[#70] fix(buffer): when mmap-ing, don't fail on empty files

This patch introduces a fix to a long-standing problem with the buffer `mmap`-ing behaviour and empty files. Previously, when trying to map an empty file, the system would throw an exception which caused a bunch of problems in implementations. Now, it checks the file size before trying to map it and if the size is zero, the empty buffer is returned.

Closes: #70

source/buffer.cc

@@ -163,6 +163,10 @@ namespace phoenix {
 	}
 
 	buffer buffer::mmap(const std::filesystem::path& path, bool readonly) {
+		auto file_size = std::filesystem::file_size(path);
+		if (file_size == 0)
+			return buffer::empty();
+
 		if (readonly) {
 			return buffer {std::make_shared<detail::mmap_backing<mio::access_mode::read>>(path)};
 		}

tests/test_buffer.cc

@@ -559,4 +559,13 @@ TEST_SUITE("buffer") {
 		auto buf = phoenix::buffer::allocate(10);
 		CHECK_FALSE(empty == buf);
 	}
+
+	TEST_CASE("buffer(mmap)") {
+		SUBCASE("empty file") {
+			auto buf = phoenix::buffer::mmap("samples/empty.txt");
+			CHECK_EQ(buf.limit(), 0);
+			CHECK_EQ(buf.capacity(), 0);
+			CHECK_THROWS_AS((void) buf.get_char(), phoenix::buffer_underflow);
+		}
+	}
 }