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Re: (ITS#7970) LMDB: Critical Heisenbug - Inconsistent reading & SIGSEGV due to the race condition.

Committed to mdb.master

> commit 6028662665c09f4d34b751bfd9b09223f6cb2433
> Author: Leo Yuriev <leo@yuriev.ru>
> Date:   2014-10-17 23:29:56 +0400
>
>      ITS#7970 for LMDB: the Heisenbug.
>
> diff --git a/libraries/liblmdb/mdb.c b/libraries/liblmdb/mdb.c
> index 3286ffb..83a15c1 100644
> --- a/libraries/liblmdb/mdb.c
> +++ b/libraries/liblmdb/mdb.c
> @@ -2522,7 +2522,12 @@ mdb_txn_renew0(MDB_txn *txn)
>    return rc;
>    }
>    }
> - txn->mt_txnid = r->mr_txnid = ti->mti_txnid;
> +
> + do /* LY: Retry on a race, ITS#7970. */
> + r->mr_txnid = ti->mti_txnid;
> + while(r->mr_txnid != ti->mti_txnid);
> +
> + txn->mt_txnid = r->mr_txnid;
>    txn->mt_u.reader = r;
>    meta = env->me_metas[txn->mt_txnid & 1];
>    }
>
>
>
>


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