Ran into this recently, it's happened on one machine running a beta test of our
software. This is a multi-threaded application, and I've run into a sequence
of steps that deadlocks hard that as far as I can tell from the documentation
shouldn't.
This is using SQLite 3.7.13 with SEE.
The source database is using WAL mode, all transactions are done as IMMEDIATE,
synchronous mode is set to 0, and it is encrypted.
The destination database for the backup is not encrypted, and is default
(non-WAL, full synchronous) modes.
There are multiple threads active:
- one performing a write
- two performing reads
- one closing a connection
- one is in the middle of a backup operation
Here are the call stacks for the threads:
Writing thread:
sqlite3_step
sqlite3VdbeExec
sqlite3VdbeHalt
sqlite3BtreeCommitPhaseOne
sqlite3PagerCommitPhaseOne
pagerWalFrames
sqlite3BackupUpdate
backupOnePage
sqlite3BtreeEnter
lockBtreeMutex
pthread_mutex_lock
__psynch_mutexwait
Closing a connection thread:
sqlite3_close
sqlite3BtreeEnterAll
sqlite3BtreeEnter
lockBtreeMutex
pthread_mutex_lock
__psynch_mutexwait
Reading thread:
sqlite3_step
sqlite3VdbeExec
sqlite3VdbeEnter
sqlite3BtreeEnter
lockBtreeMutex
pthread_mutex_lock
__psynch_mutexwait
Backing up thread:
sqlite3_backup_step
sqlite3BtreeEnter
lockBtreeMutex
pthread_mutex_lock
__psynch_mutexwait
Reading thread:
sqlite3_step
sqlite3VdbeExec
sqlite3VdbeEnter
sqlite3BtreeEnter
lockBtreeMutex
pthread_mutex_lock
__psynch_mutexwait
Also, the destination database for the backup is created on the stack by the
the thread doing the backup and is never passed out to anybody (explicitly).
What looks like is happening to me is that the writing and backing-up thread
are deadlocking with each other, with 'sqlite3BackupUpdate' attempting to
update the backup destination database. Unfortunately, this is not something
I've reproduced locally, so I can't look parameters or lock states. I'm going
to try, as a kind of hail-mary, putting a BEGIN IMMEDIATE transactions around
the backup to block writing during the database backup.
If anyone has any suggestions or ideas about what I might be doing wrong here,
I'd appreciate it.
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