/*
** 2010 November 19
**
** 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.
**
*************************************************************************
** Example code for obtaining an exclusive lock on an SQLite database
** file. This method is complicated, but works for both WAL and rollback
** mode database files. The interface to the example code in this file
** consists of the following two functions:
**
** sqlite3demo_superlock()
** sqlite3demo_superunlock()
*/
#include <sqlite3.h>
#include <string.h> /* memset(), strlen() */
#include <assert.h> /* assert() */
/*
** A structure to collect a busy-handler callback and argument and a count
** of the number of times it has been invoked.
*/
struct SuperlockBusy {
int (*xBusy)(void*,int); /* Pointer to busy-handler function */
void *pBusyArg; /* First arg to pass to xBusy */
int nBusy; /* Number of times xBusy has been invoked */
};
typedef struct SuperlockBusy SuperlockBusy;
/*
** An instance of the following structure is allocated for each active
** superlock. The opaque handle returned by sqlite3demo_superlock() is
** actually a pointer to an instance of this structure.
*/
struct Superlock {
sqlite3 *db; /* Database handle used to lock db */
int bWal; /* True if db is a WAL database */
};
typedef struct Superlock Superlock;
/*
** The pCtx pointer passed to this function is actually a pointer to a
** SuperlockBusy structure. Invoke the busy-handler function encapsulated
** by the structure and return the result.
*/
static int superlockBusyHandler(void *pCtx, int UNUSED){
SuperlockBusy *pBusy = (SuperlockBusy *)pCtx;
if( pBusy->xBusy==0 ) return 0;
return pBusy->xBusy(pBusy->pBusyArg, pBusy->nBusy++);
}
/*
** This function is used to determine if the main database file for
** connection db is open in WAL mode or not. If no error occurs and the
** database file is in WAL mode, set *pbWal to true and return SQLITE_OK.
** If it is not in WAL mode, set *pbWal to false.
**
** If an error occurs, return an SQLite error code. The value of *pbWal
** is undefined in this case.
*/
static int superlockIsWal(Superlock *pLock){
int rc; /* Return Code */
sqlite3_stmt *pStmt; /* Compiled PRAGMA journal_mode statement */
rc = sqlite3_prepare(pLock->db, "PRAGMA main.journal_mode", -1, &pStmt, 0);
if( rc!=SQLITE_OK ) return rc;
pLock->bWal = 0;
if( SQLITE_ROW==sqlite3_step(pStmt) ){
const char *zMode = (const char *)sqlite3_column_text(pStmt, 0);
if( zMode && strlen(zMode)==3 && sqlite3_strnicmp("wal", zMode, 3)==0 ){
pLock->bWal = 1;
}
}
return sqlite3_finalize(pStmt);
}
/*
** Obtain an exclusive shm-lock on nByte bytes starting at offset idx
** of the file fd. If the lock cannot be obtained immediately, invoke
** the busy-handler until either it is obtained or the busy-handler
** callback returns 0.
*/
static int superlockShmLock(
sqlite3_file *fd, /* Database file handle */
int idx, /* Offset of shm-lock to obtain */
int nByte, /* Number of consective bytes to lock */
SuperlockBusy *pBusy /* Busy-handler wrapper object */
){
int rc;
int (*xShmLock)(sqlite3_file*, int, int, int) = fd->pMethods->xShmLock;
do {
rc = xShmLock(fd, idx, nByte, SQLITE_SHM_LOCK|SQLITE_SHM_EXCLUSIVE);
}while( rc==SQLITE_BUSY && superlockBusyHandler((void *)pBusy, 0) );
return rc;
}
/*
** Obtain the extra locks on the database file required for WAL databases.
** Invoke the supplied busy-handler as required.
*/
static int superlockWalLock(
sqlite3 *db, /* Database handle open on WAL database */
SuperlockBusy *pBusy /* Busy handler wrapper object */
){
int rc; /* Return code */
sqlite3_file *fd = 0; /* Main database file handle */
void volatile *p = 0; /* Pointer to first page of shared memory */
/* Obtain a pointer to the sqlite3_file object open on the main db file. */
rc = sqlite3_file_control(db, "main", SQLITE_FCNTL_FILE_POINTER, (void *)&fd);
if( rc!=SQLITE_OK ) return rc;
/* Obtain the "recovery" lock. Normally, this lock is only obtained by
** clients running database recovery.
*/
rc = superlockShmLock(fd, 2, 1, pBusy);
if( rc!=SQLITE_OK ) return rc;
/* Zero the start of the first shared-memory page. This means that any
** clients that open read or write transactions from this point on will
** have to run recovery before proceeding. Since they need the "recovery"
** lock that this process is holding to do that, no new read or write
** transactions may now be opened. Nor can a checkpoint be run, for the
** same reason.
*/
rc = fd->pMethods->xShmMap(fd, 0, 32*1024, 1, &p);
if( rc!=SQLITE_OK ) return rc;
memset((void *)p, 0, 32);
/* Obtain exclusive locks on all the "read-lock" slots. Once these locks
** are held, it is guaranteed that there are no active reader, writer or
** checkpointer clients.
*/
rc = superlockShmLock(fd, 3, SQLITE_SHM_NLOCK-3, pBusy);
return rc;
}
/*
** Release a superlock held on a database file. The argument passed to
** this function must have been obtained from a successful call to
** sqlite3demo_superlock().
*/
void sqlite3demo_superunlock(void *pLock){
Superlock *p = (Superlock *)pLock;
if( p->bWal ){
int rc; /* Return code */
int flags = SQLITE_SHM_UNLOCK | SQLITE_SHM_EXCLUSIVE;
sqlite3_file *fd = 0;
rc = sqlite3_file_control(p->db, "main", SQLITE_FCNTL_FILE_POINTER, (void *)&fd);
if( rc==SQLITE_OK ){
fd->pMethods->xShmLock(fd, 2, 1, flags);
fd->pMethods->xShmLock(fd, 3, SQLITE_SHM_NLOCK-3, flags);
}
}
sqlite3_close(p->db);
sqlite3_free(p);
}
/*
** Obtain a superlock on the database file identified by zPath, using the
** locking primitives provided by VFS zVfs. If successful, SQLITE_OK is
** returned and output variable *ppLock is populated with an opaque handle
** that may be used with sqlite3demo_superunlock() to release the lock.
**
** If an error occurs, *ppLock is set to 0 and an SQLite error code
** (e.g. SQLITE_BUSY) is returned.
**
** If a required lock cannot be obtained immediately and the xBusy parameter
** to this function is not NULL, then xBusy is invoked in the same way
** as a busy-handler registered with SQLite (using sqlite3_busy_handler())
** until either the lock can be obtained or the busy-handler function returns
** 0 (indicating "give up").
*/
int sqlite3demo_superlock(
const char *zPath, /* Path to database file to lock */
const char *zVfs, /* VFS to use to access database file */
int (*xBusy)(void*,int), /* Busy handler callback */
void *pBusyArg, /* Context arg for busy handler */
void **ppLock /* OUT: Context to pass to superunlock() */
){
SuperlockBusy busy = {0, 0, 0}; /* Busy handler wrapper object */
int rc; /* Return code */
Superlock *pLock;
pLock = sqlite3_malloc(sizeof(Superlock));
if( !pLock ) return SQLITE_NOMEM;
memset(pLock, 0, sizeof(Superlock));
/* Open a database handle on the file to superlock. */
rc = sqlite3_open_v2(
zPath, &pLock->db, SQLITE_OPEN_READWRITE|SQLITE_OPEN_CREATE, zVfs
);
/* Install a busy-handler and execute a BEGIN EXCLUSIVE. If this is not
** a WAL database, this is all we need to do.
**
** A wrapper function is used to invoke the busy-handler instead of
** registering the busy-handler function supplied by the user directly
** with SQLite. This is because the same busy-handler function may be
** invoked directly later on when attempting to obtain the extra locks
** required in WAL mode. By using the wrapper, we are able to guarantee
** that the "nBusy" integer parameter passed to the users busy-handler
** represents the total number of busy-handler invocations made within
** this call to sqlite3demo_superlock(), including any made during the
** "BEGIN EXCLUSIVE".
*/
if( rc==SQLITE_OK ){
busy.xBusy = xBusy;
busy.pBusyArg = pBusyArg;
sqlite3_busy_handler(pLock->db, superlockBusyHandler, (void *)&busy);
rc = sqlite3_exec(pLock->db, "BEGIN EXCLUSIVE", 0, 0, 0);
}
/* If the BEGIN EXCLUSIVE was executed successfully and this is a WAL
** database, call superlockWalLock() to obtain the extra locks required
** to prevent readers, writers and/or checkpointers from accessing the
** db while this process is holding the superlock.
**
** Before attempting any WAL locks, commit the transaction started above
** to drop the WAL read and write locks currently held. Otherwise, the
** new WAL locks may conflict with the old.
*/
if( rc==SQLITE_OK ){
if( SQLITE_OK==(rc = superlockIsWal(pLock)) && pLock->bWal ){
rc = sqlite3_exec(pLock->db, "COMMIT", 0, 0, 0);
if( rc==SQLITE_OK ){
rc = superlockWalLock(pLock->db, &busy);
}
}
}
if( rc!=SQLITE_OK ){
sqlite3demo_superunlock(pLock);
*ppLock = 0;
}else{
*ppLock = pLock;
}
return rc;
}
/*
** End of example code. Everything below here is the test harness.
**************************************************************************
**************************************************************************
*************************************************************************/
#ifdef SQLITE_TEST
#include <tcl.h>
struct InterpAndScript {
Tcl_Interp *interp;
Tcl_Obj *pScript;
};
typedef struct InterpAndScript InterpAndScript;
static void superunlock_del(ClientData cd){
sqlite3demo_superunlock((void *)cd);
}
static int superunlock_cmd(
ClientData cd,
Tcl_Interp *interp,
int objc,
Tcl_Obj *CONST objv[]
){
if( objc!=1 ){
Tcl_WrongNumArgs(interp, 1, objv, "");
return TCL_ERROR;
}
Tcl_DeleteCommand(interp, Tcl_GetString(objv[0]));
return TCL_OK;
}
static int superlock_busy(void *pCtx, int nBusy){
InterpAndScript *p = (InterpAndScript *)pCtx;
Tcl_Obj *pEval; /* Script to evaluate */
int iVal = 0; /* Value to return */
pEval = Tcl_DuplicateObj(p->pScript);
Tcl_IncrRefCount(pEval);
Tcl_ListObjAppendElement(p->interp, pEval, Tcl_NewIntObj(nBusy));
Tcl_EvalObjEx(p->interp, pEval, TCL_EVAL_GLOBAL);
Tcl_GetIntFromObj(p->interp, Tcl_GetObjResult(p->interp), &iVal);
Tcl_DecrRefCount(pEval);
return iVal;
}
/*
** Tclcmd: sqlite3demo_superlock CMDNAME PATH VFS BUSY-HANDLER-SCRIPT
*/
static int superlock_cmd(
ClientData cd,
Tcl_Interp *interp,
int objc,
Tcl_Obj *CONST objv[]
){
void *pLock; /* Lock context */
char *zPath;
char *zVfs = 0;
InterpAndScript busy = {0, 0};
int (*xBusy)(void*,int) = 0; /* Busy handler callback */
int rc; /* Return code from sqlite3demo_superlock() */
if( objc<3 || objc>5 ){
Tcl_WrongNumArgs(
interp, 1, objv, "CMDNAME PATH ?VFS? ?BUSY-HANDLER-SCRIPT?");
return TCL_ERROR;
}
zPath = Tcl_GetString(objv[2]);
if( objc>3 ){
zVfs = Tcl_GetString(objv[3]);
if( strlen(zVfs)==0 ) zVfs = 0;
}
if( objc>4 ){
busy.interp = interp;
busy.pScript = objv[4];
xBusy = superlock_busy;
}
rc = sqlite3demo_superlock(zPath, zVfs, xBusy, &busy, &pLock);
assert( rc==SQLITE_OK || pLock==0 );
assert( rc!=SQLITE_OK || pLock!=0 );
if( rc!=SQLITE_OK ){
extern const char *sqlite3ErrStr(int);
Tcl_ResetResult(interp);
Tcl_AppendResult(interp, sqlite3ErrStr(rc), 0);
return TCL_ERROR;
}
Tcl_CreateObjCommand(
interp, Tcl_GetString(objv[1]), superunlock_cmd, pLock, superunlock_del
);
Tcl_SetObjResult(interp, objv[1]);
return TCL_OK;
}
int SqliteSuperlock_Init(Tcl_Interp *interp){
Tcl_CreateObjCommand(interp, "sqlite3demo_superlock", superlock_cmd, 0, 0);
return TCL_OK;
}
#endif