Enhancement request: anonymous connections
On 1/1/2006 1:05 PM, Prof Brian Ripley wrote:
On Sun, 1 Jan 2006, Duncan Murdoch wrote:
On 12/28/2005 9:50 AM, Seth Falcon wrote:
On 27 Dec 2005, murdoch at stats.uwo.ca wrote:
This is a bug in load, isn't it? load() opens the connection but doesn't close it.
Well, it may be that load needs a small fix, but that doesn't fix anonymous connections in general, IMO.
No it doesn't. However, I've committed the small fix.
It was not even a bug in load: close() and open() are not pairs. (I didn't pick the names!) Your `fix' destroys a connection, which is not the documented behaviour and far more dangerous than leaving it open. The lifecycle of a connection is (see e.g. my R-news article) create->open->close->destroy and close() does both of the last two. Please revert this change. Ideally we would close and not destroy if the connection was opened, but that needs a better C-level interface in place of this R-level one.
Sorry about that. I've done the reversion.
Is it worth putting that C-level interface in place to make load() more
compatible with ?connections which says,
'open' opens a connection. In general functions using connections
will open them if they are not open, but then close them again, so
to leave a connection open call 'open' explicitly.
? I suppose a natural way to do it would be to add a "destroy=TRUE"
argument to close(), and then have load() do
on.exit(close(con, destroy=FALSE))
but maybe it would be better to add a separate function to do this, for
better green book compatibility.
Duncan Murdoch
The loop could easily have been:
for (i in 1:50) {
print(load(url(testUrl, open="r")))
}
And it doesn't need to be related to url or load:
cat("a line of text\n", file="another-example.txt")
z <- NULL
for (i in 1:50) {
z <- c(z, readLines(file("another-example.txt", open="r")))
}
Also, connections are "in use" even if they are closed:
for (i in 1:50) {
if (isOpen(file("another-example.txt")))
stop("you will not get here")
}
I think the general problem is that R doesn't have references (or at least, they aren't in a final, documented state). If the garbage collector closed a connection, then things would go wrong when there were two copies of it: the second one would be messed up when the first was destroyed. If we had references, then opening a connection could create a connection object and a reference to it; the connection object would remain as long as there were any references to it, and could be destroyed (and automatically closed) after the last reference was gone.
However, that just isn't how connections are documented in the Green Book (referenced on all the relevant help pages, so required reading) and getConnection() allows you to create an R object pointing to a connection that previously had none. The OP has never told us what `anonymous connections' are, but it is quite possible that his unstated ideas are incompatible with the documentation.