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Fix close-port race.

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* libguile/ports.c (release_port): Fix race.
This commit is contained in:
Andy Wingo 2017-01-08 15:42:22 +01:00
parent dffe495d0d
commit b392d81c9c
1 changed files with 15 additions and 8 deletions
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23
libguile/ports.c
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@ -134,15 +134,22 @@ static void
release_port (SCM port)
{
scm_t_port *pt = SCM_PORT (port);
scm_t_uint32 prev;
prev = scm_atomic_subtract_uint32 (&pt->refcount, 1);
if (prev == 0)
/* Logic failure. */
abort ();
if (prev > 1)
/* Port still alive. */
/* It's possible for two close-port invocations to race, and since
close-port is defined to be idempotent we need to avoid
decrementing the refcount past 0. The normal case is that it's
open with a refcount of 1 and we're going to change it to 0.
Otherwise if the refcount is higher we just subtract 1 and we're
done. However if the current refcount is 0 then the port has been
closed or is closing and we just return. */
scm_t_uint32 cur = 1, next = 0;
while (!scm_atomic_compare_and_swap_uint32 (&pt->refcount, &cur, next))
{
if (cur == 0)
return;
next = cur - 1;
}
if (cur > 1)
return;
/* FIXME: `catch' around the close call? It could throw an exception,