os/buffered_xml.h: support for generating XML

This patch extends the 'Buffered_xml' utility with a new constructor
that fills the buffer with the output of an 'Xml_generator'. It thereby
presents an easy way to generate XML to be consumed locally.

The patch also add a deprecation mark to the original 'xml' accessor
because copying 'Xml_node' objects (here as return value) is dangerous.
The new 'with_xml_node' method should instead be used to access the XML
content stored in the buffer.

Fixes #3602

repos/os/include/os/buffered_xml.h

@@ -15,7 +15,9 @@
 #define _OS__BUFFERED_XML_H_
 
 /* Genode includes */
+#include <util/retry.h>
 #include <util/xml_node.h>
+#include <util/xml_generator.h>
 #include <base/allocator.h>
 
 namespace Genode { class Buffered_xml; }
@@ -25,23 +27,51 @@ class Genode::Buffered_xml
 {
 	private:
 
-		Allocator         &_alloc;
-		char const * const _ptr;   /* pointer to dynamically allocated buffer */
-		Xml_node     const _xml;   /* referring to buffer of '_ptr' */
+		Allocator &_alloc;
+
+		struct Allocation { char *ptr; size_t size; } const _allocation;
+
+		Xml_node const _xml { _allocation.ptr, _allocation.size };
 
 		/**
 		 * \throw Allocator::Out_of_memory
 		 */
-		static char const *_init_ptr(Allocator &alloc, Xml_node node)
+		Allocation _copy_xml_node(Xml_node node)
 		{
-			char *ptr = nullptr;
+			Allocation allocation { };
 
 			node.with_raw_node([&] (char const *start, size_t length) {
-				ptr = (char *)alloc.alloc(length);
-				Genode::memcpy(ptr, start, length);
+				allocation = Allocation { (char *)_alloc.alloc(length), length };
+				Genode::memcpy(allocation.ptr, start, length);
 			});
 
-			return ptr;
+			return allocation;
+		}
+
+		/**
+		 * Generate XML into allocated buffer
+		 *
+		 * \throw Allocation::Out_of_memory
+		 */
+		template <typename FN>
+		Allocation _generate(char const *node_name, FN const &fn, size_t size)
+		{
+			Allocation allocation { };
+
+			retry<Xml_generator::Buffer_exceeded>(
+
+				[&] () {
+					allocation = Allocation { (char *)_alloc.alloc(size), size };
+					Xml_generator xml(allocation.ptr, size, node_name,
+					                  [&] () { fn(xml); }); },
+
+				[&] () {
+					_alloc.free(allocation.ptr, allocation.size);
+					allocation = { };
+					size = size*2;
+				}
+			);
+			return allocation;
 		}
 
 		/*
@@ -53,18 +83,50 @@ class Genode::Buffered_xml
 	public:
 
 		/**
-		 * Constructor
+		 * Constructor for buffering a copy of the specified XML node
 		 *
 		 * \throw Allocator::Out_of_memory
 		 */
 		Buffered_xml(Allocator &alloc, Xml_node node)
 		:
-			_alloc(alloc), _ptr(_init_ptr(alloc, node)), _xml(_ptr, node.size())
+			_alloc(alloc), _allocation(_copy_xml_node(node))
 		{ }
 
-		~Buffered_xml() { _alloc.free(const_cast<char *>(_ptr), _xml.size()); }
+		struct Min_size { size_t value; };
 
+		/**
+		 * Constructor for buffering generated XML
+		 *
+		 * \param name  name of top-level XML node
+		 * \param fn    functor that takes an 'Xml_generator &' as argument
+		 * \param size  initial allocation size
+		 *
+		 * \throw Allocator::Out_of_memory
+		 */
+		template <typename FN>
+		Buffered_xml(Allocator &alloc, char const *name, FN const &fn, Min_size size)
+		:
+			_alloc(alloc), _allocation(_generate(name, fn, size.value))
+		{ }
+
+		template <typename FN>
+		Buffered_xml(Allocator &alloc, char const *name, FN const &fn)
+		:
+			Buffered_xml(alloc, name, fn, Min_size{4000})
+		{ }
+
+		~Buffered_xml() { _alloc.free(_allocation.ptr, _allocation.size); }
+
+		/*
+		 * \deprecated  Use 'with_xml_node' instead
+		 */
 		Xml_node xml() const { return _xml; }
+
+		/**
+		 * Call functor 'fn' with 'Xml_node const &' as argument
+		 */
+		template <typename FN>
+		void with_xml_node(FN const &fn) const { fn(_xml); }
 };
 
 #endif /* _OS__BUFFERED_XML_H_ */