Support memory mapped files and add File.map (#2321)

* Support memory mapped files and add File.map

---------

Co-authored-by: Christoffer Lerno <christoffer@aegik.com>

test/unit/stdlib/io/file_mmap.c3

@@ -0,0 +1,114 @@
+module std::io::file @if(env::LIBC &&& env::POSIX);
+
+import std::io;
+
+fn void write_file(String path, char[] data)
+{
+	File f = file::open(path, "wb")!!;
+	defer f.close()!!;
+	f.write(data)!!;
+}
+
+fn void map_read_only() @test
+{
+	const String FNAME = "tmp/map_test_ro";
+	char[] msg = "Hello, world";
+	write_file(FNAME, msg);
+
+	File f = file::open(FNAME, "rb")!!;
+	defer f.close()!!;
+
+	FileMmap mapped_file = file::mmap_file(f, 0, 0, vm::VirtualMemoryAccess.READ)!!;
+	defer mapped_file.destroy()!!;
+	
+	char[] view = mapped_file.bytes();
+	assert(view.len == msg.len);
+	for (usz i = 0; i < view.len; i += 1)
+	{
+		assert(view[i] == msg[i]);
+	}
+}
+
+fn void map_read_write() @test
+{
+	const String FNAME = "tmp/map_test_rw";
+	char[4] data = "ABCD";
+	write_file(FNAME, data[..]);
+
+	bool shared = true;
+	File f = file::open(FNAME, "r+")!!;
+	mmap::FileMmap region = file::mmap_file(f, 0, 0, vm::VirtualMemoryAccess.READWRITE, shared)!!;
+
+	region.bytes()[1] = 'Z';
+
+	region.destroy()!!;
+	f.close()!!;
+
+	File fr = file::open(FNAME, "r")!!;
+	defer fr.close()!!;
+	char[4] check;
+	fr.read(check[..])!!;
+	assert(check[0] == 'A' && check[1] == 'Z' && check[2] == 'C');
+}
+
+fn void map_with_offset(String fname, usz offset, usz size, char[] reference_msg)
+{
+	File f = file::open(fname, "r")!!;
+	defer f.close()!!;
+
+	mmap::FileMmap mapped_file = file::mmap_file(f, offset, size, vm::VirtualMemoryAccess.READ)!!;
+	defer mapped_file.destroy()!!;
+
+	usz expected_size = size;
+	if (size == 0)
+	{
+		expected_size = reference_msg.len - offset;
+	}
+	char[] view = mapped_file.bytes();
+	assert(view.len == expected_size);
+	assert(view[0] == reference_msg[offset]);
+}
+
+fn void map_read_only_with_offset_and_size_zero() @test
+{
+	const String FNAME = "tmp/map_test_ro_with_offset";
+	char[] msg = "Hello, world";
+	write_file(FNAME, msg);
+
+	usz map_size = 0;
+	for (usz i = 0; i < msg.len; i += 1)
+	{
+		map_with_offset(FNAME, i, map_size, msg);
+	}
+}
+
+fn void map_read_only_with_offset() @test
+{
+	const String FNAME = "tmp/map_test_ro_with_offset";
+	char[] msg = "Hello, world";
+	write_file(FNAME, msg);
+
+	usz map_size = 3;
+	for (usz i = 0; i < msg.len - map_size; i++)
+	{
+		map_with_offset(FNAME, i, map_size, msg);
+	}
+}
+
+fn void map_from_filename() @test
+{
+	const String FNAME = "tmp/map_test_ro";
+	char[] msg = "Hello, world";
+	write_file(FNAME, msg);
+
+	mmap::FileMmap mapped_file = file::mmap_open(FNAME, "rb", 0, 0, vm::VirtualMemoryAccess.READ)!!;
+	defer mapped_file.destroy()!!;
+
+	char[] view = mapped_file.bytes();
+	test::eq(view.len, msg.len);
+	for (usz i = 0; i < view.len; i++)
+	{
+		test::eq(view[i], msg[i]);
+	}
+}
+