonboard.com>
---
 include/libcamera/internal/yaml_parser.h |  5 +++++
 src/libcamera/yaml_parser.cpp            | 20 ++++++++++++++------
 test/yaml-parser.cpp                     |  6 ++++++
 3 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/include/libcamera/internal/yaml_parser.h b/include/libcamera/internal/yaml_parser.h
index 16708e488d88..33c3a96fed47 100644
--- a/include/libcamera/internal/yaml_parser.h
+++ b/include/libcamera/internal/yaml_parser.h
@@ -159,6 +159,10 @@ public:
 	{
 		return type_ == Type::Dictionary;
 	}
+	bool isEmpty() const
+	{
+		return type_ == Type::Empty;
+	}
 
 	std::size_t size() const;
 
@@ -212,6 +216,7 @@ private:
 		Dictionary,
 		List,
 		Value,
+		Empty,
 	};
 
 	template<typename T>
diff --git a/src/libcamera/yaml_parser.cpp b/src/libcamera/yaml_parser.cpp
index 8b6a403898be..eb72529136f7 100644
--- a/src/libcamera/yaml_parser.cpp
+++ b/src/libcamera/yaml_parser.cpp
@@ -38,12 +38,12 @@ static const YamlObject empty;
  * \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 can be empty, a dictionary or list of YamlObjects or a value if a
+ * tree leaf.
  */
 
 YamlObject::YamlObject()
-	: type_(Type::Value)
+	: type_(Type::Empty)
 {
 }
 
@@ -70,6 +70,13 @@ YamlObject::~YamlObject() = default;
  * \return True if the YamlObject is a dictionary, false otherwise
  */
 
+/**
+ * \fn YamlObject::isEmpty()
+ * \brief Return whether the YamlObject is a empty
+ *
+ * \return True if the YamlObject is empty, false otherwise
+ */
+
 /**
  * \fn YamlObject::size()
  * \brief Retrieve the number of elements in a dictionary or list YamlObject
@@ -443,7 +450,8 @@ template std::optional<std::vector<Size>> YamlObject::getList<Size>() const;
  *
  * 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.
+ * on other types of instances or with an invalid index results in an empty
+ * object.
  *
  * \return The YamlObject as an element of the list
  */
@@ -480,8 +488,8 @@ bool YamlObject::contains(const std::string &key) const
  *
  * 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.
+ * function on other types of instances or with a non existant key results in an
+ * empty object.
  *
  * \return The YamlObject corresponding to the \a key member
  */
diff --git a/test/yaml-parser.cpp b/test/yaml-parser.cpp
index 81c829834667..b8ae0b4c11f8 100644
--- a/test/yaml-parser.cpp
+++ b/test/yaml-parser.cpp
@@ -514,6 +514,12 @@ protected:
 			i++;
 		}
 
+		/* Test access to nonexistent member */
+		if (dictObj["nonexistent"].get<std::string>("default") != "default") {
+			cerr << "Accessing nonexistent dict fails to return default" << std::endl;
+			return TestFail;
+		}
+
 		/* Make sure utils::map_keys() works on the adapter. */
 		(void)utils::map_keys(dictObj.asDict());
 
-- 
2.43.0