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notify.c
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/*
** 2009 March 3
**
** The author disclaims copyright to this source code. In place of
** a legal notice, here is a blessing:
**
** May you do good and not evil.
** May you find forgiveness for yourself and forgive others.
** May you share freely, never taking more than you give.
**
*************************************************************************
**
** This file contains the implementation of the sqlite3_unlock_notify()
** API method and its associated functionality.
*/
#include "sqliteInt.h"
#include "btreeInt.h"
/* Omit this entire file if SQLITE_ENABLE_UNLOCK_NOTIFY is not defined. */
#ifdef SQLITE_ENABLE_UNLOCK_NOTIFY
/*
** Public interfaces:
**
** sqlite3ConnectionBlocked()
** sqlite3ConnectionUnlocked()
** sqlite3ConnectionClosed()
** sqlite3_unlock_notify()
*/
#define assertMutexHeld() \
assert( sqlite3_mutex_held(sqlite3MutexAlloc(SQLITE_MUTEX_STATIC_MASTER)) )
/*
** Head of a linked list of all sqlite3 objects created by this process
** for which either sqlite3.pBlockingConnection or sqlite3.pUnlockConnection
** is not NULL. This variable may only accessed while the STATIC_MASTER
** mutex is held.
*/
static sqlite3 *SQLITE_WSD sqlite3BlockedList = 0;
#ifndef NDEBUG
/*
** This function is a complex assert() that verifies the following
** properties of the blocked connections list:
**
** 1) Each entry in the list has a non-NULL value for either
** pUnlockConnection or pBlockingConnection, or both.
**
** 2) All entries in the list that share a common value for
** xUnlockNotify are grouped together.
**
** 3) If the argument db is not NULL, then none of the entries in the
** blocked connections list have pUnlockConnection or pBlockingConnection
** set to db. This is used when closing connection db.
*/
static void checkListProperties(sqlite3 *db){
sqlite3 *p;
for(p=sqlite3BlockedList; p; p=p->pNextBlocked){
int seen = 0;
sqlite3 *p2;
/* Verify property (1) */
assert( p->pUnlockConnection || p->pBlockingConnection );
/* Verify property (2) */
for(p2=sqlite3BlockedList; p2!=p; p2=p2->pNextBlocked){
if( p2->xUnlockNotify==p->xUnlockNotify ) seen = 1;
assert( p2->xUnlockNotify==p->xUnlockNotify || !seen );
assert( db==0 || p->pUnlockConnection!=db );
assert( db==0 || p->pBlockingConnection!=db );
}
}
}
#else
# define checkListProperties(x)
#endif
/*
** Remove connection db from the blocked connections list. If connection
** db is not currently a part of the list, this function is a no-op.
*/
static void removeFromBlockedList(sqlite3 *db){
sqlite3 **pp;
assertMutexHeld();
for(pp=&sqlite3BlockedList; *pp; pp = &(*pp)->pNextBlocked){
if( *pp==db ){
*pp = (*pp)->pNextBlocked;
break;
}
}
}
/*
** Add connection db to the blocked connections list. It is assumed
** that it is not already a part of the list.
*/
static void addToBlockedList(sqlite3 *db){
sqlite3 **pp;
assertMutexHeld();
for(
pp=&sqlite3BlockedList;
*pp && (*pp)->xUnlockNotify!=db->xUnlockNotify;
pp=&(*pp)->pNextBlocked
);
db->pNextBlocked = *pp;
*pp = db;
}
/*
** Obtain the STATIC_MASTER mutex.
*/
static void enterMutex(void){
sqlite3_mutex_enter(sqlite3MutexAlloc(SQLITE_MUTEX_STATIC_MASTER));
checkListProperties(0);
}
/*
** Release the STATIC_MASTER mutex.
*/
static void leaveMutex(void){
assertMutexHeld();
checkListProperties(0);
sqlite3_mutex_leave(sqlite3MutexAlloc(SQLITE_MUTEX_STATIC_MASTER));
}
/*
** Register an unlock-notify callback.
**
** This is called after connection "db" has attempted some operation
** but has received an SQLITE_LOCKED error because another connection
** (call it pOther) in the same process was busy using the same shared
** cache. pOther is found by looking at db->pBlockingConnection.
**
** If there is no blocking connection, the callback is invoked immediately,
** before this routine returns.
**
** If pOther is already blocked on db, then report SQLITE_LOCKED, to indicate
** a deadlock.
**
** Otherwise, make arrangements to invoke xNotify when pOther drops
** its locks.
**
** Each call to this routine overrides any prior callbacks registered
** on the same "db". If xNotify==0 then any prior callbacks are immediately
** cancelled.
*/
int sqlite3_unlock_notify(
sqlite3 *db,
void (*xNotify)(void **, int),
void *pArg
){
int rc = SQLITE_OK;
sqlite3_mutex_enter(db->mutex);
enterMutex();
if( xNotify==0 ){
removeFromBlockedList(db);
db->pBlockingConnection = 0;
db->pUnlockConnection = 0;
db->xUnlockNotify = 0;
db->pUnlockArg = 0;
}else if( 0==db->pBlockingConnection ){
/* The blocking transaction has been concluded. Or there never was a
** blocking transaction. In either case, invoke the notify callback
** immediately.
*/
xNotify(&pArg, 1);
}else{
sqlite3 *p;
for(p=db->pBlockingConnection; p && p!=db; p=p->pUnlockConnection){}
if( p ){
rc = SQLITE_LOCKED; /* Deadlock detected. */
}else{
db->pUnlockConnection = db->pBlockingConnection;
db->xUnlockNotify = xNotify;
db->pUnlockArg = pArg;
removeFromBlockedList(db);
addToBlockedList(db);
}
}
leaveMutex();
assert( !db->mallocFailed );
sqlite3Error(db, rc, (rc?"database is deadlocked":0));
sqlite3_mutex_leave(db->mutex);
return rc;
}
/*
** This function is called while stepping or preparing a statement
** associated with connection db. The operation will return SQLITE_LOCKED
** to the user because it requires a lock that will not be available
** until connection pBlocker concludes its current transaction.
*/
void sqlite3ConnectionBlocked(sqlite3 *db, sqlite3 *pBlocker){
enterMutex();
if( db->pBlockingConnection==0 && db->pUnlockConnection==0 ){
addToBlockedList(db);
}
db->pBlockingConnection = pBlocker;
leaveMutex();
}
/*
** This function is called when
** the transaction opened by database db has just finished. Locks held
** by database connection db have been released.
**
** This function loops through each entry in the blocked connections
** list and does the following:
**
** 1) If the sqlite3.pBlockingConnection member of a list entry is
** set to db, then set pBlockingConnection=0.
**
** 2) If the sqlite3.pUnlockConnection member of a list entry is
** set to db, then invoke the configured unlock-notify callback and
** set pUnlockConnection=0.
**
** 3) If the two steps above mean that pBlockingConnection==0 and
** pUnlockConnection==0, remove the entry from the blocked connections
** list.
*/
void sqlite3ConnectionUnlocked(sqlite3 *db){
void (*xUnlockNotify)(void **, int) = 0; /* Unlock-notify cb to invoke */
int nArg = 0; /* Number of entries in aArg[] */
sqlite3 **pp; /* Iterator variable */
void **aArg; /* Arguments to the unlock callback */
void **aDyn = 0; /* Dynamically allocated space for aArg[] */
void *aStatic[16]; /* Starter space for aArg[]. No malloc required */
aArg = aStatic;
enterMutex(); /* Enter STATIC_MASTER mutex */
/* This loop runs once for each entry in the blocked-connections list. */
for(pp=&sqlite3BlockedList; *pp; /* no-op */ ){
sqlite3 *p = *pp;
/* Step 1. */
if( p->pBlockingConnection==db ){
p->pBlockingConnection = 0;
}
/* Step 2. */
if( p->pUnlockConnection==db ){
assert( p->xUnlockNotify );
if( p->xUnlockNotify!=xUnlockNotify && nArg!=0 ){
xUnlockNotify(aArg, nArg);
nArg = 0;
}
sqlite3BeginBenignMalloc();
assert( aArg==aDyn || (aDyn==0 && aArg==aStatic) );
assert( nArg<=(int)ArraySize(aStatic) || aArg==aDyn );
if( (!aDyn && nArg==(int)ArraySize(aStatic))
|| (aDyn && nArg==(int)(sqlite3MallocSize(aDyn)/sizeof(void*)))
){
/* The aArg[] array needs to grow. */
void **pNew = (void **)sqlite3Malloc(nArg*sizeof(void *)*2);
if( pNew ){
memcpy(pNew, aArg, nArg*sizeof(void *));
sqlite3_free(aDyn);
aDyn = aArg = pNew;
}else{
/* This occurs when the array of context pointers that need to
** be passed to the unlock-notify callback is larger than the
** aStatic[] array allocated on the stack and the attempt to
** allocate a larger array from the heap has failed.
**
** This is a difficult situation to handle. Returning an error
** code to the caller is insufficient, as even if an error code
** is returned the transaction on connection db will still be
** closed and the unlock-notify callbacks on blocked connections
** will go unissued. This might cause the application to wait
** indefinitely for an unlock-notify callback that will never
** arrive.
**
** Instead, invoke the unlock-notify callback with the context
** array already accumulated. We can then clear the array and
** begin accumulating any further context pointers without
** requiring any dynamic allocation. This is sub-optimal because
** it means that instead of one callback with a large array of
** context pointers the application will receive two or more
** callbacks with smaller arrays of context pointers, which will
** reduce the applications ability to prioritize multiple
** connections. But it is the best that can be done under the
** circumstances.
*/
xUnlockNotify(aArg, nArg);
nArg = 0;
}
}
sqlite3EndBenignMalloc();
aArg[nArg++] = p->pUnlockArg;
xUnlockNotify = p->xUnlockNotify;
p->pUnlockConnection = 0;
p->xUnlockNotify = 0;
p->pUnlockArg = 0;
}
/* Step 3. */
if( p->pBlockingConnection==0 && p->pUnlockConnection==0 ){
/* Remove connection p from the blocked connections list. */
*pp = p->pNextBlocked;
p->pNextBlocked = 0;
}else{
pp = &p->pNextBlocked;
}
}
if( nArg!=0 ){
xUnlockNotify(aArg, nArg);
}
sqlite3_free(aDyn);
leaveMutex(); /* Leave STATIC_MASTER mutex */
}
/*
** This is called when the database connection passed as an argument is
** being closed. The connection is removed from the blocked list.
*/
void sqlite3ConnectionClosed(sqlite3 *db){
sqlite3ConnectionUnlocked(db);
enterMutex();
removeFromBlockedList(db);
checkListProperties(db);
leaveMutex();
}
#endif