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Fix the selection of events to cancel in test_workqueue.c

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Our previous algorithm had a nonzero probability of picking no
events to cancel, which is of course incorrect.  The new code uses
Vitter's good old reservoir sampling "algorithm R" from 1985.

Fixes bug 26008; bugfix on 0.2.6.3-alpha.
This commit is contained in:
Nick Mathewson
2018-05-06 21:03:26 -04:00
parent f36656cada
commit 6e3e96d2ff
2 changed files with 16 additions and 3 deletions
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src/test/test_workqueue.c
+9 -3
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@@ -224,7 +224,8 @@ add_n_work_items(threadpool_t *tp, int n)
workqueue_entry_t **to_cancel;
workqueue_entry_t *ent;
to_cancel = tor_malloc(sizeof(workqueue_entry_t*) * opt_n_cancel);
// We'll choose randomly which entries to cancel.
to_cancel = tor_calloc(opt_n_cancel, sizeof(workqueue_entry_t*));
while (n_queued++ < n) {
ent = add_work(tp);
@@ -233,9 +234,14 @@ add_n_work_items(threadpool_t *tp, int n)
tor_libevent_exit_loop_after_delay(tor_libevent_get_base(), NULL);
return -1;
}
if (n_try_cancel < opt_n_cancel &&
tor_weak_random_range(&weak_rng, n) < opt_n_cancel) {
if (n_try_cancel < opt_n_cancel) {
to_cancel[n_try_cancel++] = ent;
} else {
int p = tor_weak_random_range(&weak_rng, n_queued);
if (p < n_try_cancel) {
to_cancel[p] = ent;
}
}
}