txn.h

struct __db_txnregion

SH_TAILQ_HEAD(__active) active_txn;
SH_TAILQ_HEAD(__mvcc) mvcc_txn;

txn.c,

__txn_begin_int(),txn 加入的 buffer version 不可以被看见.

    MAX_LSN(td->visible_lsn);

__txn_commit(txn,flags),commit时,td->visible_lsn 被设置,意味着 从此时起,此txn 加入的 buffer version 可以被看见了.

...
if (DBENV_LOGGING(env) && (!IS_ZERO_LSN(td->last_lsn) ||
    STAILQ_FIRST(&txn->logs) != NULL)) {
    if (txn->parent == NULL) {
        ...
        if (ret == 0 && !IS_ZERO_LSN(td->last_lsn)) {
            ret = __txn_flush_fe_files(txn);
            if (ret == 0)
                ret = __txn_regop_log(env,txn,&td->visible_lsn,LOG_FLAGS(txn),TXN_COMMIT,(int32_t)time(NULL),id,request.obj);

txn_end(): trans从active queue里移除,如果其向 cache 加入了 mvcc page buffer,则放入mvcc queue中.

TXN_SYSTEM_LOCK(env);
td->status = is_commit ? TXN_COMMITTED : TXN_ABORTED;
SH_TAILQ_REMOVE(&region->active_txn,td,links,__txn_detail);
...
if (txn->parent != NULL) {
    ptd = txn->parent->td;
    SH_TAILQ_REMOVE(&ptd->kids,klinks,__txn_detail);
} else if ((mvcc_mtx = td->mvcc_mtx) != MUTEX_INVALID) {
    MUTEX_LOCK(env,mvcc_mtx);
    if (td->mvcc_ref != 0) {
        SH_TAILQ_INSERT_HEAD(&region->mvcc_txn,__txn_detail);
            

txn_region.c

__txn_remove_buffer(),若 mvcc txn 创建的buffer 全部被移除,则此txn可以free

MUTEX_LOCK(env,td->mvcc_mtx);
DB_ASSERT(env,td->mvcc_ref > 0);
/*

We free the transaction detail here only if this is the last
reference and td is on the list of committed snapshot transactions
with active pages.

*/

need_free = (--td->mvcc_ref == 0) && F_ISSET(td,TXN_DTL_SNAPSHOT);

MUTEX_UNLOCK(env,td->mvcc_mtx);

if (need_free) {

MUTEX_UNLOCK(env,hash_mtx);
ret = __mutex_free(env,&td->mvcc_mtx);
td->mvcc_mtx = MUTEX_INVALID;

TXN_SYSTEM_LOCK(env);
SH_TAILQ_REMOVE(&region->mvcc_txn,__txn_detail);
STAT_DEC(env,nsnapshot,region->stat.st_nsnapshot,td->txnid);
__env_alloc_free(&mgr->reginfo,td);
TXN_SYSTEM_UNLOCK(env);

MUTEX_READLOCK(env,hash_mtx);
}
return (ret);

}

（编辑：李大同）

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