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PG-225: Fix deadlock in pgss_store.

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The deadlock scenario is describe below:
1. pgss_store is called, it acquires the lock pgss->lock.
2. An error ocurr, mostly out of memory when accessing internal hash
   tables used to store internal data, on functions
   pgss_store_query_info and pgss_get_entry.
3. Call elog() to report out of memory error.
4. Our pgsm_emit_log_hook is called, it calls pgss_store_error, which in
   turn calls pgss_store.
5. Try to acquire already acquired lock pgss->lock, deadlock happens.

To fix the problem, there are two modifications worth mentioning done by
this commit:
1. We are now passing HASH_ENTER_NULL flag to hash_search, instead of
   HASH_ENTER, as read in postgresql sources, this prevents this
   function from reporting error when out of memory, but instead it will
   only return NULL if we pass HASH_ENTER_NULL, so we can handle the
   error ourselves.
2. In pgss_store, if an error happens after the pgss->lock is acquired,
   we only set a flag, then, after releasing the lock, we check if the
   flag is set and report the error accordingly.
This commit is contained in:
Diego Fronza
2021-09-03 14:11:58 -04:00
parent e541633670
commit eafb2e89a8
2 changed files with 8 additions and 5 deletions
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9
pg_stat_monitor.c
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@@ -1499,6 +1499,7 @@ pgss_store(uint64 queryid,
uint64 planid = plan_info ? plan_info->planid: 0;
uint64 appid = djb2_hash((unsigned char *)application_name, application_name_len);
char comments[512] = "";
bool out_of_memory = false;
/* Monitoring is disabled */
if (!PGSM_ENABLED)
return;
@@ -1532,7 +1533,7 @@ pgss_store(uint64 queryid,
pgssQueryEntry *query_entry;
query_entry = pgss_store_query_info(bucketid, queryid, dbid, userid, ip, appid, query, strlen(query), kind);
if (query_entry == NULL)
elog(DEBUG1, "pg_stat_monitor: out of memory");
out_of_memory = true;
break;
}
case PGSS_ERROR:
@@ -1543,13 +1544,13 @@ pgss_store(uint64 queryid,
query_entry = pgss_store_query_info(bucketid, queryid, dbid, userid, ip, appid, query, strlen(query), kind);
if (query_entry == NULL)
{
elog(DEBUG1, "pg_stat_monitor: out of memory");
out_of_memory = true;
break;
}
entry = pgss_get_entry(bucketid, userid, dbid, queryid, ip, planid, appid);
if (entry == NULL)
{
elog(DEBUG1, "pg_stat_monitor: out of memory");
out_of_memory = true;
break;
}
@@ -1576,6 +1577,8 @@ pgss_store(uint64 queryid,
break;
}
LWLockRelease(pgss->lock);
if (out_of_memory)
elog(DEBUG1, "pg_stat_monitor: out of memory");
}
/*
* Reset all statement statistics.