Horton

Re: Improve LWLock tranche name visibility across backends

Sami Imseih <samimseih@gmail.com>

View thread View on postgr.es
From: Sami Imseih <samimseih@gmail.com>
To: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Cc: Nathan Bossart <nathandbossart@gmail.com>, Rahila Syed <rahilasyed90@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2025-08-17T03:18:05Z
Lists: pgsql-hackers

Attachments

>> See v7.

I fixed an earlier issue with Windows, which was due to not initializing the
shared memory inside

```
#ifdef EXEC_BACKEND
extern void AttachSharedMemoryStructs(void);
#endif
```

But then I found another one after. LWLockTrancheNames gets forked on Linux,
but of course that will not happen on Windows without extra work. So, the
tests failed because the requested tranches (via RequestNamedLWLockTranche)
were not being found when looked up.

But of course, we already have provisions to copy these tranches for
Windows ( see inside launch_backend.c ).

```
int NamedLWLockTrancheRequests;
NamedLWLockTranche *NamedLWLockTrancheArray;
LWLockPadded *MainLWLockArray;
```

So, this means that for the local memory sync, we can actually just copy the
requested tranches (via RequestNamedLWLockTranche) and then the shared memory
tranches. This is much better, as it syncs using both possible sources
for tranche names.

```
int i = 0;

while (i < NamedLWLockTrancheRequests)
{
NamedLWLockTranche *tranche;

tranche = &NamedLWLockTrancheArray[i];

SetLocalTrancheName(i, tranche->trancheName);

i++;
}

/* Acquire shared lock on tranche names shared memory */
LWLockAcquire(&LWLockTrancheNames.shmem->lock, LW_SHARED);

while (i < LWLockTrancheNames.shmem->allocated)
{
```

So, now these tests pass locally on Windows.

Attached is v8.

--

Sami

Commits

  1. test_dsa: Avoid leaking LWLock tranches.

  2. Teach DSM registry to ERROR if attaching to an uninitialized entry.

  3. Add a test harness for the LWLock tranche code.

  4. Revert recent change to RequestNamedLWLockTranche().

  5. Move dynamically-allocated LWLock tranche names to shared memory.

  6. Prepare DSM registry for upcoming changes to LWLock tranche names.

  7. Add GetNamedDSA() and GetNamedDSHash().