Hi,
I have not reviewed the repack-related patches before. Recently, I started trying to understand how repack works and
tracethrough the code.
While tracing start_repack_decoding_worker(), I noticed something suspicious:
```
seg = dsm_create(size, 0);
shared = (DecodingWorkerShared *) dsm_segment_address(seg);
shared->lsn_upto = InvalidXLogRecPtr;
shared->done = false;
SharedFileSetInit(&shared->sfs, seg);
shared->last_exported = -1;
SpinLockInit(&shared->mutex);
shared->dbid = MyDatabaseId;
```
Here, the code creates a shared-memory segment and lets “shared" point to that memory. It then initializes some fields
of“shared". However, later code reads shared->initialized, but this field was not initialized:
```
for (;;)
{
bool initialized;
SpinLockAcquire(&shared->mutex);
initialized = shared->initialized;
SpinLockRelease(&shared->mutex);
if (initialized)
break;
ConditionVariableSleep(&shared->cv, WAIT_EVENT_REPACK_WORKER_EXPORT);
}
```
I checked the code of dsm_create(), and I did not see anything showing that the created shared-memory segment is
zeroed.
This is actually just an eyeball finding. From my tracing on my MacBook, after shared = (DecodingWorkerShared *)
dsm_segment_address(seg);,the memory always seemed to be zeroed, so I may have missed something. But given that the
codeexplicitly initializes several other fields, it seems better not to rely on that implicitly.
For the fix, since start_repack_decoding_worker() is not on a hot path, I think it is fine to zero the whole shared
structexplicitly, and then initialize the non-zero fields afterwards.
Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/
