rust/neon
1
0
Fork 0
mirror of https://github.com/neondatabase/neon.git synced 2026-05-27 18:10:37 +00:00
Code Issues Packages Projects Releases Wiki Activity

Fix issue in compaction code (#3246)

Browse Source 
If we ran `compact_prefetch_buffers` with exactly one hole in the
buffers, the code would fail to remove the last, now unused, entry from
the array.

This is now fixed. 

Also, add and adjust some comments in the compaction code so that the
algorithm used is a bit more clear.

Fixes #3192
This commit is contained in:
MMeent
2023-01-12 19:23:59 +01:00
committed by GitHub
parent 57a6e931ea
commit bb406b21a8
1 changed files with 14 additions and 5 deletions
Download Patch File Download Diff File Expand all files Collapse all files

19
pgxn/neon/pagestore_smgr.c
View File

@@ -287,12 +287,13 @@ compact_prefetch_buffers(void)
/*
* Here we have established:
* slots < search_ring_index may be unused (not scanned)
* slots >= search_ring_index and <= empty_ring_index are unused
* slots > empty_ring_index are in use, or outside our buffer's range.
* slots < search_ring_index have an unknown state (not scanned)
* slots >= search_ring_index and <= empty_ring_index are unused
* slots > empty_ring_index are in use, or outside our buffer's range.
* ... unless search_ring_index <= ring_last
*
* Therefore, there is a gap of at least one unused items between
* search_ring_index and empty_ring_index, which grows as we hit
* search_ring_index and empty_ring_index (both inclusive), which grows as we hit
* more unused items while moving backwards through the array.
*/
@@ -302,6 +303,7 @@ compact_prefetch_buffers(void)
PrefetchRequest *target_slot;
bool found;
/* update search index to an unprocessed entry */
search_ring_index--;
source_slot = GetPrfSlot(search_ring_index);
@@ -309,6 +311,7 @@ compact_prefetch_buffers(void)
if (source_slot->status == PRFS_UNUSED)
continue;
/* slot is used -- start moving slot */
target_slot = GetPrfSlot(empty_ring_index);
Assert(source_slot->status == PRFS_RECEIVED);
@@ -328,16 +331,22 @@ compact_prefetch_buffers(void)
/* Adjust the location of our known-empty slot */
empty_ring_index--;
/* empty the moved slot */
source_slot->status = PRFS_UNUSED;
source_slot->buftag = (BufferTag) {0};
source_slot->response = NULL;
source_slot->my_ring_index = 0;
source_slot->effective_request_lsn = 0;
/* update bookkeeping */
n_moved++;
}
if (MyPState->ring_last != empty_ring_index)
/*
* Only when we've moved slots we can expect trailing unused slots,
* so only then we clean up trailing unused slots.
*/
if (n_moved > 0)
{
prefetch_cleanup_trailing_unused();
return true;