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Enhanced comments on table locking logic as it relates to preparing new

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statements.  Added assert() and testcase() but no other changes to code. (CVS 6319)

FossilOrigin-Name: 4a12f5b818b769d7518c942ff3dedf453dde698e
This commit is contained in:
drh
2009-02-24 16:18:05 +00:00
parent 4d9c1dd262
commit c74d0b1dfc
4 changed files with 44 additions and 20 deletions
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28
src/prepare.c
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@@ -13,7 +13,7 @@
** interface, and routines that contribute to loading the database schema
** from disk.
**
** $Id: prepare.c,v 1.106 2009/02/19 14:39:25 danielk1977 Exp $
** $Id: prepare.c,v 1.107 2009/02/24 16:18:05 drh Exp $
*/
#include "sqliteInt.h"
@@ -540,17 +540,39 @@ static int sqlite3Prepare(
assert( !db->mallocFailed );
assert( sqlite3_mutex_held(db->mutex) );
/* If any attached database schemas are locked, do not proceed with
** compilation. Instead return SQLITE_LOCKED immediately.
/* Check to verify that it is possible to get a read lock on all
** database schemas. The inability to get a read lock indicates that
** some other database connection is holding a write-lock, which in
** turn means that the other connection has made uncommitted changes
** to the schema.
**
** Were we to proceed and prepare the statement against the uncommitted
** schema changes and if those schema changes are subsequently rolled
** back and different changes are made in their place, then when this
** prepared statement goes to run the schema cookie would fail to detect
** the schema change. Disaster would follow.
**
** This thread is currently holding mutexes on all Btrees (because
** of the sqlite3BtreeEnterAll() in sqlite3LockAndPrepare()) so it
** is not possible for another thread to start a new schema change
** while this routine is running. Hence, we do not need to hold
** locks on the schema, we just need to make sure nobody else is
** holding them.
**
** Note that setting READ_UNCOMMITTED overrides most lock detection,
** but it does *not* override schema lock detection, so this all still
** works even if READ_UNCOMMITTED is set.
*/
for(i=0; i<db->nDb; i++) {
Btree *pBt = db->aDb[i].pBt;
if( pBt ){
assert( sqlite3BtreeHoldsMutex(pBt) );
rc = sqlite3BtreeSchemaLocked(pBt);
if( rc ){
const char *zDb = db->aDb[i].zName;
sqlite3Error(db, SQLITE_LOCKED, "database schema is locked: %s", zDb);
(void)sqlite3SafetyOff(db);
testcase( db->flags & SQLITE_ReadUncommitted );
return sqlite3ApiExit(db, SQLITE_LOCKED);
}
}