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Improve a query that terminates compeling backends from citus_update_node() (#6468) 

DESCRIPTION: Improve a query that terminates compeling backends from citus_update_node()

1. Use pg_blocking_pids() function instead of self join on pg_locks. It exists since 9.6 and more accurate than pg_locks.
2. Prefix all function calls with pg_catalog schema to prevent privilege escalation by creating functions with similar names in a public schema.
3. Change logs and update comments to reflect the fact that the pg_terminate_backend() function only sends SIGTERM but not wating for the actual backend termination.
pull/6399/head^2
Alexander Kukushkin 2022-11-02 12:32:00 +01:00 committed by GitHub
parent 402a30a2b7
commit deeacfee04
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1 changed files with 19 additions and 25 deletions
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44
src/backend/distributed/utils/acquire_lock.c
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@ -211,26 +211,20 @@ LockAcquireHelperMain(Datum main_arg)
BackgroundWorkerInitializeConnectionByOid(args->DatabaseId, InvalidOid, 0);
/*
* The query below does a self join on pg_locks to find backends that are granted a
* lock our target backend (backendPid) is waiting for. Once it found such a backend
* it terminates the backend with pg_terminate_pid.
* The query below sends a SIGTERM signal to conflicting backends using
* pg_terminate_backend() function.
*
* The result is are rows of pid,bool indicating backend that is terminated and the
* success of the termination. These will be logged accordingly below for an
* administrator to correlate in the logs with the termination message.
* The result is are rows of pid,bool indicating a conflicting backend and
* whether the SIGTERM was successfully delivered. These will be logged
* accordingly below for an administrator to correlate in the logs with the
* termination message.
*/
initStringInfo(&sql);
appendStringInfo(&sql,
"SELECT \n"
" DISTINCT conflicting.pid,\n"
" pg_terminate_backend(conflicting.pid)\n"
" FROM pg_locks AS blocked\n"
" JOIN pg_locks AS conflicting\n"
" ON (conflicting.database = blocked.database\n"
" AND conflicting.objid = blocked.objid)\n"
" WHERE conflicting.granted = true\n"
" AND blocked.granted = false\n"
" AND blocked.pid = $1;");
"WITH pids AS (\n"
" SELECT DISTINCT pid\n"
" FROM pg_catalog.unnest(pg_catalog.pg_blocking_pids($1)) AS pid\n"
") SELECT pid, pg_catalog.pg_terminate_backend(pid) FROM pids");
paramValues[0] = Int32GetDatum(backendPid);
while (ShouldAcquireLock(100))
@ -256,23 +250,23 @@ LockAcquireHelperMain(Datum main_arg)
{
bool isnull = false;
int terminatedPid = DatumGetInt32(SPI_getbinval(SPI_tuptable->vals[0],
SPI_tuptable->tupdesc,
1, &isnull));
int signaledPid = DatumGetInt32(SPI_getbinval(SPI_tuptable->vals[0],
SPI_tuptable->tupdesc,
1, &isnull));
bool isTerminated = DatumGetBool(SPI_getbinval(SPI_tuptable->vals[0],
SPI_tuptable->tupdesc,
2, &isnull));
bool isSignaled = DatumGetBool(SPI_getbinval(SPI_tuptable->vals[0],
SPI_tuptable->tupdesc,
2, &isnull));
if (isTerminated)
if (isSignaled)
{
elog(WARNING, "terminated conflicting backend %d", terminatedPid);
elog(WARNING, "terminating conflicting backend %d", signaledPid);
}
else
{
elog(INFO,
"attempt to terminate conflicting backend %d was unsuccessful",
terminatedPid);
signaledPid);
}
}
}