rust/neon
1
0
Fork 0
mirror of https://github.com/neondatabase/neon.git synced 2026-01-06 21:12:55 +00:00
Code Issues Packages Projects Releases Wiki Activity

Add comment about using mdnblocks to prevent race condition

Browse Source
This commit is contained in:
Konstantin Knizhnik
2025-06-16 18:22:49 +03:00
parent f432b48cf0
commit 3821af6a6f
1 changed files with 10 additions and 1 deletions
Download Patch File Download Diff File Expand all files Collapse all files

11
pgxn/neon/pagestore_smgr.c
View File

@@ -1688,7 +1688,16 @@ neon_writev(SMgrRelation reln, ForkNumber forknum, BlockNumber blkno,
if (mdexists(reln, INIT_FORKNUM)) if (mdexists(reln, INIT_FORKNUM))
#endif #endif
{ {
/* It exists locally. Guess it's unlogged then. */ /*
* There can be race condition between backend A performing unlogged index build and some other backend B evicting page of this index.
* After `mdexists` check in backend B is completed and returns true, backend A can complete unlogged build and unlink local files
* of this relation before backend B performs `mdwrite`. It is not a problem if unlogged relation has just one segment:
* it is opened and file descriptor is cached by smgr, preventing removing file on the disk. So subsequent `mdwrite` will succeed.
* But if unlogged relation is large enough and consists of multiple segments, then first segment will be truncated and all other - removed.
* So attempt to write to some segment cause error because file doesn't exists any more and previous segment is empty or not exists as well.
* Calling `mdnblocks` cause SMGR to open and cache descriptors of all relation segments and so prevent there removal which guarantees that
* subsequent `mdwrite` will succeed.
*/
if (mdnblocks(reln, forknum) >= blkno) if (mdnblocks(reln, forknum) >= blkno)
{ {
/* prevent race condition with unlogged build end which unlinks local files */ /* prevent race condition with unlogged build end which unlinks local files */