Re: Question about an error with ports

[Chris Vine <vine24683579@gmail.com>]

On Fri, 11 Mar 2022 09:58:59 -0500
Olivier Dion <olivier.dion@polymtl.ca> wrote:
> On Fri, 11 Mar 2022, Chris Vine <vine24683579@gmail.com> wrote:
[snip]
> > Avoid using get-bytevector-n!, get-bytevector-some and
> > get-bytevector-all if you are going to use something like fibers or
> > some other asynchronous i/o, as those procedures are not suspendable
> > (they can block).  get-bytevector-n (note the difference from
> > get-bytevector-n!) and get-string-all are suspendable and should
> > generally be preferred where suspension is required.
> 
> I'm not sure this is related to the functions themself but instead the
> underlying filedescriptor opened iwth ON_NONBLOCK?

The problem I am referring to is different: it is that delimited
continuations cannot capture C code and ports are written in C.
Suspendable specializations, written in pure scheme, of some of guile's
i/o procedures are therefore provided in
modules/ice-9/suspendable-ports.scm, which is a file worth reading on
its own account, and which are brought into effect (by suppressing the
C-based equivalents) by applying the install-suspendable-ports!
procedure.  If your i/o steps out of this set of primitives (see in
particular the port-bindings variable in that file) then it is not
suspendable.

When suspendable ports were first introduced in guile-2.2.0, this
safe set did not include get-bytevector-n!, get-bytevector-some,
get-bytevector-some! and get-bytevector-all: I have just re-read the
code in guile-2.2.0 to confirm this.  I have also just checked with the
latest versions of guile-2.2 and guile-3.0 and it appears that
get-bytevector-n!, get-bytevector-some and get-bytevector-some! are now
supported so it looks as if at some point since version 2.2.0 they have
become OK.

With guile-2.2.0, this in turn meant that amongst other things,
http-get, http-put and so on could not normally be used in suspendable
code.  I would need to check whether that remains the case - I cannot
now remember which calls they made which were forbidden to suspendable
code.

Notable procedures which it appears remain as non-suspendable are
get-bytevector-all, get-string-n, read, write and display, but to be
sure about the first two of those I would also need to read the source
again.  A few years ago I had some test code to test which procedures
were suspendable and I will also have to see if I can find it again.

Chris