1 先上个图，看一下函数调用过程梗概，中间略过部分细节
初始化 walsender 和 walreceiver 相关结构 方法调用流程图
2 初始化 xlog 相关结构
话说 main()-> … ->PostmasterMain()-> … ->reset_shared() -> CreateSharedMemoryAndSemaphores()> … ->WalSndShmemInit() ，调用 ShmemInitStruct() ， 在其中 调用 hash_search() 在哈希表索引 "ShmemIndex" 中查找 "Wal Sender Ctl" ，如果没有，就在 shmemIndex 中给 "Wal Sender Ctl" 分一个 HashElement 和 ShmemIndexEnt （ entry ） ，在其中的 Entry 中写上 "Wal Sender Ctl" 。返回 ShmemInitStruct() ，再调用 ShmemAlloc() 在共享内存上给 "Wal Sender Ctl" 相关结构（见下面“ Wal Sender Ctl 和 Wal Receiver Ctl 相关结构图” ）分配空间，设置 entry （在这儿及ShmemIndexEnt 类型变量）的成员 location 指向该空间， size 成员记录该空间大小 ， 最后返回 WalSndShmemInit () ，让 WalSndCtlData * 类型 全局变量 WalSndCtl 指向 所分配内存 ，初始化WalSndCtlData 结构类型的成员值。
接着 WalRcvShmemInit() -> ShmemInitStruct() ， 在其中 调用 hash_search() 在哈希表索引 "ShmemIndex" 中查找 "Wal Receiver Ctl" ，如果没有，就在 shmemIndex 中给 "Wal Receiver Ctl" 分一个 HashElement 和 ShmemIndexEnt （ entry ） ，在其中的 Entry 中写上 "Wal Receiver Ctl" 。返回 ShmemInitStruct() ，再调用 ShmemAlloc() 在共享内存上给 "Wal Receiver Ctl" 相关结构（见下面“ Wal Sender Ctl 和 Wal Receiver Ctl 相关结构图” ）分配空间，设置 entry （在这儿及ShmemIndexEnt 类型变量）的成员 location 指向该空间， size 成员记录该空间大小 ， 最后返回 WalRcvShmemInit () ，让 WalRcvData * 类型 全局变量 WalRcv 指向 所分配内存 ，初始化WalRcvData 结构类型的成员值。
相关结构定义和图见下面：
typedef enum WalSndState
{
WALSNDSTATE_STARTUP = 0,
WALSNDSTATE_BACKUP ,
WALSNDSTATE_CATCHUP ,
WALSNDSTATE_STREAMING
} WalSndState ;
/*
* Each walsender has a WalSnd struct in shared memory.
*/
typedef struct WalSnd
{
pid_t pid ; /* this walsender's process id, or 0 */
WalSndState state ; /* this walsender's state */
XLogRecPtr sentPtr ; /* WAL has been sent up to this point */
/*
* The xlog locations that have been written, flushed, and applied by
* standby-side. These may be invalid if the standby-side has not offered
* values yet.
*/
XLogRecPtr write ;
XLogRecPtr flush ;
XLogRecPtr apply ;
/* Protects shared variables shown above. */
slock_t mutex ;
/*
* Latch used by backends to wake up this walsender when it has work to
* do.
*/
Latch latch ;
/*
* The priority order of the standby managed by this WALSender, as listed
* in synchronous_standby_names, or 0 if not-listed. Protected by
* SyncRepLock.
*/
int sync_standby_priority ;
} WalSnd ;
extern WalSnd *MyWalSnd;
/* There is one WalSndCtl struct for the whole database cluster */
typedef struct
{
/*
* Synchronous replication queue. Protected by SyncRepLock.
*/
SHM_QUEUE SyncRepQueue ;
/*
* Current location of the head of the queue. All waiters should have a
* waitLSN that follows this value. Protected by SyncRepLock.
*/
XLogRecPtr lsn ;
/*
* Are any sync standbys defined? Waiting backends can't reload the
* config file safely, so WAL writer updates this value as needed.
* Protected by SyncRepLock.
*/
bool sync_standbys_defined ;
WalSnd walsnds [1]; /* VARIABLE LENGTH ARRAY */
} WalSndCtlData ;
extern WalSndCtlData *WalSndCtl;
typedef enum
{
WALRCV_STOPPED , /* stopped and mustn't start up again */
WALRCV_STARTING , /* launched, but the process hasn't
* initialized yet */
WALRCV_RUNNING , /* walreceiver is running */
WALRCV_STOPPING /* requested to stop, but still running */
} WalRcvState ;
/* Shared memory area for management of walreceiver process */
typedef struct
{
/*
* PID of currently active walreceiver process, its current state and
* start time (actually, the time at which it was requested to be
* started).
*/
pid_t pid ;
WalRcvState walRcvState ;
pg_time_t startTime ;
/*
* receiveStart is the first byte position that will be received. When
* startup process starts the walreceiver , it sets receiveStart to the
* point where it wants the streaming to begin.
*/
XLogRecPtr receiveStart ;
/*
* receivedUpto-1 is the last byte position that has already been
* received. At the first startup of walreceiver , receivedUpto is set to
* receiveStart. After that, walreceiver updates this whenever it flushes
* the received WAL to disk.
*/
XLogRecPtr receivedUpto ;
/*
* latestChunkStart is the starting byte position of the current "batch"
* of received WAL. It's actually the same as the previous value of
* receivedUpto before the last flush to disk. Startup process can use
* this to detect whether it's keeping up or not.
*/
XLogRecPtr latestChunkStart ;
/*
* connection string; is used for walreceiver to connect with the primary.
*/
char conninfo [MAXCONNINFO];
slock_t mutex ; /* locks shared variables shown above */
} WalRcvData ;
extern WalRcvData *WalRcv;
初始化完 Wal Receiver Ctl 和 Wal Receiver Ctl 相关结构 的共享内存结构图
为了精简上图，把创建 shmem 的哈希表索引 "ShmemIndex" 时创建的 HCTL 结构删掉了，这个结构的作用是记录创建可扩展哈希表的相关信息，不过这个结构在 "ShmemIndex" 创建完成后也会由于出了对象作用域而消失。增加了左边灰色底的部分，描述 共享内存 /shmem 里各变量物理布局概览，由下往上，由低地址到高地址。 图中黄色的索引项就是本节新增加的索引项。
Wal Sender Ctl 和 Wal Receiver Ctl 相关结构图