[Rd] readLines() for non-blocking pipeline behaves differently in R 3.5

[Michael Lawrence]

Thanks for the clear explanation. At first glance seeking to the
current position seemed like it would be a no-op, but obviously things
are more complicated under the hood.

On Thu, Apr 26, 2018 at 11:35 AM, Gábor Csárdi <csardi.gabor at gmail.com> wrote:
> I suspect the reason for the seek is this:
>
> cat("1\n", file = "foobar")
> f  <- file("foobar", blocking = FALSE, open = "r")
> readLines(f)
> #> [1] "1"
>
> cat("2\n", file = "foobar", append = TRUE)
> readLines(f)
> #> [1] "2"
>
> cat("3\n", file = "foobar", append = TRUE)
> readLines(f)
> #> [1] "3"
>
> I.e. R can emulate a file connection with non-blocking reads.
> AFAICT there is no such thing, in Unix at least.
> For this  emulation, it needs to seek to the "current" position.
>
> Gabor
>
> On Thu, Apr 26, 2018 at 7:21 PM, Michael Lawrence
> <lawrence.michael at gene.com> wrote:
>> 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]]
>>>>
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>>
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