R devel - Apr 2018 - readLines() for non-blocking pipeline behaves differently in R 3.5

It seems that the behavior of readLines() in R 3.5 has changed for non-blocking
pipeline.


Consider the following R script, which reads from STDIN line by line.
```
con <- file("stdin")
open(con, blocking = FALSE)

while (TRUE) {
? ? txt <- readLines(con, 1)
? ? if (length(txt) > 0) {
? ? ? ? cat(txt, "\n", file = stdout())
? ? }
? ? Sys.sleep(0.1)
}
close(con)

```

In R 3.4.4, it works as expected.

```
(randymbpro)-Desktop$ echo "abc\nfoo" | R --slave -f test.R
abc
foo
```

In R 3.5, only the first line is printed
```
(randymbpro)-Desktop$ echo "abc\nfoo" | R --slave -f test.R
abc
```

Is this change expected? ?If I change `blocking` to `TRUE` above, the above code
would
work. But I need non-blocking connection in my use case of piping buffer from
another program.

Best,

R 3.5 @ macOS 10.13


Randy


	[[alternative HTML version deleted]]

Probably related to the switch to buffered connections. I will look
into this soon.

On Wed, Apr 25, 2018 at 2:34 PM, Randy Lai <randy.cs.lai at gmail.com>
wrote:
> It seems that the behavior of readLines() in R 3.5 has changed for
non-blocking pipeline.
>
>
> Consider the following R script, which reads from STDIN line by line.
> ```
> con <- file("stdin")
> open(con, blocking = FALSE)
>
> while (TRUE) {
>     txt <- readLines(con, 1)
>     if (length(txt) > 0) {
>         cat(txt, "\n", file = stdout())
>     }
>     Sys.sleep(0.1)
> }
> close(con)
>
> ```
>
> In R 3.4.4, it works as expected.
>
> ```
> (randymbpro)-Desktop$ echo "abc\nfoo" | R --slave -f test.R
> abc
> foo
> ```
>
> In R 3.5, only the first line is printed
> ```
> (randymbpro)-Desktop$ echo "abc\nfoo" | R --slave -f test.R
> abc
> ```
>
> Is this change expected?  If I change `blocking` to `TRUE` above, the above
code would
> work. But I need non-blocking connection in my use case of piping buffer
from
> another program.
>
> Best,
>
> R 3.5 @ macOS 10.13
>
>
> Randy
>
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

The issue is that readLines() tries to seek (for reasons I don't
understand) in the non-blocking case, but silently fails for "stdin"
since it's a stream. This confused the buffering logic. The fix is to
mark "stdin" as unable to seek, but I do wonder why readLines() is
seeking in the first place.

Anyway, I'll get this into patched ASAP. Thanks for the report.

Michael


On Wed, Apr 25, 2018 at 5:13 PM, Michael Lawrence <michafla at gene.com>
wrote:
> Probably related to the switch to buffered connections. I will look
> into this soon.
>
> On Wed, Apr 25, 2018 at 2:34 PM, Randy Lai <randy.cs.lai at
gmail.com> wrote:
>> It seems that the behavior of readLines() in R 3.5 has changed for
non-blocking pipeline.
>>
>>
>> Consider the following R script, which reads from STDIN line by line.
>> ```
>> con <- file("stdin")
>> open(con, blocking = FALSE)
>>
>> while (TRUE) {
>>     txt <- readLines(con, 1)
>>     if (length(txt) > 0) {
>>         cat(txt, "\n", file = stdout())
>>     }
>>     Sys.sleep(0.1)
>> }
>> close(con)
>>
>> ```
>>
>> In R 3.4.4, it works as expected.
>>
>> ```
>> (randymbpro)-Desktop$ echo "abc\nfoo" | R --slave -f test.R
>> abc
>> foo
>> ```
>>
>> In R 3.5, only the first line is printed
>> ```
>> (randymbpro)-Desktop$ echo "abc\nfoo" | R --slave -f test.R
>> abc
>> ```
>>
>> Is this change expected?  If I change `blocking` to `TRUE` above, the
above code would
>> work. But I need non-blocking connection in my use case of piping
buffer from
>> another program.
>>
>> Best,
>>
>> R 3.5 @ macOS 10.13
>>
>>
>> Randy
>>
>>
>>         [[alternative HTML version deleted]]
>>
>> ______________________________________________
>> R-devel at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>