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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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4
hash_query.c
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@@ -147,7 +147,7 @@ hash_entry_alloc(pgssSharedState *pgss, pgssHashKey *key,int encoding)
return NULL;
}
/* Find or create an entry with desired hash code */
entry = (pgssEntry *) hash_search(pgss_hash, key, HASH_ENTER, &found);
entry = (pgssEntry *) hash_search(pgss_hash, key, HASH_ENTER_NULL, &found);
if (!found)
{
pgss->bucket_entry[pgss->current_wbucket]++;
@@ -285,7 +285,7 @@ hash_create_query_entry(uint64 bucket_id, uint64 queryid, uint64 dbid, uint64 us
key.ip = ip;
key.appid = appid;
entry = (pgssQueryEntry *) hash_search(pgss_query_hash, &key, HASH_ENTER, &found);
entry = (pgssQueryEntry *) hash_search(pgss_query_hash, &key, HASH_ENTER_NULL, &found);
return entry;
}