diff options
Diffstat (limited to 'ext/async')
| -rw-r--r-- | ext/async/README.txt | 170 | ||||
| -rw-r--r-- | ext/async/sqlite3async.c | 1706 | ||||
| -rw-r--r-- | ext/async/sqlite3async.h | 222 |
3 files changed, 2098 insertions, 0 deletions
diff --git a/ext/async/README.txt b/ext/async/README.txt new file mode 100644 index 0000000..f62fa2f --- /dev/null +++ b/ext/async/README.txt @@ -0,0 +1,170 @@ +NOTE (2012-11-29): + +The functionality implemented by this extension has been superseded +by WAL-mode. This module is no longer supported or maintained. The +code is retained for historical reference only. + +------------------------------------------------------------------------------ + +Normally, when SQLite writes to a database file, it waits until the write +operation is finished before returning control to the calling application. +Since writing to the file-system is usually very slow compared with CPU +bound operations, this can be a performance bottleneck. This directory +contains an extension that causes SQLite to perform all write requests +using a separate thread running in the background. Although this does not +reduce the overall system resources (CPU, disk bandwidth etc.) at all, it +allows SQLite to return control to the caller quickly even when writing to +the database, eliminating the bottleneck. + + 1. Functionality + + 1.1 How it Works + 1.2 Limitations + 1.3 Locking and Concurrency + + 2. Compilation and Usage + + 3. Porting + + + +1. FUNCTIONALITY + + With asynchronous I/O, write requests are handled by a separate thread + running in the background. This means that the thread that initiates + a database write does not have to wait for (sometimes slow) disk I/O + to occur. The write seems to happen very quickly, though in reality + it is happening at its usual slow pace in the background. + + Asynchronous I/O appears to give better responsiveness, but at a price. + You lose the Durable property. With the default I/O backend of SQLite, + once a write completes, you know that the information you wrote is + safely on disk. With the asynchronous I/O, this is not the case. If + your program crashes or if a power loss occurs after the database + write but before the asynchronous write thread has completed, then the + database change might never make it to disk and the next user of the + database might not see your change. + + You lose Durability with asynchronous I/O, but you still retain the + other parts of ACID: Atomic, Consistent, and Isolated. Many + appliations get along fine without the Durablity. + + 1.1 How it Works + + Asynchronous I/O works by creating a special SQLite "vfs" structure + and registering it with sqlite3_vfs_register(). When files opened via + this vfs are written to (using the vfs xWrite() method), the data is not + written directly to disk, but is placed in the "write-queue" to be + handled by the background thread. + + When files opened with the asynchronous vfs are read from + (using the vfs xRead() method), the data is read from the file on + disk and the write-queue, so that from the point of view of + the vfs reader the xWrite() appears to have already completed. + + The special vfs is registered (and unregistered) by calls to the + API functions sqlite3async_initialize() and sqlite3async_shutdown(). + See section "Compilation and Usage" below for details. + + 1.2 Limitations + + In order to gain experience with the main ideas surrounding asynchronous + IO, this implementation is deliberately kept simple. Additional + capabilities may be added in the future. + + For example, as currently implemented, if writes are happening at a + steady stream that exceeds the I/O capability of the background writer + thread, the queue of pending write operations will grow without bound. + If this goes on for long enough, the host system could run out of memory. + A more sophisticated module could to keep track of the quantity of + pending writes and stop accepting new write requests when the queue of + pending writes grows too large. + + 1.3 Locking and Concurrency + + Multiple connections from within a single process that use this + implementation of asynchronous IO may access a single database + file concurrently. From the point of view of the user, if all + connections are from within a single process, there is no difference + between the concurrency offered by "normal" SQLite and SQLite + using the asynchronous backend. + + If file-locking is enabled (it is enabled by default), then connections + from multiple processes may also read and write the database file. + However concurrency is reduced as follows: + + * When a connection using asynchronous IO begins a database + transaction, the database is locked immediately. However the + lock is not released until after all relevant operations + in the write-queue have been flushed to disk. This means + (for example) that the database may remain locked for some + time after a "COMMIT" or "ROLLBACK" is issued. + + * If an application using asynchronous IO executes transactions + in quick succession, other database users may be effectively + locked out of the database. This is because when a BEGIN + is executed, a database lock is established immediately. But + when the corresponding COMMIT or ROLLBACK occurs, the lock + is not released until the relevant part of the write-queue + has been flushed through. As a result, if a COMMIT is followed + by a BEGIN before the write-queue is flushed through, the database + is never unlocked,preventing other processes from accessing + the database. + + File-locking may be disabled at runtime using the sqlite3async_control() + API (see below). This may improve performance when an NFS or other + network file-system, as the synchronous round-trips to the server be + required to establish file locks are avoided. However, if multiple + connections attempt to access the same database file when file-locking + is disabled, application crashes and database corruption is a likely + outcome. + + +2. COMPILATION AND USAGE + + The asynchronous IO extension consists of a single file of C code + (sqlite3async.c), and a header file (sqlite3async.h) that defines the + C API used by applications to activate and control the modules + functionality. + + To use the asynchronous IO extension, compile sqlite3async.c as + part of the application that uses SQLite. Then use the API defined + in sqlite3async.h to initialize and configure the module. + + The asynchronous IO VFS API is described in detail in comments in + sqlite3async.h. Using the API usually consists of the following steps: + + 1. Register the asynchronous IO VFS with SQLite by calling the + sqlite3async_initialize() function. + + 2. Create a background thread to perform write operations and call + sqlite3async_run(). + + 3. Use the normal SQLite API to read and write to databases via + the asynchronous IO VFS. + + Refer to sqlite3async.h for details. + + +3. PORTING + + Currently the asynchronous IO extension is compatible with win32 systems + and systems that support the pthreads interface, including Mac OSX, Linux, + and other varieties of Unix. + + To port the asynchronous IO extension to another platform, the user must + implement mutex and condition variable primitives for the new platform. + Currently there is no externally available interface to allow this, but + modifying the code within sqlite3async.c to include the new platforms + concurrency primitives is relatively easy. Search within sqlite3async.c + for the comment string "PORTING FUNCTIONS" for details. Then implement + new versions of each of the following: + + static void async_mutex_enter(int eMutex); + static void async_mutex_leave(int eMutex); + static void async_cond_wait(int eCond, int eMutex); + static void async_cond_signal(int eCond); + static void async_sched_yield(void); + + The functionality required of each of the above functions is described + in comments in sqlite3async.c. diff --git a/ext/async/sqlite3async.c b/ext/async/sqlite3async.c new file mode 100644 index 0000000..eed7c8d --- /dev/null +++ b/ext/async/sqlite3async.c @@ -0,0 +1,1706 @@ +/* +** 2005 December 14 +** +** 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. +** +************************************************************************* +** +** $Id: sqlite3async.c,v 1.7 2009/07/18 11:52:04 danielk1977 Exp $ +** +** This file contains the implementation of an asynchronous IO backend +** for SQLite. +*/ + +#if !defined(SQLITE_CORE) || defined(SQLITE_ENABLE_ASYNCIO) + +#include "sqlite3async.h" +#include "sqlite3.h" +#include <stdarg.h> +#include <string.h> +#include <assert.h> + +/* Useful macros used in several places */ +#define MIN(x,y) ((x)<(y)?(x):(y)) +#define MAX(x,y) ((x)>(y)?(x):(y)) + +#ifndef SQLITE_AMALGAMATION +/* Macro to mark parameters as unused and silence compiler warnings. */ +#define UNUSED_PARAMETER(x) (void)(x) +#endif + +/* Forward references */ +typedef struct AsyncWrite AsyncWrite; +typedef struct AsyncFile AsyncFile; +typedef struct AsyncFileData AsyncFileData; +typedef struct AsyncFileLock AsyncFileLock; +typedef struct AsyncLock AsyncLock; + +/* Enable for debugging */ +#ifndef NDEBUG +#include <stdio.h> +static int sqlite3async_trace = 0; +# define ASYNC_TRACE(X) if( sqlite3async_trace ) asyncTrace X +static void asyncTrace(const char *zFormat, ...){ + char *z; + va_list ap; + va_start(ap, zFormat); + z = sqlite3_vmprintf(zFormat, ap); + va_end(ap); + fprintf(stderr, "[%d] %s", 0 /* (int)pthread_self() */, z); + sqlite3_free(z); +} +#else +# define ASYNC_TRACE(X) +#endif + +/* +** THREAD SAFETY NOTES +** +** Basic rules: +** +** * Both read and write access to the global write-op queue must be +** protected by the async.queueMutex. As are the async.ioError and +** async.nFile variables. +** +** * The async.pLock list and all AsyncLock and AsyncFileLock +** structures must be protected by the async.lockMutex mutex. +** +** * The file handles from the underlying system are not assumed to +** be thread safe. +** +** * See the last two paragraphs under "The Writer Thread" for +** an assumption to do with file-handle synchronization by the Os. +** +** Deadlock prevention: +** +** There are three mutex used by the system: the "writer" mutex, +** the "queue" mutex and the "lock" mutex. Rules are: +** +** * It is illegal to block on the writer mutex when any other mutex +** are held, and +** +** * It is illegal to block on the queue mutex when the lock mutex +** is held. +** +** i.e. mutex's must be grabbed in the order "writer", "queue", "lock". +** +** File system operations (invoked by SQLite thread): +** +** xOpen +** xDelete +** xFileExists +** +** File handle operations (invoked by SQLite thread): +** +** asyncWrite, asyncClose, asyncTruncate, asyncSync +** +** The operations above add an entry to the global write-op list. They +** prepare the entry, acquire the async.queueMutex momentarily while +** list pointers are manipulated to insert the new entry, then release +** the mutex and signal the writer thread to wake up in case it happens +** to be asleep. +** +** +** asyncRead, asyncFileSize. +** +** Read operations. Both of these read from both the underlying file +** first then adjust their result based on pending writes in the +** write-op queue. So async.queueMutex is held for the duration +** of these operations to prevent other threads from changing the +** queue in mid operation. +** +** +** asyncLock, asyncUnlock, asyncCheckReservedLock +** +** These primitives implement in-process locking using a hash table +** on the file name. Files are locked correctly for connections coming +** from the same process. But other processes cannot see these locks +** and will therefore not honor them. +** +** +** The writer thread: +** +** The async.writerMutex is used to make sure only there is only +** a single writer thread running at a time. +** +** Inside the writer thread is a loop that works like this: +** +** WHILE (write-op list is not empty) +** Do IO operation at head of write-op list +** Remove entry from head of write-op list +** END WHILE +** +** The async.queueMutex is always held during the <write-op list is +** not empty> test, and when the entry is removed from the head +** of the write-op list. Sometimes it is held for the interim +** period (while the IO is performed), and sometimes it is +** relinquished. It is relinquished if (a) the IO op is an +** ASYNC_CLOSE or (b) when the file handle was opened, two of +** the underlying systems handles were opened on the same +** file-system entry. +** +** If condition (b) above is true, then one file-handle +** (AsyncFile.pBaseRead) is used exclusively by sqlite threads to read the +** file, the other (AsyncFile.pBaseWrite) by sqlite3_async_flush() +** threads to perform write() operations. This means that read +** operations are not blocked by asynchronous writes (although +** asynchronous writes may still be blocked by reads). +** +** This assumes that the OS keeps two handles open on the same file +** properly in sync. That is, any read operation that starts after a +** write operation on the same file system entry has completed returns +** data consistent with the write. We also assume that if one thread +** reads a file while another is writing it all bytes other than the +** ones actually being written contain valid data. +** +** If the above assumptions are not true, set the preprocessor symbol +** SQLITE_ASYNC_TWO_FILEHANDLES to 0. +*/ + + +#ifndef NDEBUG +# define TESTONLY( X ) X +#else +# define TESTONLY( X ) +#endif + +/* +** PORTING FUNCTIONS +** +** There are two definitions of the following functions. One for pthreads +** compatible systems and one for Win32. These functions isolate the OS +** specific code required by each platform. +** +** The system uses three mutexes and a single condition variable. To +** block on a mutex, async_mutex_enter() is called. The parameter passed +** to async_mutex_enter(), which must be one of ASYNC_MUTEX_LOCK, +** ASYNC_MUTEX_QUEUE or ASYNC_MUTEX_WRITER, identifies which of the three +** mutexes to lock. Similarly, to unlock a mutex, async_mutex_leave() is +** called with a parameter identifying the mutex being unlocked. Mutexes +** are not recursive - it is an error to call async_mutex_enter() to +** lock a mutex that is already locked, or to call async_mutex_leave() +** to unlock a mutex that is not currently locked. +** +** The async_cond_wait() and async_cond_signal() functions are modelled +** on the pthreads functions with similar names. The first parameter to +** both functions is always ASYNC_COND_QUEUE. When async_cond_wait() +** is called the mutex identified by the second parameter must be held. +** The mutex is unlocked, and the calling thread simultaneously begins +** waiting for the condition variable to be signalled by another thread. +** After another thread signals the condition variable, the calling +** thread stops waiting, locks mutex eMutex and returns. The +** async_cond_signal() function is used to signal the condition variable. +** It is assumed that the mutex used by the thread calling async_cond_wait() +** is held by the caller of async_cond_signal() (otherwise there would be +** a race condition). +** +** It is guaranteed that no other thread will call async_cond_wait() when +** there is already a thread waiting on the condition variable. +** +** The async_sched_yield() function is called to suggest to the operating +** system that it would be a good time to shift the current thread off the +** CPU. The system will still work if this function is not implemented +** (it is not currently implemented for win32), but it might be marginally +** more efficient if it is. +*/ +static void async_mutex_enter(int eMutex); +static void async_mutex_leave(int eMutex); +static void async_cond_wait(int eCond, int eMutex); +static void async_cond_signal(int eCond); +static void async_sched_yield(void); + +/* +** There are also two definitions of the following. async_os_initialize() +** is called when the asynchronous VFS is first installed, and os_shutdown() +** is called when it is uninstalled (from within sqlite3async_shutdown()). +** +** For pthreads builds, both of these functions are no-ops. For win32, +** they provide an opportunity to initialize and finalize the required +** mutex and condition variables. +** +** If async_os_initialize() returns other than zero, then the initialization +** fails and SQLITE_ERROR is returned to the user. +*/ +static int async_os_initialize(void); +static void async_os_shutdown(void); + +/* Values for use as the 'eMutex' argument of the above functions. The +** integer values assigned to these constants are important for assert() +** statements that verify that mutexes are locked in the correct order. +** Specifically, it is unsafe to try to lock mutex N while holding a lock +** on mutex M if (M<=N). +*/ +#define ASYNC_MUTEX_LOCK 0 +#define ASYNC_MUTEX_QUEUE 1 +#define ASYNC_MUTEX_WRITER 2 + +/* Values for use as the 'eCond' argument of the above functions. */ +#define ASYNC_COND_QUEUE 0 + +/************************************************************************* +** Start of OS specific code. +*/ +#if SQLITE_OS_WIN || defined(_WIN32) || defined(WIN32) || defined(__CYGWIN__) || defined(__MINGW32__) || defined(__BORLANDC__) + +#include <windows.h> + +/* The following block contains the win32 specific code. */ + +#define mutex_held(X) (GetCurrentThreadId()==primitives.aHolder[X]) + +static struct AsyncPrimitives { + int isInit; + DWORD aHolder[3]; + CRITICAL_SECTION aMutex[3]; + HANDLE aCond[1]; +} primitives = { 0 }; + +static int async_os_initialize(void){ + if( !primitives.isInit ){ + primitives.aCond[0] = CreateEvent(NULL, TRUE, FALSE, 0); + if( primitives.aCond[0]==NULL ){ + return 1; + } + InitializeCriticalSection(&primitives.aMutex[0]); + InitializeCriticalSection(&primitives.aMutex[1]); + InitializeCriticalSection(&primitives.aMutex[2]); + primitives.isInit = 1; + } + return 0; +} +static void async_os_shutdown(void){ + if( primitives.isInit ){ + DeleteCriticalSection(&primitives.aMutex[0]); + DeleteCriticalSection(&primitives.aMutex[1]); + DeleteCriticalSection(&primitives.aMutex[2]); + CloseHandle(primitives.aCond[0]); + primitives.isInit = 0; + } +} + +/* The following block contains the Win32 specific code. */ +static void async_mutex_enter(int eMutex){ + assert( eMutex==0 || eMutex==1 || eMutex==2 ); + assert( eMutex!=2 || (!mutex_held(0) && !mutex_held(1) && !mutex_held(2)) ); + assert( eMutex!=1 || (!mutex_held(0) && !mutex_held(1)) ); + assert( eMutex!=0 || (!mutex_held(0)) ); + EnterCriticalSection(&primitives.aMutex[eMutex]); + TESTONLY( primitives.aHolder[eMutex] = GetCurrentThreadId(); ) +} +static void async_mutex_leave(int eMutex){ + assert( eMutex==0 || eMutex==1 || eMutex==2 ); + assert( mutex_held(eMutex) ); + TESTONLY( primitives.aHolder[eMutex] = 0; ) + LeaveCriticalSection(&primitives.aMutex[eMutex]); +} +static void async_cond_wait(int eCond, int eMutex){ + ResetEvent(primitives.aCond[eCond]); + async_mutex_leave(eMutex); + WaitForSingleObject(primitives.aCond[eCond], INFINITE); + async_mutex_enter(eMutex); +} +static void async_cond_signal(int eCond){ + assert( mutex_held(ASYNC_MUTEX_QUEUE) ); + SetEvent(primitives.aCond[eCond]); +} +static void async_sched_yield(void){ + Sleep(0); +} +#else + +/* The following block contains the pthreads specific code. */ +#include <pthread.h> +#include <sched.h> + +#define mutex_held(X) pthread_equal(primitives.aHolder[X], pthread_self()) + +static int async_os_initialize(void) {return 0;} +static void async_os_shutdown(void) {} + +static struct AsyncPrimitives { + pthread_mutex_t aMutex[3]; + pthread_cond_t aCond[1]; + pthread_t aHolder[3]; +} primitives = { + { PTHREAD_MUTEX_INITIALIZER, + PTHREAD_MUTEX_INITIALIZER, + PTHREAD_MUTEX_INITIALIZER + } , { + PTHREAD_COND_INITIALIZER + } , { 0, 0, 0 } +}; + +static void async_mutex_enter(int eMutex){ + assert( eMutex==0 || eMutex==1 || eMutex==2 ); + assert( eMutex!=2 || (!mutex_held(0) && !mutex_held(1) && !mutex_held(2)) ); + assert( eMutex!=1 || (!mutex_held(0) && !mutex_held(1)) ); + assert( eMutex!=0 || (!mutex_held(0)) ); + pthread_mutex_lock(&primitives.aMutex[eMutex]); + TESTONLY( primitives.aHolder[eMutex] = pthread_self(); ) +} +static void async_mutex_leave(int eMutex){ + assert( eMutex==0 || eMutex==1 || eMutex==2 ); + assert( mutex_held(eMutex) ); + TESTONLY( primitives.aHolder[eMutex] = 0; ) + pthread_mutex_unlock(&primitives.aMutex[eMutex]); +} +static void async_cond_wait(int eCond, int eMutex){ + assert( eMutex==0 || eMutex==1 || eMutex==2 ); + assert( mutex_held(eMutex) ); + TESTONLY( primitives.aHolder[eMutex] = 0; ) + pthread_cond_wait(&primitives.aCond[eCond], &primitives.aMutex[eMutex]); + TESTONLY( primitives.aHolder[eMutex] = pthread_self(); ) +} +static void async_cond_signal(int eCond){ + assert( mutex_held(ASYNC_MUTEX_QUEUE) ); + pthread_cond_signal(&primitives.aCond[eCond]); +} +static void async_sched_yield(void){ + sched_yield(); +} +#endif +/* +** End of OS specific code. +*************************************************************************/ + +#define assert_mutex_is_held(X) assert( mutex_held(X) ) + + +#ifndef SQLITE_ASYNC_TWO_FILEHANDLES +/* #define SQLITE_ASYNC_TWO_FILEHANDLES 0 */ +#define SQLITE_ASYNC_TWO_FILEHANDLES 1 +#endif + +/* +** State information is held in the static variable "async" defined +** as the following structure. +** +** Both async.ioError and async.nFile are protected by async.queueMutex. +*/ +static struct TestAsyncStaticData { + AsyncWrite *pQueueFirst; /* Next write operation to be processed */ + AsyncWrite *pQueueLast; /* Last write operation on the list */ + AsyncLock *pLock; /* Linked list of all AsyncLock structures */ + volatile int ioDelay; /* Extra delay between write operations */ + volatile int eHalt; /* One of the SQLITEASYNC_HALT_XXX values */ + volatile int bLockFiles; /* Current value of "lockfiles" parameter */ + int ioError; /* True if an IO error has occurred */ + int nFile; /* Number of open files (from sqlite pov) */ +} async = { 0,0,0,0,0,1,0,0 }; + +/* Possible values of AsyncWrite.op */ +#define ASYNC_NOOP 0 +#define ASYNC_WRITE 1 +#define ASYNC_SYNC 2 +#define ASYNC_TRUNCATE 3 +#define ASYNC_CLOSE 4 +#define ASYNC_DELETE 5 +#define ASYNC_OPENEXCLUSIVE 6 +#define ASYNC_UNLOCK 7 + +/* Names of opcodes. Used for debugging only. +** Make sure these stay in sync with the macros above! +*/ +static const char *azOpcodeName[] = { + "NOOP", "WRITE", "SYNC", "TRUNCATE", "CLOSE", "DELETE", "OPENEX", "UNLOCK" +}; + +/* +** Entries on the write-op queue are instances of the AsyncWrite +** structure, defined here. +** +** The interpretation of the iOffset and nByte variables varies depending +** on the value of AsyncWrite.op: +** +** ASYNC_NOOP: +** No values used. +** +** ASYNC_WRITE: +** iOffset -> Offset in file to write to. +** nByte -> Number of bytes of data to write (pointed to by zBuf). +** +** ASYNC_SYNC: +** nByte -> flags to pass to sqlite3OsSync(). +** +** ASYNC_TRUNCATE: +** iOffset -> Size to truncate file to. +** nByte -> Unused. +** +** ASYNC_CLOSE: +** iOffset -> Unused. +** nByte -> Unused. +** +** ASYNC_DELETE: +** iOffset -> Contains the "syncDir" flag. +** nByte -> Number of bytes of zBuf points to (file name). +** +** ASYNC_OPENEXCLUSIVE: +** iOffset -> Value of "delflag". +** nByte -> Number of bytes of zBuf points to (file name). +** +** ASYNC_UNLOCK: +** nByte -> Argument to sqlite3OsUnlock(). +** +** +** For an ASYNC_WRITE operation, zBuf points to the data to write to the file. +** This space is sqlite3_malloc()d along with the AsyncWrite structure in a +** single blob, so is deleted when sqlite3_free() is called on the parent +** structure. +*/ +struct AsyncWrite { + AsyncFileData *pFileData; /* File to write data to or sync */ + int op; /* One of ASYNC_xxx etc. */ + sqlite_int64 iOffset; /* See above */ + int nByte; /* See above */ + char *zBuf; /* Data to write to file (or NULL if op!=ASYNC_WRITE) */ + AsyncWrite *pNext; /* Next write operation (to any file) */ +}; + +/* +** An instance of this structure is created for each distinct open file +** (i.e. if two handles are opened on the one file, only one of these +** structures is allocated) and stored in the async.aLock hash table. The +** keys for async.aLock are the full pathnames of the opened files. +** +** AsyncLock.pList points to the head of a linked list of AsyncFileLock +** structures, one for each handle currently open on the file. +** +** If the opened file is not a main-database (the SQLITE_OPEN_MAIN_DB is +** not passed to the sqlite3OsOpen() call), or if async.bLockFiles is +** false, variables AsyncLock.pFile and AsyncLock.eLock are never used. +** Otherwise, pFile is a file handle opened on the file in question and +** used to obtain the file-system locks required by database connections +** within this process. +** +** See comments above the asyncLock() function for more details on +** the implementation of database locking used by this backend. +*/ +struct AsyncLock { + char *zFile; + int nFile; + sqlite3_file *pFile; + int eLock; + AsyncFileLock *pList; + AsyncLock *pNext; /* Next in linked list headed by async.pLock */ +}; + +/* +** An instance of the following structure is allocated along with each +** AsyncFileData structure (see AsyncFileData.lock), but is only used if the +** file was opened with the SQLITE_OPEN_MAIN_DB. +*/ +struct AsyncFileLock { + int eLock; /* Internally visible lock state (sqlite pov) */ + int eAsyncLock; /* Lock-state with write-queue unlock */ + AsyncFileLock *pNext; +}; + +/* +** The AsyncFile structure is a subclass of sqlite3_file used for +** asynchronous IO. +** +** All of the actual data for the structure is stored in the structure +** pointed to by AsyncFile.pData, which is allocated as part of the +** sqlite3OsOpen() using sqlite3_malloc(). The reason for this is that the +** lifetime of the AsyncFile structure is ended by the caller after OsClose() +** is called, but the data in AsyncFileData may be required by the +** writer thread after that point. +*/ +struct AsyncFile { + sqlite3_io_methods *pMethod; + AsyncFileData *pData; +}; +struct AsyncFileData { + char *zName; /* Underlying OS filename - used for debugging */ + int nName; /* Number of characters in zName */ + sqlite3_file *pBaseRead; /* Read handle to the underlying Os file */ + sqlite3_file *pBaseWrite; /* Write handle to the underlying Os file */ + AsyncFileLock lock; /* Lock state for this handle */ + AsyncLock *pLock; /* AsyncLock object for this file system entry */ + AsyncWrite closeOp; /* Preallocated close operation */ +}; + +/* +** Add an entry to the end of the global write-op list. pWrite should point +** to an AsyncWrite structure allocated using sqlite3_malloc(). The writer +** thread will call sqlite3_free() to free the structure after the specified +** operation has been completed. +** +** Once an AsyncWrite structure has been added to the list, it becomes the +** property of the writer thread and must not be read or modified by the +** caller. +*/ +static void addAsyncWrite(AsyncWrite *pWrite){ + /* We must hold the queue mutex in order to modify the queue pointers */ + if( pWrite->op!=ASYNC_UNLOCK ){ + async_mutex_enter(ASYNC_MUTEX_QUEUE); + } + + /* Add the record to the end of the write-op queue */ + assert( !pWrite->pNext ); + if( async.pQueueLast ){ + assert( async.pQueueFirst ); + async.pQueueLast->pNext = pWrite; + }else{ + async.pQueueFirst = pWrite; + } + async.pQueueLast = pWrite; + ASYNC_TRACE(("PUSH %p (%s %s %d)\n", pWrite, azOpcodeName[pWrite->op], + pWrite->pFileData ? pWrite->pFileData->zName : "-", pWrite->iOffset)); + + if( pWrite->op==ASYNC_CLOSE ){ + async.nFile--; + } + + /* The writer thread might have been idle because there was nothing + ** on the write-op queue for it to do. So wake it up. */ + async_cond_signal(ASYNC_COND_QUEUE); + + /* Drop the queue mutex */ + if( pWrite->op!=ASYNC_UNLOCK ){ + async_mutex_leave(ASYNC_MUTEX_QUEUE); + } +} + +/* +** Increment async.nFile in a thread-safe manner. +*/ +static void incrOpenFileCount(void){ + /* We must hold the queue mutex in order to modify async.nFile */ + async_mutex_enter(ASYNC_MUTEX_QUEUE); + if( async.nFile==0 ){ + async.ioError = SQLITE_OK; + } + async.nFile++; + async_mutex_leave(ASYNC_MUTEX_QUEUE); +} + +/* +** This is a utility function to allocate and populate a new AsyncWrite +** structure and insert it (via addAsyncWrite() ) into the global list. +*/ +static int addNewAsyncWrite( + AsyncFileData *pFileData, + int op, + sqlite3_int64 iOffset, + int nByte, + const char *zByte +){ + AsyncWrite *p; + if( op!=ASYNC_CLOSE && async.ioError ){ + return async.ioError; + } + p = sqlite3_malloc(sizeof(AsyncWrite) + (zByte?nByte:0)); + if( !p ){ + /* The upper layer does not expect operations like OsWrite() to + ** return SQLITE_NOMEM. This is partly because under normal conditions + ** SQLite is required to do rollback without calling malloc(). So + ** if malloc() fails here, treat it as an I/O error. The above + ** layer knows how to handle that. + */ + return SQLITE_IOERR; + } + p->op = op; + p->iOffset = iOffset; + p->nByte = nByte; + p->pFileData = pFileData; + p->pNext = 0; + if( zByte ){ + p->zBuf = (char *)&p[1]; + memcpy(p->zBuf, zByte, nByte); + }else{ + p->zBuf = 0; + } + addAsyncWrite(p); + return SQLITE_OK; +} + +/* +** Close the file. This just adds an entry to the write-op list, the file is +** not actually closed. +*/ +static int asyncClose(sqlite3_file *pFile){ + AsyncFileData *p = ((AsyncFile *)pFile)->pData; + + /* Unlock the file, if it is locked */ + async_mutex_enter(ASYNC_MUTEX_LOCK); + p->lock.eLock = 0; + async_mutex_leave(ASYNC_MUTEX_LOCK); + + addAsyncWrite(&p->closeOp); + return SQLITE_OK; +} + +/* +** Implementation of sqlite3OsWrite() for asynchronous files. Instead of +** writing to the underlying file, this function adds an entry to the end of +** the global AsyncWrite list. Either SQLITE_OK or SQLITE_NOMEM may be +** returned. +*/ +static int asyncWrite( + sqlite3_file *pFile, + const void *pBuf, + int amt, + sqlite3_int64 iOff +){ + AsyncFileData *p = ((AsyncFile *)pFile)->pData; + return addNewAsyncWrite(p, ASYNC_WRITE, iOff, amt, pBuf); +} + +/* +** Read data from the file. First we read from the filesystem, then adjust +** the contents of the buffer based on ASYNC_WRITE operations in the +** write-op queue. +** +** This method holds the mutex from start to finish. +*/ +static int asyncRead( + sqlite3_file *pFile, + void *zOut, + int iAmt, + sqlite3_int64 iOffset +){ + AsyncFileData *p = ((AsyncFile *)pFile)->pData; + int rc = SQLITE_OK; + sqlite3_int64 filesize = 0; + sqlite3_file *pBase = p->pBaseRead; + sqlite3_int64 iAmt64 = (sqlite3_int64)iAmt; + + /* Grab the write queue mutex for the duration of the call */ + async_mutex_enter(ASYNC_MUTEX_QUEUE); + + /* If an I/O error has previously occurred in this virtual file + ** system, then all subsequent operations fail. + */ + if( async.ioError!=SQLITE_OK ){ + rc = async.ioError; + goto asyncread_out; + } + + if( pBase->pMethods ){ + sqlite3_int64 nRead; + rc = pBase->pMethods->xFileSize(pBase, &filesize); + if( rc!=SQLITE_OK ){ + goto asyncread_out; + } + nRead = MIN(filesize - iOffset, iAmt64); + if( nRead>0 ){ + rc = pBase->pMethods->xRead(pBase, zOut, (int)nRead, iOffset); + ASYNC_TRACE(("READ %s %d bytes at %d\n", p->zName, nRead, iOffset)); + } + } + + if( rc==SQLITE_OK ){ + AsyncWrite *pWrite; + char *zName = p->zName; + + for(pWrite=async.pQueueFirst; pWrite; pWrite = pWrite->pNext){ + if( pWrite->op==ASYNC_WRITE && ( + (pWrite->pFileData==p) || + (zName && pWrite->pFileData->zName==zName) + )){ + sqlite3_int64 nCopy; + sqlite3_int64 nByte64 = (sqlite3_int64)pWrite->nByte; + + /* Set variable iBeginIn to the offset in buffer pWrite->zBuf[] from + ** which data should be copied. Set iBeginOut to the offset within + ** the output buffer to which data should be copied. If either of + ** these offsets is a negative number, set them to 0. + */ + sqlite3_int64 iBeginOut = (pWrite->iOffset-iOffset); + sqlite3_int64 iBeginIn = -iBeginOut; + if( iBeginIn<0 ) iBeginIn = 0; + if( iBeginOut<0 ) iBeginOut = 0; + + filesize = MAX(filesize, pWrite->iOffset+nByte64); + + nCopy = MIN(nByte64-iBeginIn, iAmt64-iBeginOut); + if( nCopy>0 ){ + memcpy(&((char *)zOut)[iBeginOut], &pWrite->zBuf[iBeginIn], (size_t)nCopy); + ASYNC_TRACE(("OVERREAD %d bytes at %d\n", nCopy, iBeginOut+iOffset)); + } + } + } + } + +asyncread_out: + async_mutex_leave(ASYNC_MUTEX_QUEUE); + if( rc==SQLITE_OK && filesize<(iOffset+iAmt) ){ + rc = SQLITE_IOERR_SHORT_READ; + } + return rc; +} + +/* +** Truncate the file to nByte bytes in length. This just adds an entry to +** the write-op list, no IO actually takes place. +*/ +static int asyncTruncate(sqlite3_file *pFile, sqlite3_int64 nByte){ + AsyncFileData *p = ((AsyncFile *)pFile)->pData; + return addNewAsyncWrite(p, ASYNC_TRUNCATE, nByte, 0, 0); +} + +/* +** Sync the file. This just adds an entry to the write-op list, the +** sync() is done later by sqlite3_async_flush(). +*/ +static int asyncSync(sqlite3_file *pFile, int flags){ + AsyncFileData *p = ((AsyncFile *)pFile)->pData; + return addNewAsyncWrite(p, ASYNC_SYNC, 0, flags, 0); +} + +/* +** Read the size of the file. First we read the size of the file system +** entry, then adjust for any ASYNC_WRITE or ASYNC_TRUNCATE operations +** currently in the write-op list. +** +** This method holds the mutex from start to finish. +*/ +int asyncFileSize(sqlite3_file *pFile, sqlite3_int64 *piSize){ + AsyncFileData *p = ((AsyncFile *)pFile)->pData; + int rc = SQLITE_OK; + sqlite3_int64 s = 0; + sqlite3_file *pBase; + + async_mutex_enter(ASYNC_MUTEX_QUEUE); + + /* Read the filesystem size from the base file. If pMethods is NULL, this + ** means the file hasn't been opened yet. In this case all relevant data + ** must be in the write-op queue anyway, so we can omit reading from the + ** file-system. + */ + pBase = p->pBaseRead; + if( pBase->pMethods ){ + rc = pBase->pMethods->xFileSize(pBase, &s); + } + + if( rc==SQLITE_OK ){ + AsyncWrite *pWrite; + for(pWrite=async.pQueueFirst; pWrite; pWrite = pWrite->pNext){ + if( pWrite->op==ASYNC_DELETE + && p->zName + && strcmp(p->zName, pWrite->zBuf)==0 + ){ + s = 0; + }else if( pWrite->pFileData && ( + (pWrite->pFileData==p) + || (p->zName && pWrite->pFileData->zName==p->zName) + )){ + switch( pWrite->op ){ + case ASYNC_WRITE: + s = MAX(pWrite->iOffset + (sqlite3_int64)(pWrite->nByte), s); + break; + case ASYNC_TRUNCATE: + s = MIN(s, pWrite->iOffset); + break; + } + } + } + *piSize = s; + } + async_mutex_leave(ASYNC_MUTEX_QUEUE); + return rc; +} + +/* +** Lock or unlock the actual file-system entry. +*/ +static int getFileLock(AsyncLock *pLock){ + int rc = SQLITE_OK; + AsyncFileLock *pIter; + int eRequired = 0; + + if( pLock->pFile ){ + for(pIter=pLock->pList; pIter; pIter=pIter->pNext){ + assert(pIter->eAsyncLock>=pIter->eLock); + if( pIter->eAsyncLock>eRequired ){ + eRequired = pIter->eAsyncLock; + assert(eRequired>=0 && eRequired<=SQLITE_LOCK_EXCLUSIVE); + } + } + + if( eRequired>pLock->eLock ){ + rc = pLock->pFile->pMethods->xLock(pLock->pFile, eRequired); + if( rc==SQLITE_OK ){ + pLock->eLock = eRequired; + } + } + else if( eRequired<pLock->eLock && eRequired<=SQLITE_LOCK_SHARED ){ + rc = pLock->pFile->pMethods->xUnlock(pLock->pFile, eRequired); + if( rc==SQLITE_OK ){ + pLock->eLock = eRequired; + } + } + } + + return rc; +} + +/* +** Return the AsyncLock structure from the global async.pLock list +** associated with the file-system entry identified by path zName +** (a string of nName bytes). If no such structure exists, return 0. +*/ +static AsyncLock *findLock(const char *zName, int nName){ + AsyncLock *p = async.pLock; + while( p && (p->nFile!=nName || memcmp(p->zFile, zName, nName)) ){ + p = p->pNext; + } + return p; +} + +/* +** The following two methods - asyncLock() and asyncUnlock() - are used +** to obtain and release locks on database files opened with the +** asynchronous backend. +*/ +static int asyncLock(sqlite3_file *pFile, int eLock){ + int rc = SQLITE_OK; + AsyncFileData *p = ((AsyncFile *)pFile)->pData; + + if( p->zName ){ + async_mutex_enter(ASYNC_MUTEX_LOCK); + if( p->lock.eLock<eLock ){ + AsyncLock *pLock = p->pLock; + AsyncFileLock *pIter; + assert(pLock && pLock->pList); + for(pIter=pLock->pList; pIter; pIter=pIter->pNext){ + if( pIter!=&p->lock && ( + (eLock==SQLITE_LOCK_EXCLUSIVE && pIter->eLock>=SQLITE_LOCK_SHARED) || + (eLock==SQLITE_LOCK_PENDING && pIter->eLock>=SQLITE_LOCK_RESERVED) || + (eLock==SQLITE_LOCK_RESERVED && pIter->eLock>=SQLITE_LOCK_RESERVED) || + (eLock==SQLITE_LOCK_SHARED && pIter->eLock>=SQLITE_LOCK_PENDING) + )){ + rc = SQLITE_BUSY; + } + } + if( rc==SQLITE_OK ){ + p->lock.eLock = eLock; + p->lock.eAsyncLock = MAX(p->lock.eAsyncLock, eLock); + } + assert(p->lock.eAsyncLock>=p->lock.eLock); + if( rc==SQLITE_OK ){ + rc = getFileLock(pLock); + } + } + async_mutex_leave(ASYNC_MUTEX_LOCK); + } + + ASYNC_TRACE(("LOCK %d (%s) rc=%d\n", eLock, p->zName, rc)); + return rc; +} +static int asyncUnlock(sqlite3_file *pFile, int eLock){ + int rc = SQLITE_OK; + AsyncFileData *p = ((AsyncFile *)pFile)->pData; + if( p->zName ){ + AsyncFileLock *pLock = &p->lock; + async_mutex_enter(ASYNC_MUTEX_QUEUE); + async_mutex_enter(ASYNC_MUTEX_LOCK); + pLock->eLock = MIN(pLock->eLock, eLock); + rc = addNewAsyncWrite(p, ASYNC_UNLOCK, 0, eLock, 0); + async_mutex_leave(ASYNC_MUTEX_LOCK); + async_mutex_leave(ASYNC_MUTEX_QUEUE); + } + return rc; +} + +/* +** This function is called when the pager layer first opens a database file +** and is checking for a hot-journal. +*/ +static int asyncCheckReservedLock(sqlite3_file *pFile, int *pResOut){ + int ret = 0; + AsyncFileLock *pIter; + AsyncFileData *p = ((AsyncFile *)pFile)->pData; + + async_mutex_enter(ASYNC_MUTEX_LOCK); + for(pIter=p->pLock->pList; pIter; pIter=pIter->pNext){ + if( pIter->eLock>=SQLITE_LOCK_RESERVED ){ + ret = 1; + break; + } + } + async_mutex_leave(ASYNC_MUTEX_LOCK); + + ASYNC_TRACE(("CHECK-LOCK %d (%s)\n", ret, p->zName)); + *pResOut = ret; + return SQLITE_OK; +} + +/* +** sqlite3_file_control() implementation. +*/ +static int asyncFileControl(sqlite3_file *id, int op, void *pArg){ + switch( op ){ + case SQLITE_FCNTL_LOCKSTATE: { + async_mutex_enter(ASYNC_MUTEX_LOCK); + *(int*)pArg = ((AsyncFile*)id)->pData->lock.eLock; + async_mutex_leave(ASYNC_MUTEX_LOCK); + return SQLITE_OK; + } + } + return SQLITE_NOTFOUND; +} + +/* +** Return the device characteristics and sector-size of the device. It +** is tricky to implement these correctly, as this backend might +** not have an open file handle at this point. +*/ +static int asyncSectorSize(sqlite3_file *pFile){ + UNUSED_PARAMETER(pFile); + return 512; +} +static int asyncDeviceCharacteristics(sqlite3_file *pFile){ + UNUSED_PARAMETER(pFile); + return 0; +} + +static int unlinkAsyncFile(AsyncFileData *pData){ + AsyncFileLock **ppIter; + int rc = SQLITE_OK; + + if( pData->zName ){ + AsyncLock *pLock = pData->pLock; + for(ppIter=&pLock->pList; *ppIter; ppIter=&((*ppIter)->pNext)){ + if( (*ppIter)==&pData->lock ){ + *ppIter = pData->lock.pNext; + break; + } + } + if( !pLock->pList ){ + AsyncLock **pp; + if( pLock->pFile ){ + pLock->pFile->pMethods->xClose(pLock->pFile); + } + for(pp=&async.pLock; *pp!=pLock; pp=&((*pp)->pNext)); + *pp = pLock->pNext; + sqlite3_free(pLock); + }else{ + rc = getFileLock(pLock); + } + } + + return rc; +} + +/* +** The parameter passed to this function is a copy of a 'flags' parameter +** passed to this modules xOpen() method. This function returns true +** if the file should be opened asynchronously, or false if it should +** be opened immediately. +** +** If the file is to be opened asynchronously, then asyncOpen() will add +** an entry to the event queue and the file will not actually be opened +** until the event is processed. Otherwise, the file is opened directly +** by the caller. +*/ +static int doAsynchronousOpen(int flags){ + return (flags&SQLITE_OPEN_CREATE) && ( + (flags&SQLITE_OPEN_MAIN_JOURNAL) || + (flags&SQLITE_OPEN_TEMP_JOURNAL) || + (flags&SQLITE_OPEN_DELETEONCLOSE) + ); +} + +/* +** Open a file. +*/ +static int asyncOpen( + sqlite3_vfs *pAsyncVfs, + const char *zName, + sqlite3_file *pFile, + int flags, + int *pOutFlags +){ + static sqlite3_io_methods async_methods = { + 1, /* iVersion */ + asyncClose, /* xClose */ + asyncRead, /* xRead */ + asyncWrite, /* xWrite */ + asyncTruncate, /* xTruncate */ + asyncSync, /* xSync */ + asyncFileSize, /* xFileSize */ + asyncLock, /* xLock */ + asyncUnlock, /* xUnlock */ + asyncCheckReservedLock, /* xCheckReservedLock */ + asyncFileControl, /* xFileControl */ + asyncSectorSize, /* xSectorSize */ + asyncDeviceCharacteristics /* xDeviceCharacteristics */ + }; + + sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData; + AsyncFile *p = (AsyncFile *)pFile; + int nName = 0; + int rc = SQLITE_OK; + int nByte; + AsyncFileData *pData; + AsyncLock *pLock = 0; + char *z; + int isAsyncOpen = doAsynchronousOpen(flags); + + /* If zName is NULL, then the upper layer is requesting an anonymous file. + ** Otherwise, allocate enough space to make a copy of the file name (along + ** with the second nul-terminator byte required by xOpen). + */ + if( zName ){ + nName = (int)strlen(zName); + } + + nByte = ( + sizeof(AsyncFileData) + /* AsyncFileData structure */ + 2 * pVfs->szOsFile + /* AsyncFileData.pBaseRead and pBaseWrite */ + nName + 2 /* AsyncFileData.zName */ + ); + z = sqlite3_malloc(nByte); + if( !z ){ + return SQLITE_NOMEM; + } + memset(z, 0, nByte); + pData = (AsyncFileData*)z; + z += sizeof(pData[0]); + pData->pBaseRead = (sqlite3_file*)z; + z += pVfs->szOsFile; + pData->pBaseWrite = (sqlite3_file*)z; + pData->closeOp.pFileData = pData; + pData->closeOp.op = ASYNC_CLOSE; + + if( zName ){ + z += pVfs->szOsFile; + pData->zName = z; + pData->nName = nName; + memcpy(pData->zName, zName, nName); + } + + if( !isAsyncOpen ){ + int flagsout; + rc = pVfs->xOpen(pVfs, pData->zName, pData->pBaseRead, flags, &flagsout); + if( rc==SQLITE_OK + && (flagsout&SQLITE_OPEN_READWRITE) + && (flags&SQLITE_OPEN_EXCLUSIVE)==0 + ){ + rc = pVfs->xOpen(pVfs, pData->zName, pData->pBaseWrite, flags, 0); + } + if( pOutFlags ){ + *pOutFlags = flagsout; + } + } + + async_mutex_enter(ASYNC_MUTEX_LOCK); + + if( zName && rc==SQLITE_OK ){ + pLock = findLock(pData->zName, pData->nName); + if( !pLock ){ + int nByte = pVfs->szOsFile + sizeof(AsyncLock) + pData->nName + 1; + pLock = (AsyncLock *)sqlite3_malloc(nByte); + if( pLock ){ + memset(pLock, 0, nByte); + if( async.bLockFiles && (flags&SQLITE_OPEN_MAIN_DB) ){ + pLock->pFile = (sqlite3_file *)&pLock[1]; + rc = pVfs->xOpen(pVfs, pData->zName, pLock->pFile, flags, 0); + if( rc!=SQLITE_OK ){ + sqlite3_free(pLock); + pLock = 0; + } + } + if( pLock ){ + pLock->nFile = pData->nName; + pLock->zFile = &((char *)(&pLock[1]))[pVfs->szOsFile]; + memcpy(pLock->zFile, pData->zName, pLock->nFile); + pLock->pNext = async.pLock; + async.pLock = pLock; + } + }else{ + rc = SQLITE_NOMEM; + } + } + } + + if( rc==SQLITE_OK ){ + p->pMethod = &async_methods; + p->pData = pData; + + /* Link AsyncFileData.lock into the linked list of + ** AsyncFileLock structures for this file. + */ + if( zName ){ + pData->lock.pNext = pLock->pList; + pLock->pList = &pData->lock; + pData->zName = pLock->zFile; + } + }else{ + if( pData->pBaseRead->pMethods ){ + pData->pBaseRead->pMethods->xClose(pData->pBaseRead); + } + if( pData->pBaseWrite->pMethods ){ + pData->pBaseWrite->pMethods->xClose(pData->pBaseWrite); + } + sqlite3_free(pData); + } + + async_mutex_leave(ASYNC_MUTEX_LOCK); + + if( rc==SQLITE_OK ){ + pData->pLock = pLock; + } + + if( rc==SQLITE_OK && isAsyncOpen ){ + rc = addNewAsyncWrite(pData, ASYNC_OPENEXCLUSIVE, (sqlite3_int64)flags,0,0); + if( rc==SQLITE_OK ){ + if( pOutFlags ) *pOutFlags = flags; + }else{ + async_mutex_enter(ASYNC_MUTEX_LOCK); + unlinkAsyncFile(pData); + async_mutex_leave(ASYNC_MUTEX_LOCK); + sqlite3_free(pData); + } + } + if( rc!=SQLITE_OK ){ + p->pMethod = 0; + }else{ + incrOpenFileCount(); + } + + return rc; +} + +/* +** Implementation of sqlite3OsDelete. Add an entry to the end of the +** write-op queue to perform the delete. +*/ +static int asyncDelete(sqlite3_vfs *pAsyncVfs, const char *z, int syncDir){ + UNUSED_PARAMETER(pAsyncVfs); + return addNewAsyncWrite(0, ASYNC_DELETE, syncDir, (int)strlen(z)+1, z); +} + +/* +** Implementation of sqlite3OsAccess. This method holds the mutex from +** start to finish. +*/ +static int asyncAccess( + sqlite3_vfs *pAsyncVfs, + const char *zName, + int flags, + int *pResOut +){ + int rc; + int ret; + AsyncWrite *p; + sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData; + + assert(flags==SQLITE_ACCESS_READWRITE + || flags==SQLITE_ACCESS_READ + || flags==SQLITE_ACCESS_EXISTS + ); + + async_mutex_enter(ASYNC_MUTEX_QUEUE); + rc = pVfs->xAccess(pVfs, zName, flags, &ret); + if( rc==SQLITE_OK && flags==SQLITE_ACCESS_EXISTS ){ + for(p=async.pQueueFirst; p; p = p->pNext){ + if( p->op==ASYNC_DELETE && 0==strcmp(p->zBuf, zName) ){ + ret = 0; + }else if( p->op==ASYNC_OPENEXCLUSIVE + && p->pFileData->zName + && 0==strcmp(p->pFileData->zName, zName) + ){ + ret = 1; + } + } + } + ASYNC_TRACE(("ACCESS(%s): %s = %d\n", + flags==SQLITE_ACCESS_READWRITE?"read-write": + flags==SQLITE_ACCESS_READ?"read":"exists" + , zName, ret) + ); + async_mutex_leave(ASYNC_MUTEX_QUEUE); + *pResOut = ret; + return rc; +} + +/* +** Fill in zPathOut with the full path to the file identified by zPath. +*/ +static int asyncFullPathname( + sqlite3_vfs *pAsyncVfs, + const char *zPath, + int nPathOut, + char *zPathOut +){ + int rc; + sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData; + rc = pVfs->xFullPathname(pVfs, zPath, nPathOut, zPathOut); + + /* Because of the way intra-process file locking works, this backend + ** needs to return a canonical path. The following block assumes the + ** file-system uses unix style paths. + */ + if( rc==SQLITE_OK ){ + int i, j; + char *z = zPathOut; + int n = (int)strlen(z); + while( n>1 && z[n-1]=='/' ){ n--; } + for(i=j=0; i<n; i++){ + if( z[i]=='/' ){ + if( z[i+1]=='/' ) continue; + if( z[i+1]=='.' && i+2<n && z[i+2]=='/' ){ + i += 1; + continue; + } + if( z[i+1]=='.' && i+3<n && z[i+2]=='.' && z[i+3]=='/' ){ + while( j>0 && z[j-1]!='/' ){ j--; } + if( j>0 ){ j--; } + i += 2; + continue; + } + } + z[j++] = z[i]; + } + z[j] = 0; + } + + return rc; +} +static void *asyncDlOpen(sqlite3_vfs *pAsyncVfs, const char *zPath){ + sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData; + return pVfs->xDlOpen(pVfs, zPath); +} +static void asyncDlError(sqlite3_vfs *pAsyncVfs, int nByte, char *zErrMsg){ + sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData; + pVfs->xDlError(pVfs, nByte, zErrMsg); +} +static void (*asyncDlSym( + sqlite3_vfs *pAsyncVfs, + void *pHandle, + const char *zSymbol +))(void){ + sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData; + return pVfs->xDlSym(pVfs, pHandle, zSymbol); +} +static void asyncDlClose(sqlite3_vfs *pAsyncVfs, void *pHandle){ + sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData; + pVfs->xDlClose(pVfs, pHandle); +} +static int asyncRandomness(sqlite3_vfs *pAsyncVfs, int nByte, char *zBufOut){ + sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData; + return pVfs->xRandomness(pVfs, nByte, zBufOut); +} +static int asyncSleep(sqlite3_vfs *pAsyncVfs, int nMicro){ + sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData; + return pVfs->xSleep(pVfs, nMicro); +} +static int asyncCurrentTime(sqlite3_vfs *pAsyncVfs, double *pTimeOut){ + sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData; + return pVfs->xCurrentTime(pVfs, pTimeOut); +} + +static sqlite3_vfs async_vfs = { + 1, /* iVersion */ + sizeof(AsyncFile), /* szOsFile */ + 0, /* mxPathname */ + 0, /* pNext */ + SQLITEASYNC_VFSNAME, /* zName */ + 0, /* pAppData */ + asyncOpen, /* xOpen */ + asyncDelete, /* xDelete */ + asyncAccess, /* xAccess */ + asyncFullPathname, /* xFullPathname */ + asyncDlOpen, /* xDlOpen */ + asyncDlError, /* xDlError */ + asyncDlSym, /* xDlSym */ + asyncDlClose, /* xDlClose */ + asyncRandomness, /* xDlError */ + asyncSleep, /* xDlSym */ + asyncCurrentTime /* xDlClose */ +}; + +/* +** This procedure runs in a separate thread, reading messages off of the +** write queue and processing them one by one. +** +** If async.writerHaltNow is true, then this procedure exits +** after processing a single message. +** +** If async.writerHaltWhenIdle is true, then this procedure exits when +** the write queue is empty. +** +** If both of the above variables are false, this procedure runs +** indefinately, waiting for operations to be added to the write queue +** and processing them in the order in which they arrive. +** +** An artifical delay of async.ioDelay milliseconds is inserted before +** each write operation in order to simulate the effect of a slow disk. +** +** Only one instance of this procedure may be running at a time. +*/ +static void asyncWriterThread(void){ + sqlite3_vfs *pVfs = (sqlite3_vfs *)(async_vfs.pAppData); + AsyncWrite *p = 0; + int rc = SQLITE_OK; + int holdingMutex = 0; + + async_mutex_enter(ASYNC_MUTEX_WRITER); + + while( async.eHalt!=SQLITEASYNC_HALT_NOW ){ + int doNotFree = 0; + sqlite3_file *pBase = 0; + + if( !holdingMutex ){ + async_mutex_enter(ASYNC_MUTEX_QUEUE); + } + while( (p = async.pQueueFirst)==0 ){ + if( async.eHalt!=SQLITEASYNC_HALT_NEVER ){ + async_mutex_leave(ASYNC_MUTEX_QUEUE); + break; + }else{ + ASYNC_TRACE(("IDLE\n")); + async_cond_wait(ASYNC_COND_QUEUE, ASYNC_MUTEX_QUEUE); + ASYNC_TRACE(("WAKEUP\n")); + } + } + if( p==0 ) break; + holdingMutex = 1; + + /* Right now this thread is holding the mutex on the write-op queue. + ** Variable 'p' points to the first entry in the write-op queue. In + ** the general case, we hold on to the mutex for the entire body of + ** the loop. + ** + ** However in the cases enumerated below, we relinquish the mutex, + ** perform the IO, and then re-request the mutex before removing 'p' from + ** the head of the write-op queue. The idea is to increase concurrency with + ** sqlite threads. + ** + ** * An ASYNC_CLOSE operation. + ** * An ASYNC_OPENEXCLUSIVE operation. For this one, we relinquish + ** the mutex, call the underlying xOpenExclusive() function, then + ** re-aquire the mutex before seting the AsyncFile.pBaseRead + ** variable. + ** * ASYNC_SYNC and ASYNC_WRITE operations, if + ** SQLITE_ASYNC_TWO_FILEHANDLES was set at compile time and two + ** file-handles are open for the particular file being "synced". + */ + if( async.ioError!=SQLITE_OK && p->op!=ASYNC_CLOSE ){ + p->op = ASYNC_NOOP; + } + if( p->pFileData ){ + pBase = p->pFileData->pBaseWrite; + if( + p->op==ASYNC_CLOSE || + p->op==ASYNC_OPENEXCLUSIVE || + (pBase->pMethods && (p->op==ASYNC_SYNC || p->op==ASYNC_WRITE) ) + ){ + async_mutex_leave(ASYNC_MUTEX_QUEUE); + holdingMutex = 0; + } + if( !pBase->pMethods ){ + pBase = p->pFileData->pBaseRead; + } + } + + switch( p->op ){ + case ASYNC_NOOP: + break; + + case ASYNC_WRITE: + assert( pBase ); + ASYNC_TRACE(("WRITE %s %d bytes at %d\n", + p->pFileData->zName, p->nByte, p->iOffset)); + rc = pBase->pMethods->xWrite(pBase, (void *)(p->zBuf), p->nByte, p->iOffset); + break; + + case ASYNC_SYNC: + assert( pBase ); + ASYNC_TRACE(("SYNC %s\n", p->pFileData->zName)); + rc = pBase->pMethods->xSync(pBase, p->nByte); + break; + + case ASYNC_TRUNCATE: + assert( pBase ); + ASYNC_TRACE(("TRUNCATE %s to %d bytes\n", + p->pFileData->zName, p->iOffset)); + rc = pBase->pMethods->xTruncate(pBase, p->iOffset); + break; + + case ASYNC_CLOSE: { + AsyncFileData *pData = p->pFileData; + ASYNC_TRACE(("CLOSE %s\n", p->pFileData->zName)); + if( pData->pBaseWrite->pMethods ){ + pData->pBaseWrite->pMethods->xClose(pData->pBaseWrite); + } + if( pData->pBaseRead->pMethods ){ + pData->pBaseRead->pMethods->xClose(pData->pBaseRead); + } + + /* Unlink AsyncFileData.lock from the linked list of AsyncFileLock + ** structures for this file. Obtain the async.lockMutex mutex + ** before doing so. + */ + async_mutex_enter(ASYNC_MUTEX_LOCK); + rc = unlinkAsyncFile(pData); + async_mutex_leave(ASYNC_MUTEX_LOCK); + + if( !holdingMutex ){ + async_mutex_enter(ASYNC_MUTEX_QUEUE); + holdingMutex = 1; + } + assert_mutex_is_held(ASYNC_MUTEX_QUEUE); + async.pQueueFirst = p->pNext; + sqlite3_free(pData); + doNotFree = 1; + break; + } + + case ASYNC_UNLOCK: { + AsyncWrite *pIter; + AsyncFileData *pData = p->pFileData; + int eLock = p->nByte; + + /* When a file is locked by SQLite using the async backend, it is + ** locked within the 'real' file-system synchronously. When it is + ** unlocked, an ASYNC_UNLOCK event is added to the write-queue to + ** unlock the file asynchronously. The design of the async backend + ** requires that the 'real' file-system file be locked from the + ** time that SQLite first locks it (and probably reads from it) + ** until all asynchronous write events that were scheduled before + ** SQLite unlocked the file have been processed. + ** + ** This is more complex if SQLite locks and unlocks the file multiple + ** times in quick succession. For example, if SQLite does: + ** + ** lock, write, unlock, lock, write, unlock + ** + ** Each "lock" operation locks the file immediately. Each "write" + ** and "unlock" operation adds an event to the event queue. If the + ** second "lock" operation is performed before the first "unlock" + ** operation has been processed asynchronously, then the first + ** "unlock" cannot be safely processed as is, since this would mean + ** the file was unlocked when the second "write" operation is + ** processed. To work around this, when processing an ASYNC_UNLOCK + ** operation, SQLite: + ** + ** 1) Unlocks the file to the minimum of the argument passed to + ** the xUnlock() call and the current lock from SQLite's point + ** of view, and + ** + ** 2) Only unlocks the file at all if this event is the last + ** ASYNC_UNLOCK event on this file in the write-queue. + */ + assert( holdingMutex==1 ); + assert( async.pQueueFirst==p ); + for(pIter=async.pQueueFirst->pNext; pIter; pIter=pIter->pNext){ + if( pIter->pFileData==pData && pIter->op==ASYNC_UNLOCK ) break; + } + if( !pIter ){ + async_mutex_enter(ASYNC_MUTEX_LOCK); + pData->lock.eAsyncLock = MIN( + pData->lock.eAsyncLock, MAX(pData->lock.eLock, eLock) + ); + assert(pData->lock.eAsyncLock>=pData->lock.eLock); + rc = getFileLock(pData->pLock); + async_mutex_leave(ASYNC_MUTEX_LOCK); + } + break; + } + + case ASYNC_DELETE: + ASYNC_TRACE(("DELETE %s\n", p->zBuf)); + rc = pVfs->xDelete(pVfs, p->zBuf, (int)p->iOffset); + if( rc==SQLITE_IOERR_DELETE_NOENT ) rc = SQLITE_OK; + break; + + case ASYNC_OPENEXCLUSIVE: { + int flags = (int)p->iOffset; + AsyncFileData *pData = p->pFileData; + ASYNC_TRACE(("OPEN %s flags=%d\n", p->zBuf, (int)p->iOffset)); + assert(pData->pBaseRead->pMethods==0 && pData->pBaseWrite->pMethods==0); + rc = pVfs->xOpen(pVfs, pData->zName, pData->pBaseRead, flags, 0); + assert( holdingMutex==0 ); + async_mutex_enter(ASYNC_MUTEX_QUEUE); + holdingMutex = 1; + break; + } + + default: assert(!"Illegal value for AsyncWrite.op"); + } + + /* If we didn't hang on to the mutex during the IO op, obtain it now + ** so that the AsyncWrite structure can be safely removed from the + ** global write-op queue. + */ + if( !holdingMutex ){ + async_mutex_enter(ASYNC_MUTEX_QUEUE); + holdingMutex = 1; + } + /* ASYNC_TRACE(("UNLINK %p\n", p)); */ + if( p==async.pQueueLast ){ + async.pQueueLast = 0; + } + if( !doNotFree ){ + assert_mutex_is_held(ASYNC_MUTEX_QUEUE); + async.pQueueFirst = p->pNext; + sqlite3_free(p); + } + assert( holdingMutex ); + + /* An IO error has occurred. We cannot report the error back to the + ** connection that requested the I/O since the error happened + ** asynchronously. The connection has already moved on. There + ** really is nobody to report the error to. + ** + ** The file for which the error occurred may have been a database or + ** journal file. Regardless, none of the currently queued operations + ** associated with the same database should now be performed. Nor should + ** any subsequently requested IO on either a database or journal file + ** handle for the same database be accepted until the main database + ** file handle has been closed and reopened. + ** + ** Furthermore, no further IO should be queued or performed on any file + ** handle associated with a database that may have been part of a + ** multi-file transaction that included the database associated with + ** the IO error (i.e. a database ATTACHed to the same handle at some + ** point in time). + */ + if( rc!=SQLITE_OK ){ + async.ioError = rc; + } + + if( async.ioError && !async.pQueueFirst ){ + async_mutex_enter(ASYNC_MUTEX_LOCK); + if( 0==async.pLock ){ + async.ioError = SQLITE_OK; + } + async_mutex_leave(ASYNC_MUTEX_LOCK); + } + + /* Drop the queue mutex before continuing to the next write operation + ** in order to give other threads a chance to work with the write queue. + */ + if( !async.pQueueFirst || !async.ioError ){ + async_mutex_leave(ASYNC_MUTEX_QUEUE); + holdingMutex = 0; + if( async.ioDelay>0 ){ + pVfs->xSleep(pVfs, async.ioDelay*1000); + }else{ + async_sched_yield(); + } + } + } + + async_mutex_leave(ASYNC_MUTEX_WRITER); + return; +} + +/* +** Install the asynchronous VFS. +*/ +int sqlite3async_initialize(const char *zParent, int isDefault){ + int rc = SQLITE_OK; + if( async_vfs.pAppData==0 ){ + sqlite3_vfs *pParent = sqlite3_vfs_find(zParent); + if( !pParent || async_os_initialize() ){ + rc = SQLITE_ERROR; + }else if( SQLITE_OK!=(rc = sqlite3_vfs_register(&async_vfs, isDefault)) ){ + async_os_shutdown(); + }else{ + async_vfs.pAppData = (void *)pParent; + async_vfs.mxPathname = ((sqlite3_vfs *)async_vfs.pAppData)->mxPathname; + } + } + return rc; +} + +/* +** Uninstall the asynchronous VFS. +*/ +void sqlite3async_shutdown(void){ + if( async_vfs.pAppData ){ + async_os_shutdown(); + sqlite3_vfs_unregister((sqlite3_vfs *)&async_vfs); + async_vfs.pAppData = 0; + } +} + +/* +** Process events on the write-queue. +*/ +void sqlite3async_run(void){ + asyncWriterThread(); +} + +/* +** Control/configure the asynchronous IO system. +*/ +int sqlite3async_control(int op, ...){ + int rc = SQLITE_OK; + va_list ap; + va_start(ap, op); + switch( op ){ + case SQLITEASYNC_HALT: { + int eWhen = va_arg(ap, int); + if( eWhen!=SQLITEASYNC_HALT_NEVER + && eWhen!=SQLITEASYNC_HALT_NOW + && eWhen!=SQLITEASYNC_HALT_IDLE + ){ + rc = SQLITE_MISUSE; + break; + } + async.eHalt = eWhen; + async_mutex_enter(ASYNC_MUTEX_QUEUE); + async_cond_signal(ASYNC_COND_QUEUE); + async_mutex_leave(ASYNC_MUTEX_QUEUE); + break; + } + + case SQLITEASYNC_DELAY: { + int iDelay = va_arg(ap, int); + if( iDelay<0 ){ + rc = SQLITE_MISUSE; + break; + } + async.ioDelay = iDelay; + break; + } + + case SQLITEASYNC_LOCKFILES: { + int bLock = va_arg(ap, int); + async_mutex_enter(ASYNC_MUTEX_QUEUE); + if( async.nFile || async.pQueueFirst ){ + async_mutex_leave(ASYNC_MUTEX_QUEUE); + rc = SQLITE_MISUSE; + break; + } + async.bLockFiles = bLock; + async_mutex_leave(ASYNC_MUTEX_QUEUE); + break; + } + + case SQLITEASYNC_GET_HALT: { + int *peWhen = va_arg(ap, int *); + *peWhen = async.eHalt; + break; + } + case SQLITEASYNC_GET_DELAY: { + int *piDelay = va_arg(ap, int *); + *piDelay = async.ioDelay; + break; + } + case SQLITEASYNC_GET_LOCKFILES: { + int *piDelay = va_arg(ap, int *); + *piDelay = async.bLockFiles; + break; + } + + default: + rc = SQLITE_ERROR; + break; + } + va_end(ap); + return rc; +} + +#endif /* !defined(SQLITE_CORE) || defined(SQLITE_ENABLE_ASYNCIO) */ diff --git a/ext/async/sqlite3async.h b/ext/async/sqlite3async.h new file mode 100644 index 0000000..13b23bc --- /dev/null +++ b/ext/async/sqlite3async.h @@ -0,0 +1,222 @@ + +#ifndef __SQLITEASYNC_H_ +#define __SQLITEASYNC_H_ 1 + +/* +** Make sure we can call this stuff from C++. +*/ +#ifdef __cplusplus +extern "C" { +#endif + +#define SQLITEASYNC_VFSNAME "sqlite3async" + +/* +** THREAD SAFETY NOTES: +** +** Of the four API functions in this file, the following are not threadsafe: +** +** sqlite3async_initialize() +** sqlite3async_shutdown() +** +** Care must be taken that neither of these functions is called while +** another thread may be calling either any sqlite3async_XXX() function +** or an sqlite3_XXX() API function related to a database handle that +** is using the asynchronous IO VFS. +** +** These functions: +** +** sqlite3async_run() +** sqlite3async_control() +** +** are threadsafe. It is quite safe to call either of these functions even +** if another thread may also be calling one of them or an sqlite3_XXX() +** function related to a database handle that uses the asynchronous IO VFS. +*/ + +/* +** Initialize the asynchronous IO VFS and register it with SQLite using +** sqlite3_vfs_register(). If the asynchronous VFS is already initialized +** and registered, this function is a no-op. The asynchronous IO VFS +** is registered as "sqlite3async". +** +** The asynchronous IO VFS does not make operating system IO requests +** directly. Instead, it uses an existing VFS implementation for all +** required file-system operations. If the first parameter to this function +** is NULL, then the current default VFS is used for IO. If it is not +** NULL, then it must be the name of an existing VFS. In other words, the +** first argument to this function is passed to sqlite3_vfs_find() to +** locate the VFS to use for all real IO operations. This VFS is known +** as the "parent VFS". +** +** If the second parameter to this function is non-zero, then the +** asynchronous IO VFS is registered as the default VFS for all SQLite +** database connections within the process. Otherwise, the asynchronous IO +** VFS is only used by connections opened using sqlite3_open_v2() that +** specifically request VFS "sqlite3async". +** +** If a parent VFS cannot be located, then SQLITE_ERROR is returned. +** In the unlikely event that operating system specific initialization +** fails (win32 systems create the required critical section and event +** objects within this function), then SQLITE_ERROR is also returned. +** Finally, if the call to sqlite3_vfs_register() returns an error, then +** the error code is returned to the user by this function. In all three +** of these cases, intialization has failed and the asynchronous IO VFS +** is not registered with SQLite. +** +** Otherwise, if no error occurs, SQLITE_OK is returned. +*/ +int sqlite3async_initialize(const char *zParent, int isDefault); + +/* +** This function unregisters the asynchronous IO VFS using +** sqlite3_vfs_unregister(). +** +** On win32 platforms, this function also releases the small number of +** critical section and event objects created by sqlite3async_initialize(). +*/ +void sqlite3async_shutdown(void); + +/* +** This function may only be called when the asynchronous IO VFS is +** installed (after a call to sqlite3async_initialize()). It processes +** zero or more queued write operations before returning. It is expected +** (but not required) that this function will be called by a different +** thread than those threads that use SQLite. The "background thread" +** that performs IO. +** +** How many queued write operations are performed before returning +** depends on the global setting configured by passing the SQLITEASYNC_HALT +** verb to sqlite3async_control() (see below for details). By default +** this function never returns - it processes all pending operations and +** then blocks waiting for new ones. +** +** If multiple simultaneous calls are made to sqlite3async_run() from two +** or more threads, then the calls are serialized internally. +*/ +void sqlite3async_run(void); + +/* +** This function may only be called when the asynchronous IO VFS is +** installed (after a call to sqlite3async_initialize()). It is used +** to query or configure various parameters that affect the operation +** of the asynchronous IO VFS. At present there are three parameters +** supported: +** +** * The "halt" parameter, which configures the circumstances under +** which the sqlite3async_run() parameter is configured. +** +** * The "delay" parameter. Setting the delay parameter to a non-zero +** value causes the sqlite3async_run() function to sleep for the +** configured number of milliseconds between each queued write +** operation. +** +** * The "lockfiles" parameter. This parameter determines whether or +** not the asynchronous IO VFS locks the database files it operates +** on. Disabling file locking can improve throughput. +** +** This function is always passed two arguments. When setting the value +** of a parameter, the first argument must be one of SQLITEASYNC_HALT, +** SQLITEASYNC_DELAY or SQLITEASYNC_LOCKFILES. The second argument must +** be passed the new value for the parameter as type "int". +** +** When querying the current value of a paramter, the first argument must +** be one of SQLITEASYNC_GET_HALT, GET_DELAY or GET_LOCKFILES. The second +** argument to this function must be of type (int *). The current value +** of the queried parameter is copied to the memory pointed to by the +** second argument. For example: +** +** int eCurrentHalt; +** int eNewHalt = SQLITEASYNC_HALT_IDLE; +** +** sqlite3async_control(SQLITEASYNC_HALT, eNewHalt); +** sqlite3async_control(SQLITEASYNC_GET_HALT, &eCurrentHalt); +** assert( eNewHalt==eCurrentHalt ); +** +** See below for more detail on each configuration parameter. +** +** SQLITEASYNC_HALT: +** +** This is used to set the value of the "halt" parameter. The second +** argument must be one of the SQLITEASYNC_HALT_XXX symbols defined +** below (either NEVER, IDLE and NOW). +** +** If the parameter is set to NEVER, then calls to sqlite3async_run() +** never return. This is the default setting. If the parameter is set +** to IDLE, then calls to sqlite3async_run() return as soon as the +** queue of pending write operations is empty. If the parameter is set +** to NOW, then calls to sqlite3async_run() return as quickly as +** possible, without processing any pending write requests. +** +** If an attempt is made to set this parameter to an integer value other +** than SQLITEASYNC_HALT_NEVER, IDLE or NOW, then sqlite3async_control() +** returns SQLITE_MISUSE and the current value of the parameter is not +** modified. +** +** Modifying the "halt" parameter affects calls to sqlite3async_run() +** made by other threads that are currently in progress. +** +** SQLITEASYNC_DELAY: +** +** This is used to set the value of the "delay" parameter. If set to +** a non-zero value, then after completing a pending write request, the +** sqlite3async_run() function sleeps for the configured number of +** milliseconds. +** +** If an attempt is made to set this parameter to a negative value, +** sqlite3async_control() returns SQLITE_MISUSE and the current value +** of the parameter is not modified. +** +** Modifying the "delay" parameter affects calls to sqlite3async_run() +** made by other threads that are currently in progress. +** +** SQLITEASYNC_LOCKFILES: +** +** This is used to set the value of the "lockfiles" parameter. This +** parameter must be set to either 0 or 1. If set to 1, then the +** asynchronous IO VFS uses the xLock() and xUnlock() methods of the +** parent VFS to lock database files being read and/or written. If +** the parameter is set to 0, then these locks are omitted. +** +** This parameter may only be set when there are no open database +** connections using the VFS and the queue of pending write requests +** is empty. Attempting to set it when this is not true, or to set it +** to a value other than 0 or 1 causes sqlite3async_control() to return +** SQLITE_MISUSE and the value of the parameter to remain unchanged. +** +** If this parameter is set to zero, then it is only safe to access the +** database via the asynchronous IO VFS from within a single process. If +** while writing to the database via the asynchronous IO VFS the database +** is also read or written from within another process, or via another +** connection that does not use the asynchronous IO VFS within the same +** process, the results are undefined (and may include crashes or database +** corruption). +** +** Alternatively, if this parameter is set to 1, then it is safe to access +** the database from multiple connections within multiple processes using +** either the asynchronous IO VFS or the parent VFS directly. +*/ +int sqlite3async_control(int op, ...); + +/* +** Values that can be used as the first argument to sqlite3async_control(). +*/ +#define SQLITEASYNC_HALT 1 +#define SQLITEASYNC_GET_HALT 2 +#define SQLITEASYNC_DELAY 3 +#define SQLITEASYNC_GET_DELAY 4 +#define SQLITEASYNC_LOCKFILES 5 +#define SQLITEASYNC_GET_LOCKFILES 6 + +/* +** If the first argument to sqlite3async_control() is SQLITEASYNC_HALT, +** the second argument should be one of the following. +*/ +#define SQLITEASYNC_HALT_NEVER 0 /* Never halt (default value) */ +#define SQLITEASYNC_HALT_NOW 1 /* Halt as soon as possible */ +#define SQLITEASYNC_HALT_IDLE 2 /* Halt when write-queue is empty */ + +#ifdef __cplusplus +} /* End of the 'extern "C"' block */ +#endif +#endif /* ifndef __SQLITEASYNC_H_ */ |