/* ** 2004 May 22 ** ** 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 code that is specific to Unix systems. */ #include "sqliteInt.h" #include "os.h" #if OS_UNIX /* This file is used on unix only */ #include #include #include #ifndef O_LARGEFILE # define O_LARGEFILE 0 #endif #ifdef SQLITE_DISABLE_LFS # undef O_LARGEFILE # define O_LARGEFILE 0 #endif #ifndef O_NOFOLLOW # define O_NOFOLLOW 0 #endif #ifndef O_BINARY # define O_BINARY 0 #endif /* ** The DJGPP compiler environment looks mostly like Unix, but it ** lacks the fcntl() system call. So redefine fcntl() to be something ** that always succeeds. This means that locking does not occur under ** DJGPP. But its DOS - what did you expect? */ #ifdef __DJGPP__ # define fcntl(A,B,C) 0 #endif /* ** Macros used to determine whether or not to use threads. The ** SQLITE_UNIX_THREADS macro is defined if we are synchronizing for ** Posix threads and SQLITE_W32_THREADS is defined if we are ** synchronizing using Win32 threads. */ #if defined(THREADSAFE) && THREADSAFE # include # define SQLITE_UNIX_THREADS 1 #endif /* ** Include code that is common to all os_*.c files */ #include "os_common.h" #if defined(THREADSAFE) && THREADSAFE && defined(__linux__) #define getpid pthread_self #endif /* ** Here is the dirt on POSIX advisory locks: ANSI STD 1003.1 (1996) ** section 6.5.2.2 lines 483 through 490 specify that when a process ** sets or clears a lock, that operation overrides any prior locks set ** by the same process. It does not explicitly say so, but this implies ** that it overrides locks set by the same process using a different ** file descriptor. Consider this test case: ** ** int fd1 = open("./file1", O_RDWR|O_CREAT, 0644); ** int fd2 = open("./file2", O_RDWR|O_CREAT, 0644); ** ** Suppose ./file1 and ./file2 are really the same file (because ** one is a hard or symbolic link to the other) then if you set ** an exclusive lock on fd1, then try to get an exclusive lock ** on fd2, it works. I would have expected the second lock to ** fail since there was already a lock on the file due to fd1. ** But not so. Since both locks came from the same process, the ** second overrides the first, even though they were on different ** file descriptors opened on different file names. ** ** Bummer. If you ask me, this is broken. Badly broken. It means ** that we cannot use POSIX locks to synchronize file access among ** competing threads of the same process. POSIX locks will work fine ** to synchronize access for threads in separate processes, but not ** threads within the same process. ** ** To work around the problem, SQLite has to manage file locks internally ** on its own. Whenever a new database is opened, we have to find the ** specific inode of the database file (the inode is determined by the ** st_dev and st_ino fields of the stat structure that fstat() fills in) ** and check for locks already existing on that inode. When locks are ** created or removed, we have to look at our own internal record of the ** locks to see if another thread has previously set a lock on that same ** inode. ** ** The OsFile structure for POSIX is no longer just an integer file ** descriptor. It is now a structure that holds the integer file ** descriptor and a pointer to a structure that describes the internal ** locks on the corresponding inode. There is one locking structure ** per inode, so if the same inode is opened twice, both OsFile structures ** point to the same locking structure. The locking structure keeps ** a reference count (so we will know when to delete it) and a "cnt" ** field that tells us its internal lock status. cnt==0 means the ** file is unlocked. cnt==-1 means the file has an exclusive lock. ** cnt>0 means there are cnt shared locks on the file. ** ** Any attempt to lock or unlock a file first checks the locking ** structure. The fcntl() system call is only invoked to set a ** POSIX lock if the internal lock structure transitions between ** a locked and an unlocked state. ** ** 2004-Jan-11: ** More recent discoveries about POSIX advisory locks. (The more ** I discover, the more I realize the a POSIX advisory locks are ** an abomination.) ** ** If you close a file descriptor that points to a file that has locks, ** all locks on that file that are owned by the current process are ** released. To work around this problem, each OsFile structure contains ** a pointer to an openCnt structure. There is one openCnt structure ** per open inode, which means that multiple OsFiles can point to a single ** openCnt. When an attempt is made to close an OsFile, if there are ** other OsFiles open on the same inode that are holding locks, the call ** to close() the file descriptor is deferred until all of the locks clear. ** The openCnt structure keeps a list of file descriptors that need to ** be closed and that list is walked (and cleared) when the last lock ** clears. ** ** First, under Linux threads, because each thread has a separate ** process ID, lock operations in one thread do not override locks ** to the same file in other threads. Linux threads behave like ** separate processes in this respect. But, if you close a file ** descriptor in linux threads, all locks are cleared, even locks ** on other threads and even though the other threads have different ** process IDs. Linux threads is inconsistent in this respect. ** (I'm beginning to think that linux threads is an abomination too.) ** The consequence of this all is that the hash table for the lockInfo ** structure has to include the process id as part of its key because ** locks in different threads are treated as distinct. But the ** openCnt structure should not include the process id in its ** key because close() clears lock on all threads, not just the current ** thread. Were it not for this goofiness in linux threads, we could ** combine the lockInfo and openCnt structures into a single structure. ** ** 2004-Jun-28: ** On some versions of linux, threads can override each others locks. ** On others not. Sometimes you can change the behavior on the same ** system by setting the LD_ASSUME_KERNEL environment variable. The ** POSIX standard is silent as to which behavior is correct, as far ** as I can tell, so other versions of unix might show the same ** inconsistency. There is no little doubt in my mind that posix ** advisory locks and linux threads are profoundly broken. ** ** To work around the inconsistencies, we have to test at runtime ** whether or not threads can override each others locks. This test ** is run once, the first time any lock is attempted. A static ** variable is set to record the results of this test for future ** use. */ /* ** An instance of the following structure serves as the key used ** to locate a particular lockInfo structure given its inode. ** ** If threads cannot override each others locks, then we set the ** lockKey.tid field to the thread ID. If threads can override ** each others locks then tid is always set to zero. tid is also ** set to zero if we compile without threading support. */ struct lockKey { dev_t dev; /* Device number */ ino_t ino; /* Inode number */ #ifdef SQLITE_UNIX_THREADS pthread_t tid; /* Thread ID or zero if threads cannot override each other */ #endif }; /* ** An instance of the following structure is allocated for each open ** inode on each thread with a different process ID. (Threads have ** different process IDs on linux, but not on most other unixes.) ** ** A single inode can have multiple file descriptors, so each OsFile ** structure contains a pointer to an instance of this object and this ** object keeps a count of the number of OsFiles pointing to it. */ struct lockInfo { struct lockKey key; /* The lookup key */ int cnt; /* Number of SHARED locks held */ int locktype; /* One of SHARED_LOCK, RESERVED_LOCK etc. */ int nRef; /* Number of pointers to this structure */ }; /* ** An instance of the following structure serves as the key used ** to locate a particular openCnt structure given its inode. This ** is the same as the lockKey except that the thread ID is omitted. */ struct openKey { dev_t dev; /* Device number */ ino_t ino; /* Inode number */ }; /* ** An instance of the following structure is allocated for each open ** inode. This structure keeps track of the number of locks on that ** inode. If a close is attempted against an inode that is holding ** locks, the close is deferred until all locks clear by adding the ** file descriptor to be closed to the pending list. */ struct openCnt { struct openKey key; /* The lookup key */ int nRef; /* Number of pointers to this structure */ int nLock; /* Number of outstanding locks */ int nPending; /* Number of pending close() operations */ int *aPending; /* Malloced space holding fd's awaiting a close() */ }; /* ** These hash table maps inodes and process IDs into lockInfo and openCnt ** structures. Access to these hash tables must be protected by a mutex. */ static Hash lockHash = { SQLITE_HASH_BINARY, 0, 0, 0, 0, 0 }; static Hash openHash = { SQLITE_HASH_BINARY, 0, 0, 0, 0, 0 }; #ifdef SQLITE_UNIX_THREADS /* ** This variable records whether or not threads can override each others ** locks. ** ** 0: No. Threads cannot override each others locks. ** 1: Yes. Threads can override each others locks. ** -1: We don't know yet. */ static int threadsOverrideEachOthersLocks = -1; /* ** This structure holds information passed into individual test ** threads by the testThreadLockingBehavior() routine. */ struct threadTestData { int fd; /* File to be locked */ struct flock lock; /* The locking operation */ int result; /* Result of the locking operation */ }; /* ** The testThreadLockingBehavior() routine launches two separate ** threads on this routine. This routine attempts to lock a file ** descriptor then returns. The success or failure of that attempt ** allows the testThreadLockingBehavior() procedure to determine ** whether or not threads can override each others locks. */ static void *threadLockingTest(void *pArg){ struct threadTestData *pData = (struct threadTestData*)pArg; pData->result = fcntl(pData->fd, F_SETLK, &pData->lock); return pArg; } /* ** This procedure attempts to determine whether or not threads ** can override each others locks then sets the ** threadsOverrideEachOthersLocks variable appropriately. */ static void testThreadLockingBehavior(fd_orig){ int fd; struct threadTestData d[2]; pthread_t t[2]; fd = dup(fd_orig); if( fd<0 ) return; memset(d, 0, sizeof(d)); d[0].fd = fd; d[0].lock.l_type = F_RDLCK; d[0].lock.l_len = 1; d[0].lock.l_start = 0; d[0].lock.l_whence = SEEK_SET; d[1] = d[0]; d[1].lock.l_type = F_WRLCK; pthread_create(&t[0], 0, threadLockingTest, &d[0]); pthread_create(&t[1], 0, threadLockingTest, &d[1]); pthread_join(t[0], 0); pthread_join(t[1], 0); close(fd); threadsOverrideEachOthersLocks = d[0].result==0 && d[1].result==0; } #endif /* SQLITE_UNIX_THREADS */ /* ** Release a lockInfo structure previously allocated by findLockInfo(). */ static void releaseLockInfo(struct lockInfo *pLock){ pLock->nRef--; if( pLock->nRef==0 ){ sqlite3HashInsert(&lockHash, &pLock->key, sizeof(pLock->key), 0); sqliteFree(pLock); } } /* ** Release a openCnt structure previously allocated by findLockInfo(). */ static void releaseOpenCnt(struct openCnt *pOpen){ pOpen->nRef--; if( pOpen->nRef==0 ){ sqlite3HashInsert(&openHash, &pOpen->key, sizeof(pOpen->key), 0); sqliteFree(pOpen->aPending); sqliteFree(pOpen); } } /* ** Given a file descriptor, locate lockInfo and openCnt structures that ** describes that file descriptor. Create a new ones if necessary. The ** return values might be unset if an error occurs. ** ** Return the number of errors. */ static int findLockInfo( int fd, /* The file descriptor used in the key */ struct lockInfo **ppLock, /* Return the lockInfo structure here */ struct openCnt **ppOpen /* Return the openCnt structure here */ ){ int rc; struct lockKey key1; struct openKey key2; struct stat statbuf; struct lockInfo *pLock; struct openCnt *pOpen; rc = fstat(fd, &statbuf); if( rc!=0 ) return 1; memset(&key1, 0, sizeof(key1)); key1.dev = statbuf.st_dev; key1.ino = statbuf.st_ino; #ifdef SQLITE_UNIX_THREADS if( threadsOverrideEachOthersLocks<0 ){ testThreadLockingBehavior(fd); } key1.tid = threadsOverrideEachOthersLocks ? 0 : pthread_self(); #endif memset(&key2, 0, sizeof(key2)); key2.dev = statbuf.st_dev; key2.ino = statbuf.st_ino; pLock = (struct lockInfo*)sqlite3HashFind(&lockHash, &key1, sizeof(key1)); if( pLock==0 ){ struct lockInfo *pOld; pLock = sqliteMallocRaw( sizeof(*pLock) ); if( pLock==0 ) return 1; pLock->key = key1; pLock->nRef = 1; pLock->cnt = 0; pLock->locktype = 0; pOld = sqlite3HashInsert(&lockHash, &pLock->key, sizeof(key1), pLock); if( pOld!=0 ){ assert( pOld==pLock ); sqliteFree(pLock); return 1; } }else{ pLock->nRef++; } *ppLock = pLock; pOpen = (struct openCnt*)sqlite3HashFind(&openHash, &key2, sizeof(key2)); if( pOpen==0 ){ struct openCnt *pOld; pOpen = sqliteMallocRaw( sizeof(*pOpen) ); if( pOpen==0 ){ releaseLockInfo(pLock); return 1; } pOpen->key = key2; pOpen->nRef = 1; pOpen->nLock = 0; pOpen->nPending = 0; pOpen->aPending = 0; pOld = sqlite3HashInsert(&openHash, &pOpen->key, sizeof(key2), pOpen); if( pOld!=0 ){ assert( pOld==pOpen ); sqliteFree(pOpen); releaseLockInfo(pLock); return 1; } }else{ pOpen->nRef++; } *ppOpen = pOpen; return 0; } /* ** Delete the named file */ int sqlite3OsDelete(const char *zFilename){ unlink(zFilename); return SQLITE_OK; } /* ** Return TRUE if the named file exists. */ int sqlite3OsFileExists(const char *zFilename){ return access(zFilename, 0)==0; } /* ** Attempt to open a file for both reading and writing. If that ** fails, try opening it read-only. If the file does not exist, ** try to create it. ** ** On success, a handle for the open file is written to *id ** and *pReadonly is set to 0 if the file was opened for reading and ** writing or 1 if the file was opened read-only. The function returns ** SQLITE_OK. ** ** On failure, the function returns SQLITE_CANTOPEN and leaves ** *id and *pReadonly unchanged. */ int sqlite3OsOpenReadWrite( const char *zFilename, OsFile *id, int *pReadonly ){ int rc; assert( !id->isOpen ); id->dirfd = -1; id->h = open(zFilename, O_RDWR|O_CREAT|O_LARGEFILE|O_BINARY, 0644); if( id->h<0 ){ #ifdef EISDIR if( errno==EISDIR ){ return SQLITE_CANTOPEN; } #endif id->h = open(zFilename, O_RDONLY|O_LARGEFILE|O_BINARY); if( id->h<0 ){ return SQLITE_CANTOPEN; } *pReadonly = 1; }else{ *pReadonly = 0; } sqlite3OsEnterMutex(); rc = findLockInfo(id->h, &id->pLock, &id->pOpen); sqlite3OsLeaveMutex(); if( rc ){ close(id->h); return SQLITE_NOMEM; } id->locktype = 0; id->isOpen = 1; TRACE3("OPEN %-3d %s\n", id->h, zFilename); OpenCounter(+1); return SQLITE_OK; } /* ** Attempt to open a new file for exclusive access by this process. ** The file will be opened for both reading and writing. To avoid ** a potential security problem, we do not allow the file to have ** previously existed. Nor do we allow the file to be a symbolic ** link. ** ** If delFlag is true, then make arrangements to automatically delete ** the file when it is closed. ** ** On success, write the file handle into *id and return SQLITE_OK. ** ** On failure, return SQLITE_CANTOPEN. */ int sqlite3OsOpenExclusive(const char *zFilename, OsFile *id, int delFlag){ int rc; assert( !id->isOpen ); if( access(zFilename, 0)==0 ){ return SQLITE_CANTOPEN; } id->dirfd = -1; id->h = open(zFilename, O_RDWR|O_CREAT|O_EXCL|O_NOFOLLOW|O_LARGEFILE|O_BINARY, 0600); if( id->h<0 ){ return SQLITE_CANTOPEN; } sqlite3OsEnterMutex(); rc = findLockInfo(id->h, &id->pLock, &id->pOpen); sqlite3OsLeaveMutex(); if( rc ){ close(id->h); unlink(zFilename); return SQLITE_NOMEM; } id->locktype = 0; id->isOpen = 1; if( delFlag ){ unlink(zFilename); } TRACE3("OPEN-EX %-3d %s\n", id->h, zFilename); OpenCounter(+1); return SQLITE_OK; } /* ** Attempt to open a new file for read-only access. ** ** On success, write the file handle into *id and return SQLITE_OK. ** ** On failure, return SQLITE_CANTOPEN. */ int sqlite3OsOpenReadOnly(const char *zFilename, OsFile *id){ int rc; assert( !id->isOpen ); id->dirfd = -1; id->h = open(zFilename, O_RDONLY|O_LARGEFILE|O_BINARY); if( id->h<0 ){ return SQLITE_CANTOPEN; } sqlite3OsEnterMutex(); rc = findLockInfo(id->h, &id->pLock, &id->pOpen); sqlite3OsLeaveMutex(); if( rc ){ close(id->h); return SQLITE_NOMEM; } id->locktype = 0; id->isOpen = 1; TRACE3("OPEN-RO %-3d %s\n", id->h, zFilename); OpenCounter(+1); return SQLITE_OK; } /* ** Attempt to open a file descriptor for the directory that contains a ** file. This file descriptor can be used to fsync() the directory ** in order to make sure the creation of a new file is actually written ** to disk. ** ** This routine is only meaningful for Unix. It is a no-op under ** windows since windows does not support hard links. ** ** On success, a handle for a previously open file is at *id is ** updated with the new directory file descriptor and SQLITE_OK is ** returned. ** ** On failure, the function returns SQLITE_CANTOPEN and leaves ** *id unchanged. */ int sqlite3OsOpenDirectory( const char *zDirname, OsFile *id ){ if( !id->isOpen ){ /* Do not open the directory if the corresponding file is not already ** open. */ return SQLITE_CANTOPEN; } assert( id->dirfd<0 ); id->dirfd = open(zDirname, O_RDONLY|O_BINARY, 0644); if( id->dirfd<0 ){ return SQLITE_CANTOPEN; } TRACE3("OPENDIR %-3d %s\n", id->dirfd, zDirname); return SQLITE_OK; } /* ** If the following global variable points to a string which is the ** name of a directory, then that directory will be used to store ** temporary files. */ const char *sqlite3_temp_directory = 0; /* ** Create a temporary file name in zBuf. zBuf must be big enough to ** hold at least SQLITE_TEMPNAME_SIZE characters. */ int sqlite3OsTempFileName(char *zBuf){ static const char *azDirs[] = { 0, "/var/tmp", "/usr/tmp", "/tmp", ".", }; static const unsigned char zChars[] = "abcdefghijklmnopqrstuvwxyz" "ABCDEFGHIJKLMNOPQRSTUVWXYZ" "0123456789"; int i, j; struct stat buf; const char *zDir = "."; azDirs[0] = sqlite3_temp_directory; for(i=0; iisOpen ); SimulateIOError(SQLITE_IOERR); TIMER_START; got = read(id->h, pBuf, amt); TIMER_END; TRACE4("READ %-3d %7d %d\n", id->h, last_page, TIMER_ELAPSED); SEEK(0); /* if( got<0 ) got = 0; */ if( got==amt ){ return SQLITE_OK; }else{ return SQLITE_IOERR; } } /* ** Write data from a buffer into a file. Return SQLITE_OK on success ** or some other error code on failure. */ int sqlite3OsWrite(OsFile *id, const void *pBuf, int amt){ int wrote = 0; assert( id->isOpen ); SimulateIOError(SQLITE_IOERR); SimulateDiskfullError; TIMER_START; while( amt>0 && (wrote = write(id->h, pBuf, amt))>0 ){ amt -= wrote; pBuf = &((char*)pBuf)[wrote]; } TIMER_END; TRACE4("WRITE %-3d %7d %d\n", id->h, last_page, TIMER_ELAPSED); SEEK(0); if( amt>0 ){ return SQLITE_FULL; } return SQLITE_OK; } /* ** Move the read/write pointer in a file. */ int sqlite3OsSeek(OsFile *id, i64 offset){ assert( id->isOpen ); SEEK(offset/1024 + 1); lseek(id->h, offset, SEEK_SET); return SQLITE_OK; } /* ** The fsync() system call does not work as advertised on many ** unix systems. The following procedure is an attempt to make ** it work better. */ static int full_fsync(int fd){ int rc; #ifdef F_FULLFSYNC rc = fcntl(fd, F_FULLFSYNC, 0); if( rc ) rc = fsync(fd); #else rc = fsync(fd); #endif return rc; } /* ** Make sure all writes to a particular file are committed to disk. ** ** Under Unix, also make sure that the directory entry for the file ** has been created by fsync-ing the directory that contains the file. ** If we do not do this and we encounter a power failure, the directory ** entry for the journal might not exist after we reboot. The next ** SQLite to access the file will not know that the journal exists (because ** the directory entry for the journal was never created) and the transaction ** will not roll back - possibly leading to database corruption. */ int sqlite3OsSync(OsFile *id){ assert( id->isOpen ); SimulateIOError(SQLITE_IOERR); TRACE2("SYNC %-3d\n", id->h); if( full_fsync(id->h) ){ return SQLITE_IOERR; } if( id->dirfd>=0 ){ TRACE2("DIRSYNC %-3d\n", id->dirfd); full_fsync(id->dirfd); close(id->dirfd); /* Only need to sync once, so close the directory */ id->dirfd = -1; /* when we are done. */ } return SQLITE_OK; } /* ** Sync the directory zDirname. This is a no-op on operating systems other ** than UNIX. */ int sqlite3OsSyncDirectory(const char *zDirname){ int fd; int r; SimulateIOError(SQLITE_IOERR); fd = open(zDirname, O_RDONLY|O_BINARY, 0644); TRACE3("DIRSYNC %-3d (%s)\n", fd, zDirname); if( fd<0 ){ return SQLITE_CANTOPEN; } r = fsync(fd); close(fd); return ((r==0)?SQLITE_OK:SQLITE_IOERR); } /* ** Truncate an open file to a specified size */ int sqlite3OsTruncate(OsFile *id, i64 nByte){ assert( id->isOpen ); SimulateIOError(SQLITE_IOERR); return ftruncate(id->h, nByte)==0 ? SQLITE_OK : SQLITE_IOERR; } /* ** Determine the current size of a file in bytes */ int sqlite3OsFileSize(OsFile *id, i64 *pSize){ struct stat buf; assert( id->isOpen ); SimulateIOError(SQLITE_IOERR); if( fstat(id->h, &buf)!=0 ){ return SQLITE_IOERR; } *pSize = buf.st_size; return SQLITE_OK; } /* ** This routine checks if there is a RESERVED lock held on the specified ** file by this or any other process. If such a lock is held, return ** non-zero. If the file is unlocked or holds only SHARED locks, then ** return zero. */ int sqlite3OsCheckReservedLock(OsFile *id){ int r = 0; assert( id->isOpen ); sqlite3OsEnterMutex(); /* Needed because id->pLock is shared across threads */ /* Check if a thread in this process holds such a lock */ if( id->pLock->locktype>SHARED_LOCK ){ r = 1; } /* Otherwise see if some other process holds it. */ if( !r ){ struct flock lock; lock.l_whence = SEEK_SET; lock.l_start = RESERVED_BYTE; lock.l_len = 1; lock.l_type = F_WRLCK; fcntl(id->h, F_GETLK, &lock); if( lock.l_type!=F_UNLCK ){ r = 1; } } sqlite3OsLeaveMutex(); TRACE3("TEST WR-LOCK %d %d\n", id->h, r); return r; } #ifdef SQLITE_DEBUG /* ** Helper function for printing out trace information from debugging ** binaries. This returns the string represetation of the supplied ** integer lock-type. */ static const char * locktypeName(int locktype){ switch( locktype ){ case NO_LOCK: return "NONE"; case SHARED_LOCK: return "SHARED"; case RESERVED_LOCK: return "RESERVED"; case PENDING_LOCK: return "PENDING"; case EXCLUSIVE_LOCK: return "EXCLUSIVE"; } return "ERROR"; } #endif /* ** 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->locktypeh, 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; } /* ** Lower the locking level on file descriptor id to locktype. locktype ** must be either NO_LOCK or SHARED_LOCK. ** ** If the locking level of the file descriptor is already at or below ** the requested locking level, this routine is a no-op. ** ** It is not possible for this routine to fail if the second argument ** is NO_LOCK. If the second argument is SHARED_LOCK, this routine ** might return SQLITE_IOERR instead of SQLITE_OK. */ int sqlite3OsUnlock(OsFile *id, int locktype){ struct lockInfo *pLock; struct flock lock; int rc = SQLITE_OK; assert( id->isOpen ); TRACE7("UNLOCK %d %d was %d(%d,%d) pid=%d\n", id->h, locktype, id->locktype, id->pLock->locktype, id->pLock->cnt, getpid()); assert( locktype<=SHARED_LOCK ); if( id->locktype<=locktype ){ return SQLITE_OK; } sqlite3OsEnterMutex(); pLock = id->pLock; assert( pLock->cnt!=0 ); if( id->locktype>SHARED_LOCK ){ assert( pLock->locktype==id->locktype ); if( locktype==SHARED_LOCK ){ lock.l_type = F_RDLCK; lock.l_whence = SEEK_SET; lock.l_start = SHARED_FIRST; lock.l_len = SHARED_SIZE; if( fcntl(id->h, F_SETLK, &lock)!=0 ){ /* This should never happen */ rc = SQLITE_IOERR; } } lock.l_type = F_UNLCK; lock.l_whence = SEEK_SET; lock.l_start = PENDING_BYTE; lock.l_len = 2L; assert( PENDING_BYTE+1==RESERVED_BYTE ); fcntl(id->h, F_SETLK, &lock); pLock->locktype = SHARED_LOCK; } if( locktype==NO_LOCK ){ struct openCnt *pOpen; /* Decrement the shared lock counter. Release the lock using an ** OS call only when all threads in this same process have released ** the lock. */ pLock->cnt--; if( pLock->cnt==0 ){ lock.l_type = F_UNLCK; lock.l_whence = SEEK_SET; lock.l_start = lock.l_len = 0L; fcntl(id->h, F_SETLK, &lock); pLock->locktype = NO_LOCK; } /* Decrement the count of locks against this same file. When the ** count reaches zero, close any other file descriptors whose close ** was deferred because of outstanding locks. */ pOpen = id->pOpen; pOpen->nLock--; assert( pOpen->nLock>=0 ); if( pOpen->nLock==0 && pOpen->nPending>0 ){ int i; for(i=0; inPending; i++){ close(pOpen->aPending[i]); } sqliteFree(pOpen->aPending); pOpen->nPending = 0; pOpen->aPending = 0; } } sqlite3OsLeaveMutex(); id->locktype = locktype; return rc; } /* ** Close a file. */ int sqlite3OsClose(OsFile *id){ if( !id->isOpen ) return SQLITE_OK; sqlite3OsUnlock(id, NO_LOCK); if( id->dirfd>=0 ) close(id->dirfd); id->dirfd = -1; sqlite3OsEnterMutex(); if( id->pOpen->nLock ){ /* If there are outstanding locks, do not actually close the file just ** yet because that would clear those locks. Instead, add the file ** descriptor to pOpen->aPending. It will be automatically closed when ** the last lock is cleared. */ int *aNew; struct openCnt *pOpen = id->pOpen; pOpen->nPending++; aNew = sqliteRealloc( pOpen->aPending, pOpen->nPending*sizeof(int) ); if( aNew==0 ){ /* If a malloc fails, just leak the file descriptor */ }else{ pOpen->aPending = aNew; pOpen->aPending[pOpen->nPending-1] = id->h; } }else{ /* There are no outstanding locks so we can close the file immediately */ close(id->h); } releaseLockInfo(id->pLock); releaseOpenCnt(id->pOpen); sqlite3OsLeaveMutex(); id->isOpen = 0; TRACE2("CLOSE %-3d\n", id->h); OpenCounter(-1); return SQLITE_OK; } /* ** Get information to seed the random number generator. The seed ** is written into the buffer zBuf[256]. The calling function must ** supply a sufficiently large buffer. */ int sqlite3OsRandomSeed(char *zBuf){ /* We have to initialize zBuf to prevent valgrind from reporting ** errors. The reports issued by valgrind are incorrect - we would ** prefer that the randomness be increased by making use of the ** uninitialized space in zBuf - but valgrind errors tend to worry ** some users. Rather than argue, it seems easier just to initialize ** the whole array and silence valgrind, even if that means less randomness ** in the random seed. ** ** When testing, initializing zBuf[] to zero is all we do. That means ** that we always use the same random number sequence.* This makes the ** tests repeatable. */ memset(zBuf, 0, 256); #if !defined(SQLITE_TEST) { int pid; time((time_t*)zBuf); pid = getpid(); memcpy(&zBuf[sizeof(time_t)], &pid, sizeof(pid)); } #endif return SQLITE_OK; } /* ** Sleep for a little while. Return the amount of time slept. */ int sqlite3OsSleep(int ms){ #if defined(HAVE_USLEEP) && HAVE_USLEEP usleep(ms*1000); return ms; #else sleep((ms+999)/1000); return 1000*((ms+999)/1000); #endif } /* ** Static variables used for thread synchronization */ static int inMutex = 0; #ifdef SQLITE_UNIX_THREADS static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER; #endif /* ** The following pair of routine implement mutual exclusion for ** multi-threaded processes. Only a single thread is allowed to ** executed code that is surrounded by EnterMutex() and LeaveMutex(). ** ** SQLite uses only a single Mutex. There is not much critical ** code and what little there is executes quickly and without blocking. */ void sqlite3OsEnterMutex(){ #ifdef SQLITE_UNIX_THREADS pthread_mutex_lock(&mutex); #endif assert( !inMutex ); inMutex = 1; } void sqlite3OsLeaveMutex(){ assert( inMutex ); inMutex = 0; #ifdef SQLITE_UNIX_THREADS pthread_mutex_unlock(&mutex); #endif } /* ** Turn a relative pathname into a full pathname. Return a pointer ** to the full pathname stored in space obtained from sqliteMalloc(). ** The calling function is responsible for freeing this space once it ** is no longer needed. */ char *sqlite3OsFullPathname(const char *zRelative){ char *zFull = 0; if( zRelative[0]=='/' ){ sqlite3SetString(&zFull, zRelative, (char*)0); }else{ char zBuf[5000]; sqlite3SetString(&zFull, getcwd(zBuf, sizeof(zBuf)), "/", zRelative, (char*)0); } return zFull; } /* ** The following variable, if set to a non-zero value, becomes the result ** returned from sqlite3OsCurrentTime(). This is used for testing. */ #ifdef SQLITE_TEST int sqlite3_current_time = 0; #endif /* ** Find the current time (in Universal Coordinated Time). Write the ** current time and date as a Julian Day number into *prNow and ** return 0. Return 1 if the time and date cannot be found. */ int sqlite3OsCurrentTime(double *prNow){ time_t t; time(&t); *prNow = t/86400.0 + 2440587.5; #ifdef SQLITE_TEST if( sqlite3_current_time ){ *prNow = sqlite3_current_time/86400.0 + 2440587.5; } #endif return 0; } #if 0 /* NOT USED */ /* ** Find the time that the file was last modified. Write the ** modification time and date as a Julian Day number into *prNow and ** return SQLITE_OK. Return SQLITE_ERROR if the modification ** time cannot be found. */ int sqlite3OsFileModTime(OsFile *id, double *prNow){ int rc; struct stat statbuf; if( fstat(id->h, &statbuf)==0 ){ *prNow = statbuf.st_mtime/86400.0 + 2440587.5; rc = SQLITE_OK; }else{ rc = SQLITE_ERROR; } return rc; } #endif /* NOT USED */ #endif /* OS_UNIX */