R devel - Nov 2007 - isOpen on closed connections

As far as I can tell, 'isOpen' cannot return FALSE in the case when
'rw = ""'.
If the connection has already been closed by 'close' or some other
function,
then isOpen will produce an error.  The problem is that when isOpen calls 
'getConnection', the connection cannot be found and
'getConnection' produces an
error.  The check to see if it is open is never actually done.

This came up in some code where I'm trying to clean up connections after 
successfully opening them.  The problem is that if I try to close a connection 
that has already been closed, I get an error (because 'getConnection'
cannot
find it).  But then there's no way for me to find out if a connection has 
already been closed.  Perhaps there's another approach I should be taking? 
The
context is basically,

con <- file("foo", "w")

tryCatch({
	## Do stuff that might fail
	writeLines(stuff, con)
	close(con)

	file.copy("foo", "bar")
}, finally = {
	close(con)
})

So the problem is that if the block in the 'tryCatch' succeeds, the
'finally'
will produce an error.

I'm not exactly sure of what I'd want since it seems modifying
'getConnection'
would not be a great idea as it is used elsewhere.

-roger
-- 
Roger D. Peng  |  http://www.biostat.jhsph.edu/~rpeng/

"Roger D. Peng" <rpeng at jhsph.edu> writes:
> As far as I can tell, 'isOpen' cannot return FALSE in the case when
'rw = ""'.
> If the connection has already been closed by 'close' or some other
function,
> then isOpen will produce an error.  The problem is that when isOpen calls 
> 'getConnection', the connection cannot be found and
'getConnection' produces an
> error.  The check to see if it is open is never actually done.
I see this too with R-devel (r43376) {from Nov 6th}.

    con = file("example1", "w")
    isOpen(con)

    [1] TRUE

    showConnections()

      description class  mode text   isopen   can read can write
    3 "example1"  "file" "w"  "text"
"opened" "no"     "yes"

    close(con)
    isOpen(con)

    Error in isOpen(con) : invalid connection

    ## printing also fails
    con
    Error in summary.connection(x) : invalid connection
> This came up in some code where I'm trying to clean up connections
after
> successfully opening them.  The problem is that if I try to close a
connection
> that has already been closed, I get an error (because
'getConnection' cannot
> find it).  But then there's no way for me to find out if a connection
has
> already been closed.  Perhaps there's another approach I should be
taking?  The
> context is basically,
>
> con <- file("foo", "w")
>
> tryCatch({
> 	## Do stuff that might fail
> 	writeLines(stuff, con)
> 	close(con)
>
> 	file.copy("foo", "bar")
> }, finally = {
> 	close(con)
> })
This doesn't address isOpen, but why do you have the call to close
inside the tryCatch block?  Isn't the idea that finally will always be
run and so you can be reasonably sure that close gets called once?

If your real world code is more complicated, perhaps you can make use
of a work around like:

myIsOpen = function(con) tryCatch(isOpen(con), error=function(e) FALSE)

You could do similar with myClose and "close" a connection as many
times as you'd like :-)

+ seth

-- 
Seth Falcon | seth at userprimary.net | blog: http://userprimary.net/user/