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test: handle zero-length udp datagram #4344

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test: handle zero-length udp datagram #4344

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21 changes: 14 additions & 7 deletions test/test-udp-recv-in-a-row.c
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Original file line number Diff line number Diff line change
Expand Up @@ -50,11 +50,16 @@ static void sv_recv_cb(uv_udp_t* handle,
const uv_buf_t* rcvbuf,
const struct sockaddr* addr,
unsigned flags) {
if (++ recv_cnt < N) {
ASSERT_EQ(sizeof(send_data), nread);
} else {
ASSERT_OK(nread);
}
/* |nread| can be zero when the kernel drops an incoming datagram after
* marking the file descriptor as readable but before libuv has a chance
* to receive it. Libuv still invokes the uv_udp_recv_cb callback to give
* back the memory from the uv_alloc_cb callback.
*
* See https://github.com/libuv/libuv/issues/4219.
*/
recv_cnt++;
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if (nread > 0)
ASSERT_EQ(nread, sizeof(send_data));
}

static void check_cb(uv_check_t* handle) {
Expand All @@ -63,9 +68,11 @@ static void check_cb(uv_check_t* handle) {
/**
* sv_recv_cb() is called with nread set to zero to indicate
* there is no more udp packet in the kernel, so the actual
* recv_cnt is one larger than N.
* recv_cnt is up to one larger than N. UDP being what it is,
* packets can get dropped so don't assume an exact count.
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@vtjnash after looking at the code some more, I came the conclusion that the "is equal to N+1" check is wrong because any number of packets can get dropped. Even assuming a single packet makes it through is already a shaky assumption, just one that is true most of the time.

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IIUC, the kernel does end up guaranteeing reliable UDP delivery to localhost. It is sort of by-accident, but it would be more work for it to discard messages because it goes through loopback directly instead of the making a whole roundtrip through the hardware.

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Linux maybe but XNU? The CI failures are on macos buildbots.

FWIW, I can sort of reproduce it on Linux with a traffic-shaped device but at that point it's not really localhost traffic anymore so maybe not completely apples to apples.

*/
ASSERT_EQ(N+1, recv_cnt);
ASSERT_GE(recv_cnt, 1);
ASSERT_LE(recv_cnt, N+1);
check_cb_called = 1;

/* we are done */
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