MDEV-22529 thd_query_safe() isn’t, causing InnoDB to hangbb-10.2-27snc1

The function thd_query_safe() is used in the implementation of the
following INFORMATION_SCHEMA views:

    information_schema.innodb_trx
    information_schema.innodb_locks
    information_schema.innodb_lock_waits
    information_schema.rocksdb_trx

The implementation of the InnoDB views is in trx_i_s_common_fill_table().
This function invokes trx_i_s_possibly_fetch_data_into_cache(),
which will acquire lock_sys->mutex and trx_sys->mutex in order to
protect the set of active transactions and explicit locks.
While holding those mutexes, it will traverse the collection of
InnoDB transactions. For each transaction, thd_query_safe() will be
invoked.

When called via trx_i_s_common_fill_table(), thd_query_safe()
is acquiring THD::LOCK_thd_data while holding the InnoDB locks.
This will cause a deadlock with THD::awake() (such as executing
KILL QUERY), because THD::awake() could invoke lock_trx_handle_wait(),
which attempts to acquire lock_sys->mutex while already holding
THD::lock_thd_data.

thd_query_safe(): Invoke mysql_mutex_trylock() instead of
mysql_mutex_lock(). Return the empty string if the mutex
cannot be acquired without waiting.

1 files changed, 14 insertions, 4 deletions

diff --git a/sql/sql_class.cc b/sql/sql_class.cc
index 20366742084..a67a8b8e143 100644
--- a/sql/sql_class.cc
+++ b/sql/sql_class.cc
@@ -4525,6 +4525,7 @@ extern "C" LEX_STRING * thd_query_string (MYSQL_THD thd)
   @param buflen  Length of the buffer
 
   @return Length of the query
+  @retval 0 if LOCK_thd_data cannot be acquired without waiting
 
   @note This function is thread safe as the query string is
         accessed under mutex protection and the string is copied
@@ -4533,10 +4534,19 @@ extern "C" LEX_STRING * thd_query_string (MYSQL_THD thd)
 
 extern "C" size_t thd_query_safe(MYSQL_THD thd, char *buf, size_t buflen)
 {
-  mysql_mutex_lock(&thd->LOCK_thd_data);
-  size_t len= MY_MIN(buflen - 1, thd->query_length());
-  memcpy(buf, thd->query(), len);
-  mysql_mutex_unlock(&thd->LOCK_thd_data);
+  size_t len= 0;
+  /* InnoDB invokes this function while holding internal mutexes.
+  THD::awake() will hold LOCK_thd_data while invoking an InnoDB
+  function that would acquire the internal mutex. Because this
+  function is a non-essential part of information_schema view output,
+  we will break the deadlock by avoiding a mutex wait here
+  and returning the empty string if a wait would be needed. */
+  if (!mysql_mutex_trylock(&thd->LOCK_thd_data))
+  {
+    len= MY_MIN(buflen - 1, thd->query_length());
+    memcpy(buf, thd->query(), len);
+    mysql_mutex_unlock(&thd->LOCK_thd_data);
+  }
   buf[len]= '\0';
   return len;
 }