std.fs.File.Reader.seekBy() with negative offset does not work as expected

[mlarouche]

Zig Version

0.15.1

Steps to Reproduce and Observed Behavior

I'm being slowly migrating my library zigimg to 0.15.1 and the new I/O reader and I hit a snag when trying to add seekBy() to negative offsets in my streaming abstraction. Seeking negative numbers within a file reader does not give the expected behavior

test "should read and seek properly within a file" {
    const TEST_FILE_CONTENTS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    const TEST_FILENAME = "io_read_stream_test.dat";

    var temp_folder = std.testing.tmpDir(.{});
    defer temp_folder.cleanup();

    try temp_folder.dir.writeFile(.{ .sub_path = TEST_FILENAME, .data = TEST_FILE_CONTENTS });

    var read_file = try temp_folder.dir.openFile(TEST_FILENAME, .{});
    defer read_file.close();

    var read_buffer: [4096]u8 = @splat(0);
    var file_reader = read_file.reader(read_buffer[0..]);

    var reader = file_reader.interface;

    const read_inside_buffer = try reader.take(3);
    try std.testing.expectEq(3, file_reader.logicalPos());
    try std.testing.expectEq(u8, TEST_FILE_CONTENTS[0..3], read_inside_buffer[0..3]);

    try file_reader.seekBy(-3);

    // This is the test that's failling
    try std.testing.expectEq(0, file_reader.logicalPos()); 
}

Actual result: file_reader.logicalPos(): 33
Expected result: file_reader.logicalPos(): 0

Tested on Windows 11 24H2 (26100.4946)

Expected Behavior

Actual result: file_reader.logicalPos(): 33

Expected result: file_reader.logicalPos(): 0