sqlite1.0.2锁分析

锁存放在pager 这个结构体中,这个结构体是数据库页在内存中的管理单位。

其中u8 state; 就是表示锁状态的变量

在sqlite1.0.2中 目前只有三种状态

SQLITE_UNLOCK，

READLOCK ，

WRITELOCK  

#define SQLITE_UNLOCK      0
#define SQLITE_READLOCK    1
#define SQLITE_WRITELOCK   2

  SQLITE_UNLOCK     这个内存页处于初始状态，里面没有数据
  SQLITE_READLOCK     这个内存页处于被读取的状态，是不允许修改，但是允许其他进程读取
  SQLITE_WRITELOCK    这个内存页正在写入磁盘文件，其他进程不允许读取，也不允许写入

这里应该采用状态机机制来说明,因为公司笔记本没有vision，以后补上。

正常流程

sqlitepager_open 将 pager 初始化为 将 SQLITE_UNLOCK，

sqlitepager_get() 函数将pager 从 SQLITE_UNLOCK变成 SQLITE_READLOCK，

sqlitepager_write() 函数 通过调用 sqlitepager_begin()将 pager从SQLITE_READLOCK变成SQLITE_WRITELOCK，

sqlitepager_commit()函数通过pager_unwritelock()将pager从SQLITE_WRITELOCK变成SQLITE_READLOCK或者是SQLITE_UNLOCK。

sqlitepager_rollback()函数通过pager_unwritelock()将pager从SQLITE_WRITELOCK变成SQLITE_READLOCK或者是SQLITE_UNLOCK。

异常流程
(待续)

锁伴随的结构体如下：

struct Pager {
  char *zFilename;            /* Name of the database file */
  char *zJournal;             /* Name of the journal file */
  char *zDirectory;           /* Directory hold database and journal files */
  OsFile fd, jfd;             /* File descriptors for database and journal */
  OsFile cpfd;                /* File descriptor for the checkpoint journal */
  int dbSize;                 /* Number of pages in the file */
  int origDbSize;             /* dbSize before the current change */
  int ckptSize;               /* Size of database (in pages) at ckpt_begin() */
  off_t ckptJSize;            /* Size of journal at ckpt_begin() */
  int nRec;                   /* Number of pages written to the journal */
  u32 cksumInit;              /* Quasi-random value added to every checksum */
  int ckptNRec;               /* Number of records in the checkpoint journal */
  int nExtra;                 /* Add this many bytes to each in-memory page */
  void (*xDestructor)(void*); /* Call this routine when freeing pages */
  int nPage;                  /* Total number of in-memory pages */
  int nRef;                   /* Number of in-memory pages with PgHdr.nRef>0 */
  int mxPage;                 /* Maximum number of pages to hold in cache */
  int nHit, nMiss, nOvfl;     /* Cache hits, missing, and LRU overflows */
  u8 journalOpen;             /* True if journal file descriptors is valid */
  u8 journalStarted;          /* True if initial magic of journal is synced */
  u8 useJournal;              /* Do not use a rollback journal on this file */
  u8 ckptOpen;                /* True if the checkpoint journal is open */
  u8 ckptInUse;               /* True we are in a checkpoint */
  u8 ckptAutoopen;            /* Open ckpt journal when main journal is opened*/
  u8 noSync;                  /* Do not sync the journal if true */
  u8 fullSync;                /* Do extra syncs of the journal for robustness */
  u8 state;                   /* SQLITE_UNLOCK, _READLOCK or _WRITELOCK */
  u8 errMask;                 /* One of several kinds of errors */
  u8 tempFile;                /* zFilename is a temporary file */
  u8 readOnly;                /* True for a read-only database */
  u8 needSync;                /* True if an fsync() is needed on the journal */
  u8 dirtyFile;               /* True if database file has changed in any way */
  u8 alwaysRollback;          /* Disable dont_rollback() for all pages */
  u8 *aInJournal;             /* One bit for each page in the database file */
  u8 *aInCkpt;                /* One bit for each page in the database */
  PgHdr *pFirst, *pLast;      /* List of free pages */
  PgHdr *pFirstSynced;        /* First free page with PgHdr.needSync==0 */
  PgHdr *pAll;                /* List of all pages */
  PgHdr *pCkpt;               /* List of pages in the checkpoint journal */
  PgHdr *aHash[N_PG_HASH];    /* Hash table to map page number of PgHdr */
}