buffer: allow to write zero bytes

Encoder can try to write zero bytes sometimes. For example, when
it encodes an empty string. Let's simply handle this case in buffer
and iterators instead of assertion.

Along the way, replace the assertion in `iterator::get` method as well.
It should have be done in 28bae01, but we forgot about this method
because it's not used by decoder.

Author: alyapunov

src/Buffer/Buffer.hpp

@@ -703,7 +703,8 @@ template <size_t N, class allocator>
 void
 Buffer<N, allocator>::write(WData data)
 {
-	assert(data.size != 0);
+	if (data.size == 0)
+		return;
 
 	char *new_end = m_end + data.size;
 	if (TNT_LIKELY(isSameBlock(m_end, new_end))) {
@@ -1105,7 +1106,9 @@ template <bool LIGHT>
 void
 Buffer<N, allocator>::iterator_common<LIGHT>::set(WData data)
 {
-	assert(data.size > 0);
+	if (data.size == 0)
+		return;
+
 	char *pos = m_position;
 	size_t left_in_block = N - (uintptr_t) pos % N;
 	while (TNT_UNLIKELY(data.size > left_in_block)) {
@@ -1160,7 +1163,9 @@ template <bool LIGHT>
 void
 Buffer<N, allocator>::iterator_common<LIGHT>::write(WData data)
 {
-	assert(data.size > 0);
+	if (data.size == 0)
+		return;
+
 	size_t left_in_block = N - (uintptr_t) m_position % N;
 	while (TNT_UNLIKELY(data.size >= left_in_block)) {
 		std::memcpy(m_position, data.data, left_in_block);
@@ -1216,7 +1221,8 @@ template <bool LIGHT>
 void
 Buffer<N, allocator>::iterator_common<LIGHT>::get(RData data)
 {
-	assert(data.size > 0);
+	if (data.size == 0)
+		return;
 	/*
 	 * The same implementation as in ::set() method buf vice versa:
 	 * buffer and data sources are swapped.

test/BufferUnitTest.cpp

@@ -125,6 +125,12 @@ buffer_basic()
 	TEST_INIT(1, N);
 	tnt::Buffer<N> buf;
 	fail_unless(buf.empty());
+
+	/* Empty write should work and don't write anything. */
+	buf.write(typename tnt::Buffer<N>::WData {nullptr, 0});
+	fail_unless(buf.empty());
+	fail_if(buf.debugSelfCheck());
+
 	buf.write(int_sample);
 	fail_unless(! buf.empty());
 	fail_if(buf.debugSelfCheck());
@@ -378,11 +384,24 @@ buffer_iterator()
 	buf.dropBack(1);
 	fail_unless(buf.empty());
 	fail_if(buf.debugSelfCheck());
-
 	itr = buf.begin();
 	const auto& citr = itr;
 	typename tnt::Buffer<N>::iterator itr2(itr);
 
+	/* Empty writes and reads should work and don't write anything. */
+	itr.write(typename tnt::Buffer<N>::WData {nullptr, 0});
+	fail_unless(buf.empty());
+	fail_if(buf.debugSelfCheck());
+	itr.set(typename tnt::Buffer<N>::WData {nullptr, 0});
+	fail_unless(buf.empty());
+	fail_if(buf.debugSelfCheck());
+	itr.get(typename tnt::Buffer<N>::RData {nullptr, 0});
+	fail_unless(buf.empty());
+	fail_if(buf.debugSelfCheck());
+	itr.read(typename tnt::Buffer<N>::RData {nullptr, 0});
+	fail_unless(buf.empty());
+	fail_if(buf.debugSelfCheck());
+
 	auto litr1 = itr.enlight();
 	{
 		auto itr_test = litr1;

test/EncDecTest.cpp

@@ -230,6 +230,7 @@ test_basic()
 	mpp::encode(buf, std::integral_constant<bool, false>{});
 	mpp::encode(buf, std::integral_constant<bool, true>{});
 	// Strings.
+	mpp::encode(buf, "");
 	mpp::encode(buf, "abc");
 	const char *bbb = "defg";
 	mpp::encode(buf, bbb);
@@ -322,7 +323,9 @@ test_basic()
 	fail_if(bt != true);
 
 	// Strings.
-	std::string a;
+	std::string a = "nothing";
+	fail_unless(mpp::decode(run, a));
+	fail_if(a != "");
 	char b_data[10];
 	size_t b_size = 0;
 	auto b = tnt::make_ref_vector(b_data, b_size);