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register only a single poll_t with libuv 

Most of the time we only need a read _or_ a write callback registered
with libuv - for example, on a UDP request a write callback is
registered, when executed the write callback performs the write,
deregisters itself, and registers a read callback.

However there is one case where getdns registers both read and write
callbacks: when a backlog of TCP requests is going to the same upstream
resolver, we use a single fd and queue the requests. In this instance we
want to listen for both read (to get responses for requests we've
already sent) and write (to continue to send our pending requests).

libuv, like most event libraries, only allows one callback to be
registered per fd. To get notification for both reads and writes, you
should examine the event flags and have appropriate conditional logic
within the single callback. Today getdns incorrectly tries to register
two separate poll_t with libuv, one for read and one for write - this
results in a crash (internal libuv assertion guaranteeing that only a
single poll_t is registered per fd).

This was tested by building locally, creating an ALIAS to a domain known
to have large responses that truncate (forcing TCP), running trex with
--resolver-cache-size 0, and flaming at 10qps. Previously this would
crash within the first couple of seconds, now it works indefinitely.

Note that a higher qps trigger a _different_ getdns/libuv crashing bug
that occurs when the TCP backlog grows so large that requests start to
time out. That crash is not addressed in this PR.
This commit is contained in:
Eli Lindsey 2020-06-18 13:32:57 -04:00
parent e481273ff4
commit 4e0e2ee1f5
1 changed files with 30 additions and 42 deletions
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72
src/extension/libuv.c
View File

@ -73,8 +73,7 @@ getdns_libuv_cleanup(getdns_eventloop *loop)
}
typedef struct poll_timer {
uv_poll_t read;
uv_poll_t write;
uv_poll_t poll;
uv_timer_t timer;
int to_close;
struct mem_funcs mf;
@ -111,15 +110,8 @@ getdns_libuv_clear(getdns_eventloop *loop, getdns_eventloop_event *el_ev)
DEBUG_UV("enter libuv_clear(el_ev = %p, my_ev = %p, to_close = %d)\n"
, el_ev, my_ev, my_ev->to_close);
if (el_ev->read_cb) {
my_poll = &my_ev->read;
uv_poll_stop(my_poll);
my_ev->to_close += 1;
my_poll->data = my_ev;
uv_close((uv_handle_t *)my_poll, getdns_libuv_close_cb);
}
if (el_ev->write_cb) {
my_poll = &my_ev->write;
if (el_ev->read_cb || el_ev->write_cb) {
my_poll = &my_ev->poll;
uv_poll_stop(my_poll);
my_ev->to_close += 1;
my_poll->data = my_ev;
@ -139,29 +131,28 @@ getdns_libuv_clear(getdns_eventloop *loop, getdns_eventloop_event *el_ev)
}
static void
getdns_libuv_read_cb(uv_poll_t *poll, int status, int events)
getdns_libuv_cb(uv_poll_t *poll, int status, int events)
{
getdns_eventloop_event *el_ev = (getdns_eventloop_event *)poll->data;
(void)status; (void)events;
assert(el_ev->read_cb);
DEBUG_UV("enter libuv_read_cb(el_ev = %p, el_ev->ev = %p)\n"
, el_ev, el_ev->ev);
el_ev->read_cb(el_ev->userarg);
DEBUG_UV("exit libuv_read_cb(el_ev = %p, el_ev->ev = %p)\n"
, el_ev, el_ev->ev);
}
getdns_eventloop_event *el_ev = (getdns_eventloop_event *)poll->data;
(void)status;
static void
getdns_libuv_write_cb(uv_poll_t *poll, int status, int events)
{
getdns_eventloop_event *el_ev = (getdns_eventloop_event *)poll->data;
(void)status; (void)events;
assert(el_ev->write_cb);
DEBUG_UV("enter libuv_write_cb(el_ev = %p, el_ev->ev = %p)\n"
, el_ev, el_ev->ev);
el_ev->write_cb(el_ev->userarg);
DEBUG_UV("exit libuv_write_cb(el_ev = %p, el_ev->ev = %p)\n"
, el_ev, el_ev->ev);
if (events & UV_READABLE) {
assert(el_ev->read_cb);
DEBUG_UV("enter libuv_read_cb(el_ev = %p, el_ev->ev = %p)\n"
, el_ev, el_ev->ev);
el_ev->read_cb(el_ev->userarg);
DEBUG_UV("exit libuv_read_cb(el_ev = %p, el_ev->ev = %p)\n"
, el_ev, el_ev->ev);
} else if (events & UV_WRITABLE) {
assert(el_ev->write_cb);
DEBUG_UV("enter libuv_write_cb(el_ev = %p, el_ev->ev = %p)\n"
, el_ev, el_ev->ev);
el_ev->write_cb(el_ev->userarg);
DEBUG_UV("exit libuv_write_cb(el_ev = %p, el_ev->ev = %p)\n"
, el_ev, el_ev->ev);
} else {
assert(ASSERT_UNREACHABLE);
}
}
static void
@ -205,18 +196,15 @@ getdns_libuv_schedule(getdns_eventloop *loop,
my_ev->to_close = 0;
my_ev->mf = ext->mf;
el_ev->ev = my_ev;
if (el_ev->read_cb) {
my_poll = &my_ev->read;
if (el_ev->read_cb || el_ev->write_cb) {
my_poll = &my_ev->poll;
my_poll->data = el_ev;
uv_poll_init(ext->loop, my_poll, fd);
uv_poll_start(my_poll, UV_READABLE, getdns_libuv_read_cb);
}
if (el_ev->write_cb) {
my_poll = &my_ev->write;
my_poll->data = el_ev;
uv_poll_init(ext->loop, my_poll, fd);
uv_poll_start(my_poll, UV_WRITABLE, getdns_libuv_write_cb);
int events =
(el_ev->read_cb ? UV_READABLE : 0) |
(el_ev->write_cb ? UV_WRITABLE : 0);
uv_poll_start(my_poll, events, getdns_libuv_cb);
}
if (el_ev->timeout_cb) {
my_timer = &my_ev->timer;