Bytes are being dropped by cat command -- likely an issue with applications/app_io or libs/stdio

[vikrammullick]

The 256th (indexed at 0) byte is being dropped when writing to stdout (and possibly when reading from stdin though this hasn't been tested).

To reproduce, run cat on a text file that is longer than 256 bytes to observe that every 256th byte is dropped.

It seems likely that this is an off-by-one error either in applications/app_io or libs/stdio.

[kevinaboos]

Ok, finally found the culprit! (Took me a while since i wasn't the original author of that stdio code)

The code responsible for this is in the try_read() function here:

Theseus/libs/stdio/src/lib.rs

Line 255 in 18ea998

while let (Some(buf_elem), Some(queue_elem)) = (buf_iter.next(), locked_ring_buf.queue.pop_front()) {

Basically, it unconditionally pops the front item off of the stdio buffer, even when there is no next item in the buffer.

The fix is to only call pop_front() iff there is another element in the destination buf.

I am fixing this for now since it's an obvious bug. Also, note that a full stdio redesign is in the works too, which will be more akin to an arbitrary byte stream, so this code will eventually be removed anyway.