How to corrupt an sqlite databaseFrom:An SQLite database is highly resistant to corruption. If an application crash, or an operating-system crash, or even a power failure occurs in the middle of a transaction, the partially written transaction should be automatically rolled back the next time the database file is accessed. The recovery process is fully automatic and does not require any action on the part of the user or the application. Though SQLite is resistant to database corruption, it is not immune.
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This document describes the various ways that an SQLite database might go corrupt.1. File overwrite by a rogue thread or processSQLite database files are ordinary disk files.That means that any process can open the file andoverwrite it with garbage. There is nothing that the SQLitelibrary can do to defend against this. Continuing to use a file descriptor after it has been closedWe have seen multiple cases where a file descriptor was open on a file,then that file descriptor was closed and reopened on an SQLite database.Later, some other thread continued to write into theold file descriptor, not realizing that the original file had been closedalready. But because the file descriptor had been reopened by SQLite,the information that was intended to go into the original file ended upoverwriting parts of the SQLite database, leading to corruption of thedatabase.One example of this occurred circa 2013-08-30 on the canonical repositoryfor the. In that event,file descriptor 2 (standard error) was being erroneously closed (by, we suspect) prior toso that the file descriptor used for therepository database file was 2. Later, an applicationbug caused an assert statement to emitan error message by invoking write(2.).
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But since file descriptor 2 wasnow connected to a database file, the error messageoverwrote part of the database. To guard against this kind of problem,SQLite (2013-10-17)and later refuse to use low-numbered file descriptorsfor database files.(See for additional information.)Another example of corruption caused by using a closed filedescriptor wasin a blog post on 2014-08-12. Backup or restore while a transaction is activeSystems that run automatic backups in the background might try tomake a backup copy of an SQLite database file while it is in the middleof a transaction.
The backup copy then might contain some old and somenew content, and thus be corrupt.The best approach to make reliable backup copies of an SQLite databaseis to make use of the that is part of the SQLite library.Failing that, it is safe to make a copy of an SQLite database file as longas there are no transactions in progress by any process. If the previoustransaction failed, then it is important that any rollback journal(the.-journal file) or write-ahead log (the.-wal file)be copied together with the database file itself. Deleting a hot journalSQLite normally stores all content in a single disk file. However,while performing a transaction, information necessary to recover thedatabase following a crash or power failure is stored in auxiliaryjournal files.
Repair Corrupt SQLite Database. This tool helps to recover the corrupt and damaged file of SQLite Database. It scans the corrupt database and recovers all the possible database objects from it. There could be multiple reasons for a corrupt SQLite database like viruses, abnormal shutdowns etc. Apr 06, 2017 There’s an easy manual trick you can use here to repair SQLite database. Also, you can use a professional recovery solution like SQLite Database Recovery tool, to perform SQLite data recovery from corrupt database files. SQLite – Relational Database Management System.
Such journal files are described as.The journal files have the same name as theoriginal database file with the additionof -journal or -wal suffix.SQLite must see the journal files in order to recover from a crashor power failure. If the are moved, deleted, or renamedafter a crash or power failure, then automatic recovery will not workand the database may go corrupt.Another manifestation of this problem is.
Mispairing database files and hot journalsThe previous example is a specific case of a more general problem:The state of an SQLite database is controlled by both thedatabase file and the journal file. In a quiescent state, the journalfile does not exist and only the database file matters.But if the journal file does exist, it must bekept together with the database to avoid corruption. The followingactions are all likely to lead to corruption:. Swapping journal files between two different databases. Overwritting a journal file with a different journal file. Moving a journal file from one database to another. Copying a database file without also copying its journal.
Overwriting a database file with another without alsodeleting any hot journal associated with the original database.2. File locking problemsSQLite uses file locks on the database file, and on theor file, to coordinate access between concurrentprocesses. Without coordination, two threads or processes might tryto make incompatible changes to a database file at the same time,resulting in database corruption.
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Filesystems with broken or missing lock implementationsSQLite depends on the underlying filesystem to do locking as thedocumentation says it will. But some filesystems contain bugs in theirlocking logic such that the locks do not always behave as advertised.This is especially true of network filesystems and NFS in particular.If SQLite is used on a filesystem where the locking primitives containbugs, and if two or more threads or processes try to access the samedatabase at the same time, then database corruption might result. Posix advisory locks canceled by a separate thread doing closeThe default locking mechanism used by SQLite on unix platforms isPOSIX advisory locking. Unfortunately, POSIX advisory locking has designquirks that make it prone to misuse and failure. In particular, anythread in the same process with a file descriptor that is holding a POSIXadvisory lock can override that lock using a different file descriptor.One particularly pernicious problem is that the close systemcall will cancel all POSIX advisory locks on the same file for allthreads and all file descriptors in the process.So, for example, suppose a multi-thread process hastwo or more threads with separate SQLite database connections to thesame database file. Then a third thread comes along and wants to readsomething out of that same database file on its own, without using theSQLite library.The third thread does an open, a read and thena close.One would think this would be harmless.But the close system call caused thelocks held on the database by all the other threads to be dropped.
Thoseother threads have no way of knowing that their locks have just beentrashed (POSIX does not provide any mechanism to determine this) and sothey keep on running under the assumption that their locks are still valid.This can lead to two or more threads or processes trying to write to thedatabase at the same time, resulting in database corruption.Note that it is perfectly safe for two or more threads to access thesame SQLite database file using the SQLite library. The unix drivers forSQLite know about the POSIX advisory locking quirks and work around them.This problem only arises when a thread tries to bypass the SQLite libraryand read the database file directly. Multiple copies of SQLite linked into the same applicationAs pointed out in the previous paragraph, SQLite takes steps to workaround the quirks of POSIX advisory locking. An SQLite database is highly resistant to corruption. If an application crash, or an operating-system crash, or even a power failure occurs in the middle of a transaction, the partially written transaction should be automatically rolled back the next time the database file is accessed.Though SQLite is resistant to database corruption, it is not immune and in some cases a special tool like SQLiteDoctor is required in order to be able to fully restore your damaged sqlite database.A single license is valid for up to 2 computers (if not used at the same time). No subscription required.
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