postgressql——ReadBuffer_common函数（7）

PostgreSQL中ReadBuffer_common函数

数据结构

BufferDesc
共享缓冲区的共享描述符(状态)数据

typedef struct BufferDesc
{
    //buffer tag
    BufferTag   tag;            /* ID of page contained in buffer */
    //buffer索引编号(0开始)
    int         buf_id;         /* buffer's index number (from 0) */
    /* state of the tag, containing flags, refcount and usagecount */
    //tag状态,包括flags/refcount和usagecount
    pg_atomic_uint32 state;
    //pin-count等待进程ID
    int         wait_backend_pid;   /* backend PID of pin-count waiter */
    //空闲链表链中下一个空闲的buffer
    int         freeNext;       /* link in freelist chain */
    //缓冲区内容锁
    LWLock      content_lock;   /* to lock access to buffer contents */
} BufferDesc;

BufferTag
Buffer tag标记了buffer存储的是磁盘中哪个block

typedef struct buftag
{
    //物理relation标识符
    RelFileNode rnode;          /* physical relation identifier */
    ForkNumber  fo