PostgreSQL启动过程中的那些事七：初始化共享内存和信号六：shmem中初始化twophase
初始化twophase方法调用流程图
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话说main()->…->PostmasterMain()->…->reset_shared() -> CreateSharedMemoryAndSemaphores()>…->TwoPhaseShmemInit()，初始化支持分布式事务的两阶段提交/TwoPhase相关数据结构TwoPhaseState等，用作内存里管理和缓存两阶段提交/TwoPhase（对应于存放在"data/ pg_twophase"文件夹里的文件）。
TwoPhaseShmemInit()->ShmemInitStruct()，在其中调用hash_search()在哈希表索引"ShmemIndex"中查找"Prepared Transaction Table"，如果没有，就在shmemIndex中给"Prepared Transaction Table"分一个HashElement和ShmemIndexEnt（entry），在其中的Entry中写上"Prepared Transaction Table"。返回ShmemInitStruct()，再调用ShmemAlloc()在共享内存上给"Prepared Transaction Table"相关结构（见下面“TwoPhase相关结构图”）分配空间，设置entry（在这儿即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ShmemInitStruct()，让TwoPhaseStateData *类型静态全局变量TwoPhaseState指向TwoPhaseStateData结构，TwoPhaseStateData的起始地址就是在shmem里给"Prepared Transaction Table"相关结构分配的内存起始地址，设置其中TwoPhaseStateData结构类型的成员值。
相关变量、结构定义和初始化完成后数据结构图在下面。
typedef struct TwoPhaseStateData
{
??? /* Head of linked list of free GlobalTransactionDatastructs */
??? GlobalTransactionfreeGXacts;
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??? /* Number of valid prepXacts entries. */
??? int???????? numPrepXacts;
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??? /*
??? ?* There are max_prepared_xacts items in thisarray, but C wants a
??? ?* fixed-size array.
??? ?*/
??? GlobalTransactionprepXacts[1];???? /*VARIABLE LENGTH ARRAY */
} TwoPhaseStateData;??????????? /* VARIABLE LENGTH STRUCT */
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static TwoPhaseStateData*TwoPhaseState;
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/*
?*GlobalTransactionData is defined in twophase.c; other places have no
?* business knowingthe internal definition.
?*/
typedef structGlobalTransactionData *GlobalTransaction;
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typedef struct GlobalTransactionData
{
??? PGPROC????? proc;?????????? /* dummy proc */
??? TimestampTzprepared_at;??? /*time of preparation */
??? XLogRecPtr? prepare_lsn;??? /* XLOG offset of prepare record */
??? Oid???????? owner;????????? /* ID of user that executed the xact */
??? TransactionIdlocking_xid;? /*top-level XID of backend working on xact */
??? bool??????? valid;????????? /* TRUE iffully prepared */
??? char??????? gid[GIDSIZE];?? /* The GID assignedto the prepared xact */
} GlobalTransactionData;
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struct PGPROC
{
??? /* proc->linksMUST BE FIRST IN STRUCT (see ProcSleep,ProcWakeup,etc) */
??? SHM_QUEUE?? links;????????? /*list link if process is in a list */
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??? PGSemaphoreData sem;??????? /* ONE semaphore to sleep on */
??? int???????? waitStatus;???? /* STATUS_WAITING, STATUS_OK or STATUS_ERROR */
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??? LocalTransactionIdlxid;??? /* local id of top-leveltransaction currently
??????????????????????????????? ?* being executed by this proc, if running;
??????????????????????????????? ?* else InvalidLocalTransactionId */
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??? TransactionId xid;????????? /* id of top-level transactioncurrently being
??????????????????????????????? ?* executed by this proc, if running and XID
??????????????????????????????? ?* is assigned; else InvalidTransactionId */
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??? TransactionId xmin;???????? /* minimal running XID as it was whenwe were
??????????????????????????????? ?* starting our xact, excluding LAZY VACUUM:
??????????????????????????????? ?* vacuum must not remove tuples deleted by
??????????????????????????????? ?* xid >= xmin ! */
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??? int???????? pid;??????????? /*Backend's process ID; 0 if prepared xact */
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??? /* These fields arezero while a backend is still starting up: */
??? BackendId?? backendId;????? /*This backend's backend ID (if assigned) */
??? Oid???????? databaseId;???? /* OID of database this backend is using */
??? Oid???????? roleId;???????? /*OID of role using this backend */
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??? bool??????? inCommit;?????? /* true if within commit critical section */
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??? uint8?????? vacuumFlags;??? /* vacuum-related flags, see above */
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??? /*
??? ?* While in hot standby mode, shows that aconflict signal has been sent
??? ?* for the current transaction. Set/clearedwhile holding ProcArrayLock,
??? ?* though not required. Accessed without lock,if needed.
??? ?*/
??? bool??????? recoveryConflictPending;
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??? /* Info about LWLockthe process is currently waiting for, if any. */
??? bool??????? lwWaiting;????? /* true if waiting for an LW lock */
??? bool??????? lwExclusive;??? /* true if waiting for exclusive access */
??? struct PGPROC*lwWaitLink;? /* next waiter for same LWlock */
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??? /* Info about lockthe process is currently waiting for, if any. */
??? /* waitLock andwaitProcLock are NULL if not currently waiting. */
??? LOCK??? ??*waitLock;?????? /* Lock objectwe're sleeping on ... */
??? PROCLOCK?? *waitProcLock;?? /* Per-holder info for awaited lock */
??? LOCKMODE??? waitLockMode;?? /* type of lock we're waiting for */
??? LOCKMASK??? heldLocks;????? /*bitmask for lock types already held on this
??????????????????????????????? ?* lock object by this backend */
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??? Latch?????? procLatch;????? /* generic latch for process */
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??? /*
??? ?* Info to allow us to wait for synchronousreplication, if needed.
??? ?* waitLSN is InvalidXLogRecPtr if not waiting;set only by user backend.
??? ?* syncRepState must not be touched except byowning process or WALSender.
??? ?* syncRepLinks used only while holdingSyncRepLock.
??? ?*/
??? XLogRecPtr? waitLSN;??????? /*waiting for this LSN or higher */
??? int ??????? syncRepState;?? /* wait state for sync rep */
??? SHM_QUEUE?? syncRepLinks;?? /*list link if process is in syncrep queue */
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??? /*
??? ?* All PROCLOCK objects for locks held orawaited by this backend are
??? ?* linked into one of these lists, according tothe partition number of
??? ?* their lock.
??? ?*/
??? SHM_QUEUE?? myProcLocks[NUM_LOCK_PARTITIONS];
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??? struct XidCachesubxids;??? /* cache for subtransactionXIDs */
};
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/* shmqueue.c */
typedef struct SHM_QUEUE
{
??? struct SHM_QUEUE *prev;
??? struct SHM_QUEUE *next;
} SHM_QUEUE;
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关于上面的PGPROC结构这里再说一下。
每一个后台进程（backend）在共享内存里有一个PGPROC结构。还有一个当前未使用的PGPROC结构列表可以从中给新的backend分配。
Links：任何PGPROC在列表里。当等待锁时，PGPROC被链接到锁的等待进程队列里（lock's waitProcs queue）。一个回收的PGPROC被链接到ProcGlobal的freeProcs列表里。
pg的两阶段提交事务管理器还为每一个当前prepared事务创建一个假PGPROC结构。这些PGPROC结构出现在ProcArray数据结构以至于/目的是prepared事务显示/看起来仍然在运行且正确的显示其持有锁。通过它的pid等于0这个事实，一个prepared事务PGPROC能够和一个真正的进程在需要的时候能够被区别。在一个prepared事务PGPROC里的信号和lock-activity字段是不用的，但它的myProcLocks[]数组列表是有效的。
下面看看初始化完"Prepared Transaction Table"相关结构后在内存中的结构图
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初始化完twophase相关结构的内存结构图
?????? 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。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。其中的"twophase"相关结构图在下面给出，要不上面的图太大太复杂了。
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twophase相关结构图
