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


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