PostgreSQL数据库事务系统——Push\Pop快照

迈不过友情╰ 2022-09-11 11:25 15阅读 0赞

快照使用快照链栈来保存获取的多个快照。栈的元素是ActiveSnapshotElt结构体，ActiveSnapshot指针指向栈顶，OldestActiveSnapshot指向栈底。

/* Elements of the active snapshot stack. * Each element here accounts for exactly one active_count on SnapshotData. * NB: the code assumes that elements in this list are in non-increasing * order of as_level; also, the list must be NULL-terminated. */
    typedef struct ActiveSnapshotElt { 
    	Snapshot	as_snap;
    	int			as_level;
    	struct ActiveSnapshotElt *as_next;
    } ActiveSnapshotElt;
    /* Top of the stack of active snapshots */
    static ActiveSnapshotElt *ActiveSnapshot = NULL;
    /* Bottom of the stack of active snapshots */
    static ActiveSnapshotElt *OldestActiveSnapshot = NULL;

## PushActiveSnapshot ##

PushActiveSnapshot第一步先申请ActiveSnapshotElt内存，第二步判定形参snap是否是CurrentSnapshot或者SecondarySnapshot，或者没有拷贝过，则需要将该快照拷贝到ActiveSnapshotElt的as\_snap快照成员，否则直接将snap赋值给ActiveSnapshotElt的as\_snap快照成员。

/* PushActiveSnapshot Set the given snapshot as the current active snapshot * If the passed snapshot is a statically-allocated one, or it is possibly * subject to a future command counter update, create a new long-lived copy * with active refcount=1. Otherwise, only increment the refcount. */
    void PushActiveSnapshot(Snapshot snap) { 
    	ActiveSnapshotElt *newactive;
    	Assert(snap != InvalidSnapshot);
    	newactive = MemoryContextAlloc(TopTransactionContext, sizeof(ActiveSnapshotElt));
    	/* Checking SecondarySnapshot is probably useless here, but it seems better to be sure. */
    	if (snap == CurrentSnapshot || snap == SecondarySnapshot || !snap->copied)
    		newactive->as_snap = CopySnapshot(snap);
    	else
    		newactive->as_snap = snap;
    
    	newactive->as_next = ActiveSnapshot;
    	newactive->as_level = GetCurrentTransactionNestLevel();
    	newactive->as_snap->active_count++;
    
    	ActiveSnapshot = newactive;
    	if (OldestActiveSnapshot == NULL)
    		OldestActiveSnapshot = ActiveSnapshot;
    }

第三步将ActiveSnapshotElt的as\_next指向当前栈顶，ActiveSnapshotElt作为当前快照的栈顶，为as\_level赋值为GetCurrentTransactionNestLevel()，将快照的active\_count自增。（If the passed snapshot is a statically-allocated one, or it is possibly subject to a future command counter update, create a new long-lived copy with active refcount=1. Otherwise, only increment the refcount.）  
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## PopActiveSnapshot ##

从快照链栈中删除栈顶的元素，并减少快照引用计数，如果引用计数为零，需要释放该栈顶快照。

/* PopActiveSnapshot * Remove the topmost snapshot from the active snapshot stack, decrementing the * reference count, and free it if this was the last reference. */
    void PopActiveSnapshot(void) { 
    	ActiveSnapshotElt *newstack;
    	newstack = ActiveSnapshot->as_next;
    	Assert(ActiveSnapshot->as_snap->active_count > 0);
    	ActiveSnapshot->as_snap->active_count--;
    	if (ActiveSnapshot->as_snap->active_count == 0 && ActiveSnapshot->as_snap->regd_count == 0)
    		FreeSnapshot(ActiveSnapshot->as_snap);
    	pfree(ActiveSnapshot);
    	ActiveSnapshot = newstack;
    	if (ActiveSnapshot == NULL) OldestActiveSnapshot = NULL;
    	SnapshotResetXmin();
    }

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