[YSQL] Postgres crash in 2.12.7, 2.12.8 and 2.12.9 release.

[sushantrmishra]

Jira Link: DB-2980

Analysis

On a change to show transaction's priority to users (#12775), the backport to 2.12 introduced a bug that allows for queries to access txn_ field, which can be null.

This crash happens when the transaction has not started and yb_transaction_priority is retrieved.

The crash can be reproduced by following commands:

• select * from pg_catalog.pg_settings;
• show yb_transaction_priority;

2.14 and 2.15 releases are not impacted by this.

Description

We have observed crash in 2.12.7, 2.12.8 and 2.12.9 release. This is regression caused by GH #12775 .

yugabyte=# select * from pg_catalog.pg_settings;
server closed the connection unexpectedly
	This probably means the server terminated abnormally
	before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
!> 

yugabyte=# show yb_transaction_priority;
server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
!> \q

If the transaction is started then retrieving the transaction priority does not result in the crash.

yugabyte=# begin;
BEGIN
yugabyte=# insert into t values (4);
INSERT 0 1
yugabyte=# begin;
yugabyte=# show yb_transaction_priority;
          yb_transaction_priority
-------------------------------------------
 0.179627565 (Normal priority transaction)
(1 row)

Relevant stack trace:

Call stack:
yb::client::YBTransaction::GetPriority() const (@yb::client::YBTransaction::GetPriority() const:5)
yb::pggate::PgTxnManager::GetTransactionPriorityType() const (@yb::pggate::PgTxnManager::GetTransactionPriorityType() const:7)
show_transaction_priority (@show_transaction_priority:9)
_ShowOption (@_ShowOption:101)
GetConfigOptionByNum (@GetConfigOptionByNum:41)
show_all_settings (@show_all_settings:179)
ExecMakeTableFunctionResult (@ExecMakeTableFunctionResult:123)
FunctionNext (@FunctionNext:29)
standard_ExecutorRun (@standard_ExecutorRun:90)
pgss_ExecutorRun (@pgss_ExecutorRun:44)

Resolution

We should use the equivalent of saved_priority_ as in the master to retrieve the priority.