Hello!
I just discovered undesirable side effects of commit
b5cb4464ca4e23d077a9777bbc17835feb0f4374 "Make multi-byte reads on
unbuffered ports more efficient."
An example application that breaks in the presence of this patch are
"custom binary input ports" (aka. CBIPs [0]) in Guile-R6RS-Libs [1]. The
CBIP implementation [2] works as follows:
1. make_cbip ()
/* Create a bytevector for use as the CBIP's internal buffer. */
SCM bv = scm_r6rs_c_make_bytevector (c_len);
c_bv = (char *) SCM_R6RS_BYTEVECTOR_CONTENTS (bv);
c_port->read_pos = c_port->read_buf = (unsigned char *) c_bv;
c_port->read_end = (unsigned char *) c_bv;
/* Store BV for later reuse. */
SCM_SETSTREAM (port, SCM_UNPACK (bv, and other things));
2. cbip_fill_input (port)
if (c_port->read_pos >= c_port->read_end)
{
/* Invoke the user's `read!' procedure. */
bv = SCM_R6RS_CBIP_BYTEVECTOR (port);
octets = scm_call_3 (read_proc, bv, SCM_INUM0,
SCM_I_MAKINUM (CBIP_BUFFER_SIZE));
c_octets = scm_to_uint (octets);
c_port->read_pos = (unsigned char *) SCM_R6RS_BYTEVECTOR_CONTENTS
(bv);
c_port->read_end = (unsigned char *) c_port->read_pos +
c_port->c_octets;
}
IOW, the CBIP `fill_input' method does *not* directly pass
`c_port->read_buf' to the user's `read!' method but instead passes it
its bytevector, which it assumes to wrap its internal. Thus, if
`c_port->read_buf' happens to point to something other than BV's
contents, it is just left untouched.
Worse, `cbip_fill_input ()' updates `read_pos' and `read_end' but does
not touch `read_buf', leading to an inconsistent state that will confuse
later `scm_fill_input ()' calls on that port (e.g., in the loop for
`scm_c_read ()'), and possibly to heap corruption.
So where to go from here? I think this example shows that the
`swap_buffer' trick is too risky, unfortunately. Thus, we may need to
revert it, at least in 1.8. Second, I think that a `read' method as a
replacement for `fill_input', as I proposed back then [3], would be
safer; maybe 1.9 would be a nice place to add it. Neil: what do you
think?
Thanks,
Ludo'.
[0]
http://www.r6rs.org/final/html/r6rs-lib/r6rs-lib-Z-H-1.html#node_toc_node_sec_8.2.7
[1] http://www.fdn.fr/~lcourtes/software/guile/guile-r6rs-libs-0.1.tar.gz
and http://repo.or.cz/w/guile-r6rs-libs.git
[2] http://repo.or.cz/w/guile-r6rs-libs.git?a=blob;f=src/ports.c;#l210
[3] http://thread.gmane.org/gmane.lisp.guile.devel/7292
