make stack_allocator::format_string() grow the buffer for large strings

src/stack_allocator.cpp

@@ -31,6 +31,7 @@ POSSIBILITY OF SUCH DAMAGE.
 */
 
 #include "libtorrent/stack_allocator.hpp"
+#include <cstdarg> // for va_list, va_copy, va_end
 
 namespace libtorrent {
 namespace aux {
@@ -56,28 +57,44 @@ namespace aux {
 
 	allocation_slot stack_allocator::format_string(char const* fmt, va_list v)
 	{
-		int const ret = int(m_storage.size());
-		int const max_size = 1024;
-		m_storage.resize(ret + max_size);
+		int const pos = int(m_storage.size());
+		int len = 512;
+
+		for(;;)
+		{
+			m_storage.resize(pos + len + 1);
+
+			va_list args;
+			va_copy(args, v);
 
 #ifdef __clang__
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wformat-nonliteral"
 #endif
-		int const len = std::vsnprintf(m_storage.data() + ret, max_size, fmt, v);
+			int const ret = std::vsnprintf(m_storage.data() + pos, static_cast<std::size_t>(len + 1), fmt, args);
 #ifdef __clang__
 #pragma clang diagnostic pop
 #endif
 
-		if (len < 0)
-		{
-			m_storage.resize(ret);
-			return copy_string("(format error)");
+			va_end(args);
+
+			if (ret < 0)
+			{
+				m_storage.resize(pos);
+				return copy_string("(format error)");
+			}
+			if (ret > len)
+			{
+				// try again
+				len = ret;
+				continue;
+			}
+			break;
 		}
 
 		// +1 is to include the 0-terminator
-		m_storage.resize(ret + std::min(len, max_size) + 1);
-		return allocation_slot(ret);
+		m_storage.resize(pos + len + 1);
+		return allocation_slot(pos);
 	}
 
 	allocation_slot stack_allocator::copy_buffer(span<char const> buf)

test/test_stack_allocator.cpp

@@ -33,6 +33,7 @@ POSSIBILITY OF SUCH DAMAGE.
 #include "test.hpp"
 #include "libtorrent/stack_allocator.hpp"
 #include "libtorrent/string_view.hpp"
+#include <cstdarg> // for va_list, va_start, va_end
 
 using lt::aux::stack_allocator;
 using lt::aux::allocation_slot;
@@ -107,3 +108,34 @@ TORRENT_TEST(swap)
 	TEST_CHECK(a2.ptr(idx1) == "testing"_sv);
 }
 
+namespace {
+
+TORRENT_FORMAT(2,3)
+allocation_slot format_string_helper(stack_allocator& stack, char const* fmt, ...)
+{
+		va_list v;
+		va_start(v, fmt);
+		auto const ret = stack.format_string(fmt, v);
+		va_end(v);
+		return ret;
+}
+
+}
+
+TORRENT_TEST(format_string_long)
+{
+	stack_allocator a;
+	std::string long_string;
+	for (int i = 0; i < 1024; ++i) long_string += "foobar-";
+	auto const idx = format_string_helper(a, "%s", long_string.c_str());
+
+	TEST_EQUAL(a.ptr(idx), long_string);
+}
+
+TORRENT_TEST(format_string)
+{
+	stack_allocator a;
+	auto const idx = format_string_helper(a, "%d", 10);
+
+	TEST_EQUAL(a.ptr(idx), "10"_sv);
+}