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Fix a race condition in OP_ParseSchema.

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FossilOrigin-Name: 71a799b02a3b3cf2e12758dea29fd2465bbec3e1
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dan
2011-04-02 09:44:43 +00:00
parent 82346d9873
commit 6c15487b7e
3 changed files with 24 additions and 21 deletions
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31
src/vdbe.c
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@@ -4625,23 +4625,26 @@ case OP_ParseSchema: {
iDb = pOp->p1;
assert( iDb>=0 && iDb<db->nDb );
/* Although the mutex on the BtShared object that corresponds to
** database iDb (the database containing the sqlite_master table
** read by this instruction) is currently held, it is necessary to
** obtain the mutexes on all attached databases before checking if
** the schema of iDb is loaded. This is because, at the start of
** the sqlite3_exec() call below, SQLite will invoke
** sqlite3BtreeEnterAll(). If all mutexes are not already held, the
** iDb mutex may be temporarily released to avoid deadlock. If
** this happens, then some other thread may delete the in-memory
** schema of database iDb before the SQL statement runs. The schema
** will not be reloaded becuase the db->init.busy flag is set. This
** can result in a "no such table: sqlite_master" or "malformed
** database schema" error being returned to the user.
/* When this opcode is invoked, it is guaranteed that the b-tree mutex
** is held and the schema is loaded for database iDb. However, at the
** start of the sqlite3_exec() call below, SQLite will invoke
** sqlite3BtreeEnterAll(). If all mutexes are not already held, the iDb
** mutex may be temporarily released to avoid deadlock. If this happens,
** then some other thread may delete the in-memory schema of database iDb
** before the SQL statement runs. The schema will not be reloaded because
** the db->init.busy flag is set. This can result in a "no such table:
** sqlite_master" or "malformed database schema" error being returned to
** the user.
**
** To avoid this, obtain all mutexes and check that no other thread has
** deleted the schema before calling sqlite3_exec(). If we find that the
** another thread has deleted the schema, there is no need to update it.
** The updated schema will be loaded from disk when it is next required.
*/
assert( sqlite3BtreeHoldsMutex(db->aDb[iDb].pBt) );
assert( DbHasProperty(db, iDb, DB_SchemaLoaded) );
sqlite3BtreeEnterAll(db);
if( ALWAYS(DbHasProperty(db, iDb, DB_SchemaLoaded)) ){
if( DbHasProperty(db, iDb, DB_SchemaLoaded) ){
zMaster = SCHEMA_TABLE(iDb);
initData.db = db;
initData.iDb = pOp->p1;