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

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


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