dd: reports incorrect whole/partial records written

[jfinkels]

GNU dd:

$ mkfifo inpipe
$ dd ibs=3 obs=3 if=inpipe &
$ (printf 'ab'; sleep 1; printf 'cd') > inpipe
abcd0+2 records in
1+1 records out
4 bytes copied, 1.00373 s, 0.0 kB/s

uutils dd:

$ mkfifo inpipe
$ ./target/debug/dd ibs=3 obs=3 if=inpipe &
$ (printf 'ab'; sleep 1; printf 'cd') > inpipe
0+2 records in
2+0 records out
4 bytes (4 B, 4 B) copied, 1.0 s, 0 B/s
abcd

There's a couple of minor differences here, but this issue is about the difference in the number of "records out":
1+1 records out versus 2+0 records out. The number on the left of the + sign is the number of complete blocks written, and the number on the right is the number of partial blocks written. In this case, the input block size is 3 and the output block size is 3, so 1+1 records out means that 1 complete block was written ('abc') and 1 partial block was written ('d'). However, the uutils dd is reporting 2 complete blocks written (presumably 'ab' and 'cd'), which is incorrect.

I believe this is happening because we are not buffering the output appropriately. I'm thinking that by maintaining a buffer of size obs, only flushing it when full, and only incrementing the number of complete blocks on flush, then we can get the correct behavior.

One caveat: if bs=3 is given instead of ibs=3 obs=3 on the command-line, then the behavior is different, as described in https://www.gnu.org/software/coreutils/manual/html_node/dd-invocation.html :

This makes dd read and write bytes per block, overriding any ‘ibs’ and ‘obs’ settings. In addition, if no data-transforming conv operand is specified, input is copied to the output as soon as it’s read, even if it is smaller than the block size.

In that case:

$ mkfifo inpipe
$ dd bs=3 if=inpipe &
$ (printf 'ab'; sleep 1; printf 'cd') > inpipe
abcd0+2 records in
0+2 records out
4 bytes copied, 1.00381 s, 0.0 kB/s

and you can see when you run it that the first partial block 'ab' gets written, then there is a 1 second pause, then the second partial block 'cd' gets written.

[jfinkels]

Here's a simpler demonstration:

GNU dd:

$ printf 'abc' | dd bs=2 status=noxfer > /dev/null
1+1 records in
1+1 records out

uutils dd:

$ printf 'abc' | ./target/debug/dd bs=2 status=noxfer > /dev/null
1+1 records in
2+0 records out

[jfinkels]

Pull request #3129 resolved the simpler test case $ printf 'abc' | dd bs=2 status=noxfer > /dev/null, but not the one I originally reported.

[jfinkels]

I can no longer reproduce the issue as originally reported, so I think this is fixed.