static
int little_lock(sqlite3_file *file, int lock) {
int res;
little_file *self = (little_file*)file;
trace("LOCK UP %s...\n", locktypeName(lock));
fflush(stdout);
switch (lock) {
case SQLITE_LOCK_SHARED:
res = get_shared(self->name);
if (res < 0) trace ("Optimistic locking! forge ahead\n");
else self->shared_lock_number = res;
// don't worry if we don't get the lock, we have versioning
res = get_version(self->name, &(self->version), &(self->nextfreeblock));
if (errno == ENOENT) res = 0;
break;
case SQLITE_LOCK_RESERVED:
res = get_reserved(self->name);
break;
case SQLITE_LOCK_EXCLUSIVE:
res = get_exclusive(self->name, self->shared_lock_number);
self->shared_lock_number = -1;
++(self->version);
break;
default: return SQLITE_ERROR;
}
if (res == -EAGAIN) return SQLITE_BUSY;
if (res < 0) return SQLITE_ERROR;
trace("LOCK UP %s OK\n", locktypeName(lock));
return SQLITE_OK;
}
static
int little_unlock(sqlite3_file *file, int lock) {
int res;
little_file *self = (little_file*)file;
trace("LOCK DOWN %s...\n", locktypeName(lock));
switch (lock) {
case SQLITE_LOCK_NONE:
free_shared(self->name, self->shared_lock_number);
flush(self);
self->lastblock = -1;
self->shared_lock_number = -1;
break;
case SQLITE_LOCK_SHARED:
set_version(self);
res = free_exclusive(self->name);
if (res < 0) return SQLITE_ERROR;
self->shared_lock_number = res;
break;
default: return SQLITE_ERROR;
}
trace("LOCK DOWN %s OK\n", locktypeName(lock));
return SQLITE_OK;
}
/*
** Lock the file with the lock specified by parameter locktype - one
** of the following:
**
** (1) SHARED_LOCK
** (2) RESERVED_LOCK
** (3) PENDING_LOCK
** (4) EXCLUSIVE_LOCK
**
** Sometimes when requesting one lock state, additional lock states
** are inserted in between. The locking might fail on one of the later
** transitions leaving the lock state different from what it started but
** still short of its goal. The following chart shows the allowed
** transitions and the inserted intermediate states:
**
** UNLOCKED -> SHARED
** SHARED -> RESERVED
** SHARED -> (PENDING) -> EXCLUSIVE
** RESERVED -> (PENDING) -> EXCLUSIVE
** PENDING -> EXCLUSIVE
**
** This routine will only increase a lock. Use the sqlite3OsUnlock()
** routine to lower a locking level.
*/
int sqlite3OsLock(OsFile *id, int locktype){
/* The following describes the implementation of the various locks and
** lock transitions in terms of the POSIX advisory shared and exclusive
** lock primitives (called read-locks and write-locks below, to avoid
** confusion with SQLite lock names). The algorithms are complicated
** slightly in order to be compatible with windows systems simultaneously
** accessing the same database file, in case that is ever required.
**
** Symbols defined in os.h indentify the 'pending byte' and the 'reserved
** byte', each single bytes at well known offsets, and the 'shared byte
** range', a range of 510 bytes at a well known offset.
**
** To obtain a SHARED lock, a read-lock is obtained on the 'pending
** byte'. If this is successful, a random byte from the 'shared byte
** range' is read-locked and the lock on the 'pending byte' released.
**
** A process may only obtain a RESERVED lock after it has a SHARED lock.
** A RESERVED lock is implemented by grabbing a write-lock on the
** 'reserved byte'.
**
** A process may only obtain a PENDING lock after it has obtained a
** SHARED lock. A PENDING lock is implemented by obtaining a write-lock
** on the 'pending byte'. This ensures that no new SHARED locks can be
** obtained, but existing SHARED locks are allowed to persist. A process
** does not have to obtain a RESERVED lock on the way to a PENDING lock.
** This property is used by the algorithm for rolling back a journal file
** after a crash.
**
** An EXCLUSIVE lock, obtained after a PENDING lock is held, is
** implemented by obtaining a write-lock on the entire 'shared byte
** range'. Since all other locks require a read-lock on one of the bytes
** within this range, this ensures that no other locks are held on the
** database.
**
** The reason a single byte cannot be used instead of the 'shared byte
** range' is that some versions of windows do not support read-locks. By
** locking a random byte from a range, concurrent SHARED locks may exist
** even if the locking primitive used is always a write-lock.
*/
int rc = SQLITE_OK;
struct lockInfo *pLock = id->pLock;
struct flock lock;
int s;
assert( id->isOpen );
TRACE7("LOCK %d %s was %s(%s,%d) pid=%d\n", id->h, locktypeName(locktype),
locktypeName(id->locktype), locktypeName(pLock->locktype), pLock->cnt
,getpid() );
/* If there is already a lock of this type or more restrictive on the
** OsFile, do nothing. Don't use the end_lock: exit path, as
** sqlite3OsEnterMutex() hasn't been called yet.
*/
if( id->locktype>=locktype ){
TRACE3("LOCK %d %s ok (already held)\n", id->h, locktypeName(locktype));
return SQLITE_OK;
}
/* Make sure the locking sequence is correct
*/
assert( id->locktype!=NO_LOCK || locktype==SHARED_LOCK );
assert( locktype!=PENDING_LOCK );
assert( locktype!=RESERVED_LOCK || id->locktype==SHARED_LOCK );
/* This mutex is needed because id->pLock is shared across threads
*/
sqlite3OsEnterMutex();
/* If some thread using this PID has a lock via a different OsFile*
** handle that precludes the requested lock, return BUSY.
*/
if( (id->locktype!=pLock->locktype &&
(pLock->locktype>=PENDING_LOCK || locktype>SHARED_LOCK))
){
rc = SQLITE_BUSY;
goto end_lock;
}
/* If a SHARED lock is requested, and some thread using this PID already
** has a SHARED or RESERVED lock, then increment reference counts and
** return SQLITE_OK.
*/
if( locktype==SHARED_LOCK &&
(pLock->locktype==SHARED_LOCK || pLock->locktype==RESERVED_LOCK) ){
assert( locktype==SHARED_LOCK );
assert( id->locktype==0 );
assert( pLock->cnt>0 );
id->locktype = SHARED_LOCK;
pLock->cnt++;
id->pOpen->nLock++;
goto end_lock;
}
lock.l_len = 1L;
lock.l_whence = SEEK_SET;
/* A PENDING lock is needed before acquiring a SHARED lock and before
** acquiring an EXCLUSIVE lock. For the SHARED lock, the PENDING will
** be released.
*/
if( locktype==SHARED_LOCK
|| (locktype==EXCLUSIVE_LOCK && id->locktype<PENDING_LOCK)
){
lock.l_type = (locktype==SHARED_LOCK?F_RDLCK:F_WRLCK);
lock.l_start = PENDING_BYTE;
s = fcntl(id->h, F_SETLK, &lock);
if( s ){
rc = (errno==EINVAL) ? SQLITE_NOLFS : SQLITE_BUSY;
goto end_lock;
}
}
/* If control gets to this point, then actually go ahead and make
** operating system calls for the specified lock.
*/
if( locktype==SHARED_LOCK ){
assert( pLock->cnt==0 );
assert( pLock->locktype==0 );
/* Now get the read-lock */
lock.l_start = SHARED_FIRST;
lock.l_len = SHARED_SIZE;
s = fcntl(id->h, F_SETLK, &lock);
/* Drop the temporary PENDING lock */
lock.l_start = PENDING_BYTE;
lock.l_len = 1L;
lock.l_type = F_UNLCK;
fcntl(id->h, F_SETLK, &lock);
if( s ){
rc = (errno==EINVAL) ? SQLITE_NOLFS : SQLITE_BUSY;
}else{
id->locktype = SHARED_LOCK;
id->pOpen->nLock++;
pLock->cnt = 1;
}
}else if( locktype==EXCLUSIVE_LOCK && pLock->cnt>1 ){
/* We are trying for an exclusive lock but another thread in this
** same process is still holding a shared lock. */
rc = SQLITE_BUSY;
}else{
/* The request was for a RESERVED or EXCLUSIVE lock. It is
** assumed that there is a SHARED or greater lock on the file
** already.
*/
assert( 0!=id->locktype );
lock.l_type = F_WRLCK;
switch( locktype ){
case RESERVED_LOCK:
lock.l_start = RESERVED_BYTE;
break;
case EXCLUSIVE_LOCK:
lock.l_start = SHARED_FIRST;
lock.l_len = SHARED_SIZE;
break;
default:
assert(0);
}
s = fcntl(id->h, F_SETLK, &lock);
if( s ){
rc = (errno==EINVAL) ? SQLITE_NOLFS : SQLITE_BUSY;
}
}
if( rc==SQLITE_OK ){
id->locktype = locktype;
pLock->locktype = locktype;
}else if( locktype==EXCLUSIVE_LOCK ){
id->locktype = PENDING_LOCK;
pLock->locktype = PENDING_LOCK;
}
end_lock:
sqlite3OsLeaveMutex();
TRACE4("LOCK %d %s %s\n", id->h, locktypeName(locktype),
rc==SQLITE_OK ? "ok" : "failed");
return rc;
}
