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Implement an experimental defensive measure to try to prevent the -wal and

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-shm files of an active WAL-mode database connection from being deleted out
from under the database connection in the case where the database SHARED
lock has been released by an accidental call to close() in a different
thread, or similar.

FossilOrigin-Name: ed1b9206640f66cb8abff444809ccab8a4a5157b5c90e9aacddc422210e63423
This commit is contained in:
drh
2025-10-27 14:24:02 +00:00
parent 6e27846323
commit 626351d444
4 changed files with 46 additions and 11 deletions
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34
src/os_unix.c
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@@ -1832,6 +1832,9 @@ static int unixFileLock(unixFile *pFile, struct flock *pLock){
return rc;
}
/* Forward reference */
static int unixIsSharingShmNode(unixFile*);
/*
** Lock the file with the lock specified by parameter eFileLock - one
** of the following:
@@ -2020,7 +2023,9 @@ static int unixLock(sqlite3_file *id, int eFileLock){
pInode->nLock++;
pInode->nShared = 1;
}
}else if( eFileLock==EXCLUSIVE_LOCK && pInode->nShared>1 ){
}else if( (eFileLock==EXCLUSIVE_LOCK && pInode->nShared>1)
|| unixIsSharingShmNode(pFile)
){
/* We are trying for an exclusive lock but another thread in this
** same process is still holding a shared lock. */
rc = SQLITE_BUSY;
@@ -4640,6 +4645,33 @@ static int unixFcntlExternalReader(unixFile *pFile, int *piOut){
return rc;
}
/*
** If pFile has a -shm file open and it is sharing that file with some
** other connection, either in the same process or in a separate process,
** then return true. Return false if either pFile does not have a -shm
** file open or if it is the only connection to that -shm file across the
** entire system.
*/
static int unixIsSharingShmNode(unixFile *pFile){
int rc;
unixShmNode *pShmNode;
if( pFile->pShm==0 ) return 0;
pShmNode = pFile->pShm->pShmNode;
rc = 1;
unixEnterMutex();
if( pShmNode->nRef==1 ){
struct flock lock;
lock.l_whence = SEEK_SET;
lock.l_start = UNIX_SHM_DMS;
lock.l_len = 1;
lock.l_type = F_WRLCK;
if( osFcntl(pShmNode->hShm, F_GETLK, &lock)==0 && lock.l_type==F_UNLCK ){
rc = 0;
}
}
unixLeaveMutex();
return rc;
}
/*
** Apply posix advisory locks for all bytes from ofst through ofst+n-1.