BUG #14929: Unchecked AllocateDir() return value in restoreTwoPhaseData()

The following bug has been logged on the website:

Bug reference: 14929
Logged by: Pan Bian
Email address: bianpan2016(at)163(dot)com
PostgreSQL version: 10.1
Operating system: Linux
Description:

File: src/backend/access/transam/twophase.c
Function: restoreTwoPhaseData
Line: 1738

AllocateDir() will return a NULL pointer if it fails to open the specified
directory. However, in function restoreTwoPhaseData(), its return value is
not checked. This may result in a NULL pointer dereference when trying to
free it (see line 1759).

For your convenience, I copy and paste related codes as follows:

1732 void
1733 restoreTwoPhaseData(void)
1734 {
1735 DIR *cldir;
1736 struct dirent *clde;
1737
1738 cldir = AllocateDir(TWOPHASE_DIR);
1739 LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);
1740 while ((clde = ReadDir(cldir, TWOPHASE_DIR)) != NULL)
1741 {
...
1758 LWLockRelease(TwoPhaseStateLock);
1759 FreeDir(cldir);
1760 }

Thank you!

Pan Bian