本节介绍了PostgreSQL获取事务快照的主实现逻辑,相应的实现函数是GetTransactionSnapshot。
全局/静态变量
/*
* Currently registered Snapshots. Ordered in a heap by xmin, so that we can
* quickly find the one with lowest xmin, to advance our MyPgXact->xmin.
* 当前已注册的快照.
* 按照xmin堆排序，这样我们可以快速找到xmin最小的一个，从而可以设置MyPgXact->xmin。
*/
static int xmin_cmp(const pairingheap_node *a, const pairingheap_node *b,
void *arg);
static pairingheap RegisteredSnapshots = {&xmin_cmp, NULL, NULL};
/* first GetTransactionSnapshot call in a transaction? */
bool FirstSnapshotSet = false;
/*
* Remember the serializable transaction snapshot, if any. We cannot trust
* FirstSnapshotSet in combination with IsolationUsesXactSnapshot(), because
* GUC may be reset before us, changing the value of IsolationUsesXactSnapshot.
* 如存在则记下serializable事务快照.
* 我们不能信任与IsolationUsesXactSnapshot()结合使用的FirstSnapshotSet，
* 因为GUC可能会在我们之前重置，改变IsolationUsesXactSnapshot的值。
*/
static Snapshot FirstXactSnapshot = NULL;
/*
* CurrentSnapshot points to the only snapshot taken in transaction-snapshot
* mode, and to the latest one taken in a read-committed transaction.
* SecondarySnapshot is a snapshot that's always up-to-date as of the current
* instant, even in transaction-snapshot mode. It should only be used for
* special-purpose code (say, RI checking.) CatalogSnapshot points to an
* MVCC snapshot intended to be used for catalog scans; we must invalidate it
* whenever a system catalog change occurs.
* CurrentSnapshot指向在transaction-snapshot模式下获取的唯一快照/在read-committed事务中获取的最新快照。
* SecondarySnapshot是即使在transaction-snapshot模式下，也总是最新的快照。它应该只用于特殊用途码(例如，RI检查)。
* CatalogSnapshot指向打算用于catalog扫描的MVCC快照;
* 无论何时发生system catalog更改，我们都必须马上使其失效。
*
* These SnapshotData structs are static to simplify memory allocation
* (see the hack in GetSnapshotData to avoid repeated malloc/free).
* 这些SnapshotData结构体是静态的便于简化内存分配.
* (可以回过头来看GetSnapshotData函数如何避免重复的malloc/free)
*/
static SnapshotData CurrentSnapshotData = {HeapTupleSatisfiesMVCC};
static SnapshotData SecondarySnapshotData = {HeapTupleSatisfiesMVCC};
SnapshotData CatalogSnapshotData = {HeapTupleSatisfiesMVCC};
/* Pointers to valid snapshots */
//指向有效的快照
static Snapshot CurrentSnapshot = NULL;
static Snapshot SecondarySnapshot = NULL;
static Snapshot CatalogSnapshot = NULL;
static Snapshot HistoricSnapshot = NULL;
/*
* These are updated by GetSnapshotData. We initialize them this way
* for the convenience of TransactionIdIsInProgress: even in bootstrap
* mode, we don't want it to say that BootstrapTransactionId is in progress.
* 这些变量通过函数GetSnapshotData更新.
* 为了便于TransactionIdIsInProgress，以这种方式初始化它们:
* 即使在引导模式下，我们也不希望表示BootstrapTransactionId正在进行中。
*
* RecentGlobalXmin and RecentGlobalDataXmin are initialized to
* InvalidTransactionId, to ensure that no one tries to use a stale
* value. Readers should ensure that it has been set to something else
* before using it.
* RecentGlobalXmin和RecentGlobalDataXmin初始化为InvalidTransactionId,
* 以确保没有人尝试使用过时的值。
* 在使用它之前，读取进程应确保它已经被设置为其他值。
*/
TransactionId TransactionXmin = FirstNormalTransactionId;
TransactionId RecentXmin = FirstNormalTransactionId;
TransactionId RecentGlobalXmin = InvalidTransactionId;
TransactionId RecentGlobalDataXmin = InvalidTransactionId;
/* (table, ctid) => (cmin, cmax) mapping during timetravel */
static HTAB *tuplecid_data = NULL;
MyPgXact
当前的事务信息.
/*
* Flags for PGXACT->vacuumFlags
* PGXACT->vacuumFlags标记
*
* Note: If you modify these flags, you need to modify PROCARRAY_XXX flags
* in src/include/storage/procarray.h.
* 注意:如果修改了这些标记,需要更新src/include/storage/procarray.h中的PROCARRAY_XXX标记
*
* PROC_RESERVED may later be assigned for use in vacuumFlags, but its value is
* used for PROCARRAY_SLOTS_XMIN in procarray.h, so GetOldestXmin won't be able
* to match and ignore processes with this flag set.
* PROC_RESERVED可能在接下来分配给vacuumFlags使用,
* 但是它在procarray.h中用于标识PROCARRAY_SLOTS_XMIN,
* 因此GetOldestXmin不能匹配和忽略使用此标记的进程.
*/
//是否auto vacuum worker?
#define PROC_IS_AUTOVACUUM 0x01 /* is it an autovac worker? */
//正在运行lazy vacuum
#define PROC_IN_VACUUM 0x02 /* currently running lazy vacuum */
//正在运行analyze
#define PROC_IN_ANALYZE 0x04 /* currently running analyze */
//只能通过auto vacuum设置
#define PROC_VACUUM_FOR_WRAPAROUND 0x08 /* set by autovac only */
//在事务外部正在执行逻辑解码
#define PROC_IN_LOGICAL_DECODING 0x10 /* currently doing logical
* decoding outside xact */
//保留用于procarray
#define PROC_RESERVED 0x20 /* reserved for procarray */
/* flags reset at EOXact */
//在EOXact时用于重置标记的MASK
#define PROC_VACUUM_STATE_MASK \
(PROC_IN_VACUUM | PROC_IN_ANALYZE | PROC_VACUUM_FOR_WRAPAROUND)
/*
* Prior to PostgreSQL 9.2, the fields below were stored as part of the
* PGPROC. However, benchmarking revealed that packing these particular
* members into a separate array as tightly as possible sped up GetSnapshotData
* considerably on systems with many CPU cores, by reducing the number of
* cache lines needing to be fetched. Thus, think very carefully before adding
* anything else here.
*/
typedef struct PGXACT
{
//当前的顶层事务ID(非子事务)
//出于优化的目的,只读事务并不会分配事务号(xid = 0)
TransactionId xid; /* id of top-level transaction currently being
* executed by this proc, if running and XID
* is assigned; else InvalidTransactionId */
//在启动事务时,当前正在执行的最小事务号XID,但不包括LAZY VACUUM
//vacuum不能清除删除事务号xid >= xmin的元组
TransactionId xmin; /* minimal running XID as it was when we were
* starting our xact, excluding LAZY VACUUM:
* vacuum must not remove tuples deleted by
* xid >= xmin ! */
//vacuum相关的标记
uint8 vacuumFlags; /* vacuum-related flags, see above */
bool overflowed;
bool delayChkpt; /* true if this proc delays checkpoint start;
* previously called InCommit */
uint8 nxids;
} PGXACT;
extern PGDLLIMPORT struct PGXACT *MyPgXact;
Snapshot
SnapshotData结构体指针,SnapshotData结构体可表达的信息囊括了所有可能的快照.
有以下几种不同类型的快照:
1.常规的MVCC快照
2.在恢复期间的MVCC快照(处于Hot-Standby模式)
3.在逻辑解码过程中使用的历史MVCC快照
4.作为参数传递给HeapTupleSatisfiesDirty()函数的快照
5.作为参数传递给HeapTupleSatisfiesNonVacuumable()函数的快照
6.用于在没有成员访问情况下SatisfiesAny、Toast和Self的快照
//SnapshotData结构体指针
typedef struct SnapshotData *Snapshot;
//无效的快照
#define InvalidSnapshot ((Snapshot) NULL)
/*
* We use SnapshotData structures to represent both "regular" (MVCC)
* snapshots and "special" snapshots that have non-MVCC semantics.
* The specific semantics of a snapshot are encoded by the "satisfies"
* function.
* 我们使用SnapshotData结构体表示"regular" (MVCC) snapshots和具有非MVCC语义的"special" snapshots。
*/
//测试函数
typedef bool (*SnapshotSatisfiesFunc) (HeapTuple htup,
Snapshot snapshot, Buffer buffer);
//常见的有:
//HeapTupleSatisfiesMVCC：判断元组对某一快照版本是否有效
//HeapTupleSatisfiesUpdate：判断元组是否可更新(同时更新同一个元组)
//HeapTupleSatisfiesDirty：判断当前元组是否存在脏数据
//HeapTupleSatisfiesSelf：判断tuple对自身信息是否有效
//HeapTupleSatisfiesToast：判断是否TOAST表
//HeapTupleSatisfiesVacuum：判断元组是否能被VACUUM删除
//HeapTupleSatisfiesAny：所有元组都可见
//HeapTupleSatisfiesHistoricMVCC：用于CATALOG 表
/*
* Struct representing all kind of possible snapshots.
* 该结构体可表达的信息囊括了所有可能的快照.
*
* There are several different kinds of snapshots:
* * Normal MVCC snapshots
* * MVCC snapshots taken during recovery (in Hot-Standby mode)
* * Historic MVCC snapshots used during logical decoding
* * snapshots passed to HeapTupleSatisfiesDirty()
* * snapshots passed to HeapTupleSatisfiesNonVacuumable()
* * snapshots used for SatisfiesAny, Toast, Self where no members are
* accessed.
* 有以下几种不同类型的快照:
* * 常规的MVCC快照
* * 在恢复期间的MVCC快照(处于Hot-Standby模式)
* * 在逻辑解码过程中使用的历史MVCC快照
* * 作为参数传递给HeapTupleSatisfiesDirty()函数的快照
* * 作为参数传递给HeapTupleSatisfiesNonVacuumable()函数的快照
* * 用于在没有成员访问情况下SatisfiesAny、Toast和Self的快照
*
* TODO: It's probably a good idea to split this struct using a NodeTag
* similar to how parser and executor nodes are handled, with one type for
* each different kind of snapshot to avoid overloading the meaning of
* individual fields.
* TODO: 使用类似于parser/executor nodes的处理,使用NodeTag来拆分结构体会是一个好的做法,
* 使用OO(面向对象继承)的方法.
*/
typedef struct SnapshotData
{
//测试tuple是否可见的函数
SnapshotSatisfiesFunc satisfies; /* tuple test function */
/*
* The remaining fields are used only for MVCC snapshots, and are normally
* just zeroes in special snapshots. (But xmin and xmax are used
* specially by HeapTupleSatisfiesDirty, and xmin is used specially by
* HeapTupleSatisfiesNonVacuumable.)
* 余下的字段仅用于MVCC快照，在特殊快照中通常为0。
* (xmin和xmax可用于HeapTupleSatisfiesDirty，xmin可用于HeapTupleSatisfiesNonVacuumable)
*
* An MVCC snapshot can never see the effects of XIDs >= xmax. It can see
* the effects of all older XIDs except those listed in the snapshot. xmin
* is stored as an optimization to avoid needing to search the XID arrays
* for most tuples.
* XIDs >= xmax的事务,对该快照是不可见的(没有任何影响).
* 对该快照可见的是小于xmax,但不在snapshot列表中的XIDs.
* 记录xmin是出于优化的目的,避免为大多数tuples搜索XID数组.
*/
//XID ∈ [2,min)是可见的
TransactionId xmin; /* all XID < xmin are visible to me */
//XID ∈ [xmax,∞)是不可见的
TransactionId xmax; /* all XID >= xmax are invisible to me */
/*
* For normal MVCC snapshot this contains the all xact IDs that are in
* progress, unless the snapshot was taken during recovery in which case
* it's empty. For historic MVCC snapshots, the meaning is inverted, i.e.
* it contains *committed* transactions between xmin and xmax.
* 对于普通的MVCC快照,xip存储了所有正在进行中的XIDs,除非在恢复期间产生的快照(这时候数组为空)
* 对于历史MVCC快照,意义相反，即它包含xmin和xmax之间的*已提交*事务。
*
* note: all ids in xip[] satisfy xmin <= xip[i] < xmax
* 注意: 所有在xip数组中的XIDs满足xmin <= xip[i] < xmax
*/
TransactionId *xip;
//xip数组中的元素个数
uint32 xcnt; /* # of xact ids in xip[] */
/*
* For non-historic MVCC snapshots, this contains subxact IDs that are in
* progress (and other transactions that are in progress if taken during
* recovery). For historic snapshot it contains *all* xids assigned to the
* replayed transaction, including the toplevel xid.
* 对于非历史MVCC快照,下面这些域含有活动的subxact IDs.
* (以及在恢复过程中状态为进行中的事务).
* 对于历史MVCC快照,这些域字段含有*所有*用于回放事务的快照,包括顶层事务XIDs.
*
* note: all ids in subxip[] are >= xmin, but we don't bother filtering
* out any that are >= xmax
* 注意:sbuxip数组中的元素均≥ xmin,但我们不需要过滤掉任何>= xmax的项
*/
TransactionId *subxip;
//subxip数组元素个数
int32 subxcnt; /* # of xact ids in subxip[] */
//是否溢出?
bool suboverflowed; /* has the subxip array overflowed? */
//在Recovery期间的快照?
bool takenDuringRecovery; /* recovery-shaped snapshot? */
//如为静态快照,则该值为F
bool copied; /* false if it's a static snapshot */
//在自身的事务中,CID < curcid是可见的
CommandId curcid; /* in my xact, CID < curcid are visible */
/*
* An extra return value for HeapTupleSatisfiesDirty, not used in MVCC
* snapshots.
* HeapTupleSatisfiesDirty返回的值,在MVCC快照中无用
*/
uint32 speculativeToken;
/*
* Book-keeping information, used by the snapshot manager
* 用于快照管理器的Book-keeping信息
*/
//在ActiveSnapshot栈中的引用计数
uint32 active_count; /* refcount on ActiveSnapshot stack */
//在RegisteredSnapshots中的引用计数
uint32 regd_count; /* refcount on RegisteredSnapshots */
//RegisteredSnapshots堆中的链接
pairingheap_node ph_node; /* link in the RegisteredSnapshots heap */
//快照"拍摄"时间戳
TimestampTz whenTaken; /* timestamp when snapshot was taken */
//拍照时WAL stream中的位置
XLogRecPtr lsn; /* position in the WAL stream when taken */
} SnapshotData;
GetTransactionSnapshot函数在事务处理中为新查询获得相应的快照.
/*
* GetTransactionSnapshot
* Get the appropriate snapshot for a new query in a transaction.
* 在事务处理中为新查询获得相应的快照
*
* Note that the return value may point at static storage that will be modified
* by future calls and by CommandCounterIncrement(). Callers should call
* RegisterSnapshot or PushActiveSnapshot on the returned snap if it is to be
* used very long.
* 注意返回值可能会指向将来调用和CommandCounterIncrement()函数修改的静态存储区.
* 如需要长时间保持快照,调用者需要调用RegisterSnapshot或者PushActiveSnapshot函数记录快照信息.
*/
Snapshot
GetTransactionSnapshot(void)
{
/*
* Return historic snapshot if doing logical decoding. We'll never need a
* non-historic transaction snapshot in this (sub-)transaction, so there's
* no need to be careful to set one up for later calls to
* GetTransactionSnapshot().
* 如执行逻辑解码,则返回历史快照.
* 在该事务中,我们不需要非历史快照,因此不需要为后续的GetTransactionSnapshot()调用小心配置
*/
if (HistoricSnapshotActive())
{
Assert(!FirstSnapshotSet);
return HistoricSnapshot;
}
/* First call in transaction? */
//首次调用?
if (!FirstSnapshotSet)
{
/*
* Don't allow catalog snapshot to be older than xact snapshot. Must
* do this first to allow the empty-heap Assert to succeed.
* 不允许catalog快照比事务快照更旧.
* 必须首次执行该函数以确保empty-heap验证是成功的.
*/
InvalidateCatalogSnapshot();
Assert(pairingheap_is_empty(&RegisteredSnapshots));
Assert(FirstXactSnapshot == NULL);
if (IsInParallelMode())
elog(ERROR,
"cannot take query snapshot during a parallel operation");
/*
* In transaction-snapshot mode, the first snapshot must live until
* end of xact regardless of what the caller does with it, so we must
* make a copy of it rather than returning CurrentSnapshotData
* directly. Furthermore, if we're running in serializable mode,
* predicate.c needs to wrap the snapshot fetch in its own processing.
* 在transaction-snapshot模式下,无论调用者对它做什么，第一个快照必须一直存在到xact事务结束，
* 因此我们必须复制它，而不是直接返回CurrentSnapshotData。
*/
if (IsolationUsesXactSnapshot())
{
//transaction-snapshot模式
/* First, create the snapshot in CurrentSnapshotData */
//首先,在CurrentSnapshotData中创建快照
if (IsolationIsSerializable())
//隔离级别 = Serializable
CurrentSnapshot = GetSerializableTransactionSnapshot(&CurrentSnapshotData);
else
//其他隔离级别
CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);
/* Make a saved copy */
//拷贝快照
CurrentSnapshot = CopySnapshot(CurrentSnapshot);
FirstXactSnapshot = CurrentSnapshot;
/* Mark it as "registered" in FirstXactSnapshot */
//在FirstXactSnapshot中标记该快照已注册
FirstXactSnapshot->regd_count++;
pairingheap_add(&RegisteredSnapshots, &FirstXactSnapshot->ph_node);
}
else
//非transaction-snapshot模式,直接获取
CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);
//设置标记
FirstSnapshotSet = true;
return CurrentSnapshot;
}
//transaction-snapshot模式
if (IsolationUsesXactSnapshot())
return CurrentSnapshot;
/* Don't allow catalog snapshot to be older than xact snapshot. */
//不允许catalog快照比事务快照旧
InvalidateCatalogSnapshot();
//获取快照
CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);
//返回
return CurrentSnapshot;
}
执行简单查询,可触发获取快照逻辑.
16:35:08 (xdb@[local]:5432)testdb=# begin;
BEGIN
16:35:13 (xdb@[local]:5432)testdb=#* select 1;
启动gdb,设置断点
(gdb) b GetTransactionSnapshot
Breakpoint 1 at 0xa9492e: file snapmgr.c, line 312.
(gdb) c
Continuing.
Breakpoint 1, GetTransactionSnapshot () at snapmgr.c:312
312 if (HistoricSnapshotActive())
(gdb)
如执行逻辑解码,则返回历史快照(本例不是).
(gdb) n
319 if (!FirstSnapshotSet)
(gdb)
首次调用?是,进入相应的逻辑
319 if (!FirstSnapshotSet)
(gdb) n
325 InvalidateCatalogSnapshot();
(gdb)
327 Assert(pairingheap_is_empty(&RegisteredSnapshots));
(gdb)
328 Assert(FirstXactSnapshot == NULL);
(gdb) n
330 if (IsInParallelMode())
(gdb)
非transaction-snapshot模式,直接调用GetSnapshotData获取
(gdb)
341 if (IsolationUsesXactSnapshot())
(gdb)
356 CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);
(gdb) p CurrentSnapshotData
$1 = {satisfies = 0xa9310d <HeapTupleSatisfiesMVCC>, xmin = 2342, xmax = 2350, xip = 0x14bee40, xcnt = 2,
subxip = 0x1514fa0, subxcnt = 0, suboverflowed = false, takenDuringRecovery = false, copied = false, curcid = 0,
speculativeToken = 0, active_count = 0, regd_count = 0, ph_node = {first_child = 0x0, next_sibling = 0x0,
prev_or_parent = 0x0}, whenTaken = 0, lsn = 0}
(gdb)
函数执行成功,查看CurrentSnapshot
注:2342事务所在的进程已被kill
(gdb) n
358
(gdb) p CurrentSnapshot
$2 = (Snapshot) 0xf9be60 <CurrentSnapshotData>
(gdb) p *CurrentSnapshot
$3 = {satisfies = 0xa9310d <HeapTupleSatisfiesMVCC>, xmin = 2350, xmax = 2350, xip = 0x14bee40, xcnt = 0,
subxip = 0x1514fa0, subxcnt = 0, suboverflowed = false, takenDuringRecovery = false, copied = false, curcid = 0,
speculativeToken = 0, active_count = 0, regd_count = 0, ph_node = {first_child = 0x0, next_sibling = 0x0,
prev_or_parent = 0x0}, whenTaken = 0, lsn = 0}
(gdb)
执行成功
(gdb) n
359 return CurrentSnapshot;
(gdb)
371 }
(gdb)
exec_simple_query (query_string=0x149aec8 "select 1;") at postgres.c:1059
1059 snapshot_set = true;
(gdb)
查看全局变量MyPgXact
(gdb) p MyPgXact
$7 = (struct PGXACT *) 0x7f47103c01f4
(gdb) p *MyPgXact
$8 = {xid = 0, xmin = 2350, vacuumFlags = 0 '\000', overflowed = false, delayChkpt = false, nxids = 0 '\000'}
(gdb)
注意:
1.xid = 0,表示未分配事务号.出于优化的理由,PG在修改数据时才会分配事务号.
2.txid_current()函数会分配事务号;txid_current_if_assigned()函数不会.
DONE!
遗留问题:
1.CurrentSnapshotData全局变量中的信息何时初始化/更改?
2.GetSnapshotData函数的实现(下节介绍).
PG Source Code
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