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CDRIVER-5589 add option to prefer TCP for SRV lookup #1625

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Merged
merged 7 commits into from
Jun 4, 2024
Merged

CDRIVER-5589 add option to prefer TCP for SRV lookup #1625

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dbdb1a0
add option to prefer TCP for SRV lookup
kevinAlbs Jun 2, 2024
02cf25c
add documentation
kevinAlbs Jun 4, 2024
2320ccd
fix formatting
kevinAlbs Jun 4, 2024
8df2a3e
rename env var to `MONGOC_EXPERIMENTAL_SRV_PREFER_TCP`
kevinAlbs Jun 4, 2024
5677157
document env var as experimental
kevinAlbs Jun 4, 2024
27e97da
check for presence of env var
kevinAlbs Jun 4, 2024
c9218bb
rename to `srv_prefer_tcp`
kevinAlbs Jun 4, 2024
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2 changes: 2 additions & 0 deletions src/libmongoc/doc/mongoc_uri_t.rst
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Expand Up @@ -79,6 +79,8 @@ The driver prefixes the service name with "_mongodb._tcp.", then performs a DNS

On Unix, the MongoDB C Driver relies on libresolv to look up SRV and TXT records. If libresolv is unavailable, then using a "mongodb+srv" URI will cause an error. If your libresolv lacks ``res_nsearch`` then the driver will fall back to ``res_search``, which is not thread-safe.

Set the environment variable ``MONGOC_EXPERIMENTAL_SRV_PREFER_TCP`` to prefer TCP for the initial queries. The environment variable is ignored for ``res_search``. Large DNS responses over UDP may be truncated due to UDP size limitations. DNS resolvers are expected to retry over TCP if the UDP response indicates truncation. Some observed DNS environments do not set the truncation flag (TC), preventing the TCP retry. This environment variable is currently experimental and subject to change.

IPv4 and IPv6
-------------

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1 change: 1 addition & 0 deletions src/libmongoc/src/mongoc/mongoc-client-private.h
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Expand Up @@ -146,6 +146,7 @@ _mongoc_client_get_rr (const char *hostname,
mongoc_rr_type_t rr_type,
mongoc_rr_data_t *rr_data,
size_t initial_buffer_size,
bool prefer_tcp,
bson_error_t *error);

mongoc_client_t *
Expand Down
18 changes: 14 additions & 4 deletions src/libmongoc/src/mongoc/mongoc-client.c
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Expand Up @@ -170,7 +170,8 @@ txt_callback (const char *hostname, PDNS_RECORD pdns, mongoc_rr_data_t *rr_data,
*/

static bool
_mongoc_get_rr_dnsapi (const char *hostname, mongoc_rr_type_t rr_type, mongoc_rr_data_t *rr_data, bson_error_t *error)
_mongoc_get_rr_dnsapi (
const char *hostname, mongoc_rr_type_t rr_type, mongoc_rr_data_t *rr_data, bool prefer_tcp, bson_error_t *error)
{
const char *rr_type_name;
WORD nst;
Expand Down Expand Up @@ -198,7 +199,11 @@ _mongoc_get_rr_dnsapi (const char *hostname, mongoc_rr_type_t rr_type, mongoc_rr
callback = txt_callback;
}

res = DnsQuery_UTF8 (hostname, nst, DNS_QUERY_BYPASS_CACHE, NULL /* IP Address */, &pdns, 0 /* reserved */);
DWORD options = DNS_QUERY_BYPASS_CACHE;
if (prefer_tcp) {
options |= DNS_QUERY_USE_TCP_ONLY;
}
res = DnsQuery_UTF8 (hostname, nst, options, NULL /* IP Address */, &pdns, 0 /* reserved */);

if (res) {
DWORD flags = FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS;
Expand Down Expand Up @@ -386,6 +391,7 @@ _mongoc_get_rr_search (const char *hostname,
mongoc_rr_type_t rr_type,
mongoc_rr_data_t *rr_data,
size_t initial_buffer_size,
bool prefer_tcp,
bson_error_t *error)
{
#ifdef MONGOC_HAVE_RES_NSEARCH
Expand Down Expand Up @@ -438,6 +444,9 @@ _mongoc_get_rr_search (const char *hostname,
#ifdef MONGOC_HAVE_RES_NSEARCH
/* thread-safe */
res_ninit (&state);
if (prefer_tcp) {
state.options |= RES_USEVC;
}
size = res_nsearch (&state, hostname, ns_c_in, nst, search_buf, buffer_size);
#elif defined(MONGOC_HAVE_RES_SEARCH)
size = res_search (hostname, ns_c_in, nst, search_buf, buffer_size);
Expand Down Expand Up @@ -551,6 +560,7 @@ _mongoc_client_get_rr (const char *hostname,
mongoc_rr_type_t rr_type,
mongoc_rr_data_t *rr_data,
size_t initial_buffer_size,
bool prefer_tcp,
bson_error_t *error)
{
BSON_ASSERT (rr_data);
Expand All @@ -563,9 +573,9 @@ _mongoc_client_get_rr (const char *hostname,
"libresolv unavailable, cannot use mongodb+srv URI");
return false;
#elif defined(MONGOC_HAVE_DNSAPI)
return _mongoc_get_rr_dnsapi (hostname, rr_type, rr_data, error);
return _mongoc_get_rr_dnsapi (hostname, rr_type, rr_data, prefer_tcp, error);
#elif (defined(MONGOC_HAVE_RES_NSEARCH) || defined(MONGOC_HAVE_RES_SEARCH))
return _mongoc_get_rr_search (hostname, rr_type, rr_data, initial_buffer_size, error);
return _mongoc_get_rr_search (hostname, rr_type, rr_data, initial_buffer_size, prefer_tcp, error);
#else
#error No SRV library is available, but ENABLE_SRV is true!
#endif
Expand Down
6 changes: 6 additions & 0 deletions src/libmongoc/src/mongoc/mongoc-topology-private.h
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Expand Up @@ -85,6 +85,7 @@ typedef bool (*_mongoc_rr_resolver_fn) (const char *hostname,
mongoc_rr_type_t rr_type,
mongoc_rr_data_t *rr_data,
size_t initial_buffer_size,
bool prefer_tcp,
bson_error_t *error);

/**
Expand Down Expand Up @@ -217,6 +218,11 @@ typedef struct _mongoc_topology_t {
// `mongoc_client_set_usleep_impl`.
mongoc_usleep_func_t usleep_fn;
void *usleep_data;

// `srv_prefer_tcp` determines if DNS lookup for SRV tries TCP first instead of UDP.
// DNS implementations are expected to try UDP first, then retry with TCP if the UDP response indicates truncation.
// Some DNS servers truncate UDP responses without setting the truncated (TC) flag. This may result in no TCP retry.
bool srv_prefer_tcp;
} mongoc_topology_t;

mongoc_topology_t *
Expand Down
32 changes: 26 additions & 6 deletions src/libmongoc/src/mongoc/mongoc-topology.c
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Expand Up @@ -383,6 +383,14 @@ mongoc_topology_new (const mongoc_uri_t *uri, bool single_threaded)
#endif

topology = (mongoc_topology_t *) bson_malloc0 (sizeof *topology);
// Check if requested to use TCP for SRV lookup.
{
char *srv_prefer_tcp = _mongoc_getenv ("MONGOC_EXPERIMENTAL_SRV_PREFER_TCP");
if (srv_prefer_tcp) {
topology->srv_prefer_tcp = true;
}
bson_free (srv_prefer_tcp);
}
topology->usleep_fn = mongoc_usleep_default_impl;
topology->session_pool = mongoc_server_session_pool_new_with_params (
_server_session_init, _server_session_destroy, _server_session_should_prune, topology);
Expand Down Expand Up @@ -476,16 +484,24 @@ mongoc_topology_new (const mongoc_uri_t *uri, bool single_threaded)

/* a mongodb+srv URI. try SRV lookup, if no error then also try TXT */
prefixed_hostname = bson_strdup_printf ("_%s._tcp.%s", mongoc_uri_get_srv_service_name (uri), srv_hostname);
if (!topology->rr_resolver (
prefixed_hostname, MONGOC_RR_SRV, &rr_data, MONGOC_RR_DEFAULT_BUFFER_SIZE, &topology->scanner->error)) {
if (!topology->rr_resolver (prefixed_hostname,
MONGOC_RR_SRV,
&rr_data,
MONGOC_RR_DEFAULT_BUFFER_SIZE,
topology->srv_prefer_tcp,
&topology->scanner->error)) {
GOTO (srv_fail);
}

/* Failure to find TXT records will not return an error (since it is only
* for options). But _mongoc_client_get_rr may return an error if
* there is more than one TXT record returned. */
if (!topology->rr_resolver (
srv_hostname, MONGOC_RR_TXT, &rr_data, MONGOC_RR_DEFAULT_BUFFER_SIZE, &topology->scanner->error)) {
if (!topology->rr_resolver (srv_hostname,
MONGOC_RR_TXT,
&rr_data,
MONGOC_RR_DEFAULT_BUFFER_SIZE,
topology->srv_prefer_tcp,
&topology->scanner->error)) {
GOTO (srv_fail);
}

Expand Down Expand Up @@ -812,8 +828,12 @@ mongoc_topology_rescan_srv (mongoc_topology_t *topology)
prefixed_hostname =
bson_strdup_printf ("_%s._tcp.%s", mongoc_uri_get_srv_service_name (topology->uri), srv_hostname);

ret = topology->rr_resolver (
prefixed_hostname, MONGOC_RR_SRV, &rr_data, MONGOC_RR_DEFAULT_BUFFER_SIZE, &topology->scanner->error);
ret = topology->rr_resolver (prefixed_hostname,
MONGOC_RR_SRV,
&rr_data,
MONGOC_RR_DEFAULT_BUFFER_SIZE,
topology->srv_prefer_tcp,
&topology->scanner->error);

td = mc_tpld_take_ref (topology);
topology->srv_polling_last_scan_ms = bson_get_monotonic_time () / 1000;
Expand Down
16 changes: 13 additions & 3 deletions src/libmongoc/tests/test-mongoc-dns.c
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Expand Up @@ -599,9 +599,9 @@ test_small_initial_buffer (void *unused)
BSON_UNUSED (unused);

memset (&rr_data, 0, sizeof (rr_data));
ASSERT_OR_PRINT (
_mongoc_client_get_rr ("_mongodb._tcp.test1.test.build.10gen.cc", rr_type, &rr_data, small_buffer_size, &error),
error);
ASSERT_OR_PRINT (_mongoc_client_get_rr (
"_mongodb._tcp.test1.test.build.10gen.cc", rr_type, &rr_data, small_buffer_size, false, &error),
error);
ASSERT_CMPINT (rr_data.count, ==, 2);
bson_free (rr_data.txt_record_opts);
_mongoc_host_list_destroy_all (rr_data.hosts);
Expand All @@ -612,12 +612,14 @@ _mock_rr_resolver_prose_test_9 (const char *service,
mongoc_rr_type_t rr_type,
mongoc_rr_data_t *rr_data,
size_t initial_buffer_size,
bool prefer_tcp,
bson_error_t *error)
{
BSON_UNUSED (service);
BSON_UNUSED (rr_type);
BSON_UNUSED (rr_data);
BSON_UNUSED (initial_buffer_size);
BSON_UNUSED (prefer_tcp);
BSON_UNUSED (error);

test_error ("Expected mock resolver to not be called");
Expand Down Expand Up @@ -853,12 +855,14 @@ _mock_rr_resolver_prose_test_10 (const char *service,
mongoc_rr_type_t rr_type,
mongoc_rr_data_t *rr_data,
size_t initial_buffer_size,
bool prefer_tcp,
bson_error_t *error)
{
BSON_UNUSED (initial_buffer_size);

BSON_ASSERT_PARAM (service);
BSON_ASSERT_PARAM (rr_data);
BSON_UNUSED (prefer_tcp);
BSON_ASSERT_PARAM (error);

if (rr_type == MONGOC_RR_SRV) {
Expand Down Expand Up @@ -953,12 +957,14 @@ _mock_rr_resolver_prose_test_11 (const char *service,
mongoc_rr_type_t rr_type,
mongoc_rr_data_t *rr_data,
size_t initial_buffer_size,
bool prefer_tcp,
bson_error_t *error)
{
BSON_UNUSED (initial_buffer_size);

BSON_ASSERT_PARAM (service);
BSON_ASSERT_PARAM (rr_data);
BSON_UNUSED (prefer_tcp);
BSON_ASSERT_PARAM (error);

if (rr_type == MONGOC_RR_SRV) {
Expand Down Expand Up @@ -1050,12 +1056,14 @@ _mock_rr_resolver_prose_test_12 (const char *service,
mongoc_rr_type_t rr_type,
mongoc_rr_data_t *rr_data,
size_t initial_buffer_size,
bool prefer_tcp,
bson_error_t *error)
{
BSON_UNUSED (initial_buffer_size);

BSON_ASSERT_PARAM (service);
BSON_ASSERT_PARAM (rr_data);
BSON_UNUSED (prefer_tcp);
BSON_ASSERT_PARAM (error);

if (rr_type == MONGOC_RR_SRV) {
Expand Down Expand Up @@ -1198,12 +1206,14 @@ _mock_rr_resolver_with_override (const char *service,
mongoc_rr_type_t rr_type,
mongoc_rr_data_t *rr_data,
size_t initial_buffer_size,
bool prefer_tcp,
bson_error_t *error)
{
BSON_UNUSED (initial_buffer_size);

BSON_ASSERT_PARAM (service);
BSON_ASSERT_PARAM (rr_data);
BSON_UNUSED (prefer_tcp);
BSON_ASSERT_PARAM (error);

if (rr_type == MONGOC_RR_SRV) {
Expand Down