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Re: (ITS#8339) LMDB robust mutex fix
by hyc＠symas.com 02 Jun '16

02 Jun '16

openldap(a)gulag.ch wrote: > Hi Hallvard > > Thanks for the hint. Your are perfectly right, the mutex memory has to > be zeroed in case of a robust mutex. Applied against 0.9.18 and it seems > to work. My proposed patch is broken. I've pushed a tweaked version of Hallvard's patch. While the Studio compiler defines __SunOS_5_10 it appears that gcc doesn't, so we took a different detection route. > > There is a slight inconsistency if subsequent calls after a successfull > pthread_mutexattr_init() fail (in the ORed if statement). This might > result in a memory leak as the mutex attributes are not destroyed. But > this is not related to the topic of this ITS. Feel free to submit another ITS for that, thanks. > > Thank you > > Rolf > > On 06/02/16 07:56 AM, Hallvard Breien Furuseth wrote: >> Rolf, please try this instead, like the manpage you referred to >> says under Notes. Do not catch EBUSY/EINVAL, they do mean the >> mutex was not (re)inited. >> >> diff --git a/libraries/liblmdb/mdb.c b/libraries/liblmdb/mdb.c >> index 0209c09..5ae93bc 100644 >> --- a/libraries/liblmdb/mdb.c >> +++ b/libraries/liblmdb/mdb.c >> @@ -304,7 +304,7 @@ union semun { >> # else >> # define MDB_USE_ROBUST 1 >> /* glibc < 2.12 only provided _np API */ >> -# if defined(__GLIBC__) && GLIBC_VER < 0x02000c >> +# if (defined(__GLIBC__) && GLIBC_VER < 0x02000c) || defined(__SunOS_5_10) >> # define PTHREAD_MUTEX_ROBUST PTHREAD_MUTEX_ROBUST_NP >> # define pthread_mutexattr_setrobust(attr, flag) pthread_mutexattr_setrobust_np(attr, flag) >> # define pthread_mutex_consistent(mutex) pthread_mutex_consistent_np(mutex) >> @@ -4963,6 +4963,13 @@ mdb_env_setup_locks(MDB_env *env, char *lpath, int mode, int *excl) >> #else /* MDB_USE_POSIX_MUTEX: */ >> pthread_mutexattr_t mattr; >> >> + /* Solaris needs this before initing a robust mutex. Otherwise >> + * it may skip the init and return EBUSY "seems someone already >> + * inited" or EINVAL "it was inited differently". >> + */ >> + memset(env->me_txns->mti_rmutex, 0, sizeof(*env->me_txns->mti_rmutex)); >> + memset(env->me_txns->mti_wmutex, 0, sizeof(*env->me_txns->mti_wmutex)); >> + >> if ((rc = pthread_mutexattr_init(&mattr)) >> || (rc = pthread_mutexattr_setpshared(&mattr, PTHREAD_PROCESS_SHARED)) >> #ifdef MDB_ROBUST_SUPPORTED >> > > > > > -- -- Howard Chu CTO, Symas Corp. http://www.symas.com Director, Highland Sun http://highlandsun.com/hyc/ Chief Architect, OpenLDAP http://www.openldap.org/project/

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Re: (ITS#8339) LMDB robust mutex fix
by openldap＠gulag.ch 02 Jun '16

02 Jun '16

Hi Hallvard Thanks for the hint. Your are perfectly right, the mutex memory has to be zeroed in case of a robust mutex. Applied against 0.9.18 and it seems to work. My proposed patch is broken. There is a slight inconsistency if subsequent calls after a successfull pthread_mutexattr_init() fail (in the ORed if statement). This might result in a memory leak as the mutex attributes are not destroyed. But this is not related to the topic of this ITS. Thank you Rolf On 06/02/16 07:56 AM, Hallvard Breien Furuseth wrote: > Rolf, please try this instead, like the manpage you referred to > says under Notes. Do not catch EBUSY/EINVAL, they do mean the > mutex was not (re)inited. > > diff --git a/libraries/liblmdb/mdb.c b/libraries/liblmdb/mdb.c > index 0209c09..5ae93bc 100644 > --- a/libraries/liblmdb/mdb.c > +++ b/libraries/liblmdb/mdb.c > @@ -304,7 +304,7 @@ union semun { > # else > # define MDB_USE_ROBUST 1 > /* glibc < 2.12 only provided _np API */ > -# if defined(__GLIBC__) && GLIBC_VER < 0x02000c > +# if (defined(__GLIBC__) && GLIBC_VER < 0x02000c) || defined(__SunOS_5_10) > # define PTHREAD_MUTEX_ROBUST PTHREAD_MUTEX_ROBUST_NP > # define pthread_mutexattr_setrobust(attr, flag) pthread_mutexattr_setrobust_np(attr, flag) > # define pthread_mutex_consistent(mutex) pthread_mutex_consistent_np(mutex) > @@ -4963,6 +4963,13 @@ mdb_env_setup_locks(MDB_env *env, char *lpath, int mode, int *excl) > #else /* MDB_USE_POSIX_MUTEX: */ > pthread_mutexattr_t mattr; > > + /* Solaris needs this before initing a robust mutex. Otherwise > + * it may skip the init and return EBUSY "seems someone already > + * inited" or EINVAL "it was inited differently". > + */ > + memset(env->me_txns->mti_rmutex, 0, sizeof(*env->me_txns->mti_rmutex)); > + memset(env->me_txns->mti_wmutex, 0, sizeof(*env->me_txns->mti_wmutex)); > + > if ((rc = pthread_mutexattr_init(&mattr)) > || (rc = pthread_mutexattr_setpshared(&mattr, PTHREAD_PROCESS_SHARED)) > #ifdef MDB_ROBUST_SUPPORTED >

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Re: (ITS#8339) LMDB robust mutex fix
by h.b.furuseth＠usit.uio.no 02 Jun '16

02 Jun '16

Rolf, please try this instead, like the manpage you referred to says under Notes. Do not catch EBUSY/EINVAL, they do mean the mutex was not (re)inited. diff --git a/libraries/liblmdb/mdb.c b/libraries/liblmdb/mdb.c index 0209c09..5ae93bc 100644 --- a/libraries/liblmdb/mdb.c +++ b/libraries/liblmdb/mdb.c @@ -304,7 +304,7 @@ union semun { # else # define MDB_USE_ROBUST 1 /* glibc < 2.12 only provided _np API */ -# if defined(__GLIBC__) && GLIBC_VER < 0x02000c +# if (defined(__GLIBC__) && GLIBC_VER < 0x02000c) || defined(__SunOS_5_10) # define PTHREAD_MUTEX_ROBUST PTHREAD_MUTEX_ROBUST_NP # define pthread_mutexattr_setrobust(attr, flag) pthread_mutexattr_setrobust_np(attr, flag) # define pthread_mutex_consistent(mutex) pthread_mutex_consistent_np(mutex) @@ -4963,6 +4963,13 @@ mdb_env_setup_locks(MDB_env *env, char *lpath, int mode, int *excl) #else /* MDB_USE_POSIX_MUTEX: */ pthread_mutexattr_t mattr; + /* Solaris needs this before initing a robust mutex. Otherwise + * it may skip the init and return EBUSY "seems someone already + * inited" or EINVAL "it was inited differently". + */ + memset(env->me_txns->mti_rmutex, 0, sizeof(*env->me_txns->mti_rmutex)); + memset(env->me_txns->mti_wmutex, 0, sizeof(*env->me_txns->mti_wmutex)); + if ((rc = pthread_mutexattr_init(&mattr)) || (rc = pthread_mutexattr_setpshared(&mattr, PTHREAD_PROCESS_SHARED)) #ifdef MDB_ROBUST_SUPPORTED

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Re: (ITS#8339) LMDB robust mutex fix
by openldap＠gulag.ch 02 Jun '16

02 Jun '16

On 06/02/16 01:48 AM, Howard Chu wrote: > My last SPARC/Solaris dev machine was decommissioned sometime last year > so I haven't had any system available to test this on. Hi Howard Thanks for the reply. Maybe I can help out and setup two systems for you (accessible via Internet): -Solaris 11.3 Intel with Solaris Studio 12.3/4 -Solaris 10 Intel with Solaris Studio 12.3/4 We already use this patch (applied to LMDB 0.9.18) with OpenLDAP 2.4.44 and as part of our application in production on 11.3/Intel and 10/Intel+Sparc. So I could do some further testing for you on 10/Sparc (Solaris Studio 12.3/4 as well) if required. Does this sound feasible? Thank you Rolf

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Re: (ITS#8339) LMDB robust mutex fix
by hyc＠symas.com 02 Jun '16

02 Jun '16

openldap(a)gulag.ch wrote: > Hi > > I'm fully aware of the fact that Solaris is not the most widely-used > platform on earth... Is there any chance to get this patch evaluated? > I'm glad to help with whatever is needed. My last SPARC/Solaris dev machine was decommissioned sometime last year so I haven't had any system available to test this on. -- -- Howard Chu CTO, Symas Corp. http://www.symas.com Director, Highland Sun http://highlandsun.com/hyc/ Chief Architect, OpenLDAP http://www.openldap.org/project/

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Re: (ITS#8339) LMDB robust mutex fix
by openldap＠gulag.ch 02 Jun '16

02 Jun '16

Hi I'm fully aware of the fact that Solaris is not the most widely-used platform on earth... Is there any chance to get this patch evaluated? I'm glad to help with whatever is needed. Thank you Rolf On 12/20/15 02:54 AM, openldap-its(a)OpenLDAP.org wrote: > *** THIS IS AN AUTOMATICALLY GENERATED REPLY *** > > Thanks for your report to the OpenLDAP Issue Tracking System. Your > report has been assigned the tracking number ITS#8339. > > One of our support engineers will look at your report in due course. > Note that this may take some time because our support engineers > are volunteers. They only work on OpenLDAP when they have spare > time. > > If you need to provide additional information in regards to your > issue report, you may do so by replying to this message. Note that > any mail sent to openldap-its(a)openldap.org with (ITS#8339) > in the subject will automatically be attached to the issue report. > > mailto:openldap-its@openldap.org?subject=(ITS#8339) > > You may follow the progress of this report by loading the following > URL in a web browser: > http://www.OpenLDAP.org/its/index.cgi?findid=8339 > > Please remember to retain your issue tracking number (ITS#8339) > on any further messages you send to us regarding this report. If > you don't then you'll just waste our time and yours because we > won't be able to properly track the report. > > Please note that the Issue Tracking System is not intended to > be used to seek help in the proper use of OpenLDAP Software. > Such requests will be closed. > > OpenLDAP Software is user supported. > http://www.OpenLDAP.org/support/ > > -------------- > Copyright 1998-2007 The OpenLDAP Foundation, All Rights Reserved. >

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Re: (ITS#8431) LMDB: Access newly opened database from another transaction
by h.b.furuseth＠usit.uio.no 30 May '16

30 May '16

On 30. mai 2016 18:08, Juerg Bircher wrote: > If I got it right it is all about protecting against invalid DBIs > which can happen for example if someone closes database "A" which was > assigned DBI "5" and later the database "B" is opened and the free > slot 5 is then assigned to "B". So we could be misled using DBI "5" > having in mind database "A". > > So why don't we extend the MDB_DBI by adding the sequence in the lower > or higher bits. (...) Ugh. I didn't expect all my words to miss that badly. Yes, you caught one issue. There are more. The central point is what I wrote here: >> Multi-threaded use of structures, like the ones behind DBIs, must be >> synchronized. But DBIs and mdb_dbi_open() are not mutex-protected, >> nor are read-only txns (after the per-thread setup). So it's up to >> the user. And LMDB mostly can't catch users at less than txn-sized >> threading bugs - like the DBI workarounds people keep trying. I shouldn't really be the one to answer this, Howard might answer differently if he didn't just fetch a flamethrower. But anyway: It's about keeping a clean model of what is going on, which helps understanding and correctly coding both LMDB itself and user programs. When you don't work against the model, anyway. It's about keeping LMDB's internal data structures consistent. LMDB must know what the heck is going on when it manages and uses DBIs. Things like ensuring that two threads do not run mdb_dbi_open() at the same time, overwriting each others' work and creating a mess. It's about LMDB being able to know it delivers correct results, at least if the user doesn't work too hard at thwarting it. This usually means serializing various parts of the code using locks or something similar. But for the sake of speed and simplicity, LMDB has no relevant locks. It relies on the user to lock and has some code to catch some locking errors. One key word is "tries" - without locks or other expensive code, it can't always catch them even when it does try. So DBI errors are not "this DBI is broken, so drop/refresh it" but "your program is buggy, so fix it". And it's about living with the design choices and tradeoffs which LMDB has made, e.g. not slowing down read-only txns for anything like locking or extensive error checking. So anyway, this ITS is about changes to LMDB which will not happen. Instead it reminded me to add some checks for user errors with DBIs. -- Hallvard

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Re: (ITS#8431) LMDB: Access newly opened database from another transaction
by juerg.bircher＠gmail.com 30 May '16

30 May '16

On Mon, May 30, 2016 at 2:44 PM, Hallvard Breien Furuseth <h.b.furuseth(a)usit.uio.no> wrote: > In ITS#8430, Juerg Bircher wrote: >> >> So if you drop ITS#8431 it would at least be very helpful to get an >> separate error like MDB_INVALID_DPI instead of EINVAL So the >> application sharing and caching handles (dbi) could handle the >> situation appropriate like use mdb_env_open() so that the dbi becomes >> valid in that txn as well. > > > This is ITS#8431 in a new guise, so I'm replying here. We know DBI > handling is inconvenient. We'd have fixed it if it were easy without > adding a bunch of locks. I'll walk through some of it below, I expect > it'll be useful to have for future reference anyway: > > > You mean like this? > > if (mdb_something(txn, dbi, ...) == MDB_INVALID_DBI) { > /* Get <dbi>, which some other txn committed after <txn> began */ > mdb_dbi_open(txn, "database name of <dbi>", 0, &dbi2); > assert(dbi2 == dbi); > } > > That breaks this requirement from the mdb_dbi_open() doc: > > * A transaction that uses > * this function must finish (either commit or abort) before > * any other transaction in the process may use this function. > Just to illustrate the simplified sequence of calls (using MDB_NOTLS): mdb_txn_begin(env, NULL, MDB_RDONLY, &txn2); mdb_txn_begin(env, NULL, MDB_RDONLY, &txn); MDB_dbi dbi; mdb_dbi_open(txn, "db1", 0, &dbi); // dbi becomes now valid and is propagated to the env mdb_txn_commit(txn); // At this point dbi for txn2 is invalid as txn2 started before opening the database if (mdb_something(txn2, dbi, ...) == MDB_INVALID_DBI) { MDB_dbi dbi2; mdb_dbi_open(txn2, "db1", 0, &dbi2); assert(dbi2 == dbi); // now dbi is valid for txn2 as with the mdb_dbi_open we propagated it to txn2! mdb_something(txn2, dbi, ...); } mdb_txn_commit(txn2); This code is really not preferred and is only a workaround. > To fulfill that, you must serialize txns which use mdb_dbi_open(). > (Unless they're all write txns, which LMDB serializes.) One reason > you must, is that there is no sync anywhere else in your suggestion. > > Multi-threaded use of structures, like the ones behind DBIs, must be > synchronized. But DBIs and mdb_dbi_open() are not mutex-protected, > nor are read-only txns (after the per-thread setup). So it's up to > the user. And LMDB mostly can't catch users at less than txn-sized > threading bugs - like the DBI workarounds people keep trying. > > I suppose we should add some of the above more clearly in lmdb.h doc, > since people keep trying to hack around it. And "EINVAL/MDB_BAD_DBI > often indicate that LMDB detected a bug in your program, not that you > can safely work around the problem when LMDB returns them." > (Not always, unless we walk carefully through which error codes are > returned when first.) > > > For a txn to pick up a DBI committed after the txn started, LMDB > would need to mutex-protect DBI operations. Something like this: > > - Add an env+txn flag MDB_ADMIN_LOCKS. If it is set in the env, > then mdb_dbi_open() fails if it is not also set in the txn. > The txn flag wraps DBI handling in a new mutex in txn_begin(), > dbi_open(), commit/abort() of a txn which opened a DBI. > The env flag wraps dbi_close() in the mutex. > > - Add mdb_dbi_pickup(MDB_txn*, MDB_dbi of a named DB): Pick up a DBI > which was committed after the txn began. It will lock the mutex, > and maybe it'll require MDB_ADMIN_LOCKS in the txn. > (I don't know if pickup(main DBI) would be safe.) > > Maybe that's more ACID-friendly - a separate function to step > outside and look at the current env. If I've gotten it right... > > Of course, if the txn which calls dbi_pickup() began before that > named DB was created, then the txn picks up a DBI which it cannot > use. That's OK as far as LMDB is concerned, it does safely catch > use of a non-existent named DB. But makes it less useful to users. > > So I wonder, will people optimize away the mutexing in the way you > suggested above? Try to use a DBI in a txn without MDB_ADMIN_LOCKS, > and if that fails call mdb_dbi_pickup(). If so, they reintroduce > the heisenbug: The test for whether the DBI was OK, was not mutexed > vs. mdb_txn_commit(txn which created the DBI). > > Or simpler, the new flag could just tell MDB to wrap a mutex around > every single txn_begin and txn_commit. That won't tempt anyone to > optimize away locks, since there are so much locking already. But > then, why are they using LMDB? LMDB has made a lot of tradeoffs to > avoid locking in read-only txns. See Caveats in lmdb.h. > > > And it'd still be unsafe to allow overlapping txns to mdb_dbi_open() > the same DB. txn1 creates the DBI, txn2 opens it too, txn1 aborts but > must not close the DBI because txn2 is using it... So MDB_dbx would > need a refcount of how many existing txns opened the DB during DBI > creation. Also I suppose ADMIN_LOCKS would need to disable > mdb_set_compare() & co. A new function mdb_dbi_open2() could init the > entire MDB_dbx while holding the mutex, so other txns would not use > the half-initialized DBX. > > Alternatively, dbi_open2(named DB) above can take a flag which says > "make the DBI immediately available in the MDB_env". That'll be > unsafe for txns where the user does not synchronize txn_begin against > the dbi_open2(). And it has its own sets of issues with multiple > txns opening a DB, nested txns, etc. > > > Then there's another approach: "Transactions are isolated from each > other, provided you use LMDB correctly. That's all. Sorry about > the DBI inconveniences." > If I got it right it is all about protecting against invalid DBIs which can happen for example if someone closes database "A" which was assigned DBI "5" and later the database "B" is opened and the free slot 5 is then assigned to "B". So we could be misled using DBI "5" having in mind database "A". So why don't we extend the MDB_DBI by adding the sequence in the lower or higher bits. When addressing the slot we simply strip the sequence bits and verify the slot's sequence against the sequence in the MDB_DBI to ensure the DBI has not changed. So picking up a DPI (as suggested in the patch) can be verified. The restriction with mdb_dbi_open() will stay. But the following would work: MDB_dbi dbi; mdb_txn_begin(env, NULL, MDB_RDONLY, &txn2); mdb_txn_begin(env, NULL, MDB_RDONLY, &txn1); mdb_dbi_open(txn1, "db1", 0, &dbi); // dbi becomes now valid and is propagated to the env // ==> and therefore is also valid for any other transaction! mdb_txn_commit(txn1); // use dbi opened in txn1 (which is committed) mdb_something(txn2, dbi, ...); .... mdb_txn_commit(txn2); So the patch could look something like that. static int mdb_update_db_info(MDB_txn *txn, MDB_dbi dbi) { unsigned int dbi_index = dbi & MDB_DBI_INDXE_MASK; unsigned int dbi_seq = dbi & MDB_DBI_SEQ_MASK << 16; if (txn->mt_env->me_dbiseqs[dbi_index] != dbi_seq) { return 0; } /* propagate newly opened db from env to current txn. Mark them as unused */ if (txn->mt_numdbs < txn->mt_env->me_numdbs) { memset(txn->mt_dbflags + txn->mt_numdbs, DB_UNUSED, txn->mt_env->me_numdbs - txn->mt_numdbs); } txn->mt_numdbs = txn->mt_env->me_numdbs; return dbi_index < txn->mt_numdbs; }

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Re: (ITS#8430) Improved handling for large number of databases
by h.b.furuseth＠usit.uio.no 30 May '16

30 May '16

On 30. mai 2016 13:51, Hallvard Breien Furuseth wrote: > Copied from a private mail: >> Maybe a good way in between could be to memcpy txn->mt_env->me_dbflags. >> (...) memcpy uses SSE instructions for x86_64. > > Yes. If we make MDB_txn.mt_dbflags an uint16_t*, we can memcpy to it. > That costs <maxdbs> bytes/txn + one more reserved env flag bit in > PERSISTENT_FLAGS: Reserve DB_UNUSED, and keep it always set in > me_dbflags so the memcpy will set it in mt_dbflags. Oops, that was too clever. Then a bunch of code which assumes a DBI is valid if various txn flags are set, must also check DB_UNUSED. Or invert that to DB_USED which might be easier - (flags & DB_DIRTY) becomes F_ISSET(flags, DB_DIRTY|DB_USED) which checks if both are set. Maybe the simple solution using 2*<maxdbs> bytes/txn would be better. -- Hallvard

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Re: (ITS#8431) LMDB: Access newly opened database from another transaction
by h.b.furuseth＠usit.uio.no 30 May '16

30 May '16

In ITS#8430, Juerg Bircher wrote: > So if you drop ITS#8431 it would at least be very helpful to get an > separate error like MDB_INVALID_DPI instead of EINVAL So the > application sharing and caching handles (dbi) could handle the > situation appropriate like use mdb_env_open() so that the dbi becomes > valid in that txn as well. This is ITS#8431 in a new guise, so I'm replying here. We know DBI handling is inconvenient. We'd have fixed it if it were easy without adding a bunch of locks. I'll walk through some of it below, I expect it'll be useful to have for future reference anyway: You mean like this? if (mdb_something(txn, dbi, ...) == MDB_INVALID_DBI) { /* Get <dbi>, which some other txn committed after <txn> began */ mdb_dbi_open(txn, "database name of <dbi>", 0, &dbi2); assert(dbi2 == dbi); } That breaks this requirement from the mdb_dbi_open() doc: * A transaction that uses * this function must finish (either commit or abort) before * any other transaction in the process may use this function. To fulfill that, you must serialize txns which use mdb_dbi_open(). (Unless they're all write txns, which LMDB serializes.) One reason you must, is that there is no sync anywhere else in your suggestion. Multi-threaded use of structures, like the ones behind DBIs, must be synchronized. But DBIs and mdb_dbi_open() are not mutex-protected, nor are read-only txns (after the per-thread setup). So it's up to the user. And LMDB mostly can't catch users at less than txn-sized threading bugs - like the DBI workarounds people keep trying. I suppose we should add some of the above more clearly in lmdb.h doc, since people keep trying to hack around it. And "EINVAL/MDB_BAD_DBI often indicate that LMDB detected a bug in your program, not that you can safely work around the problem when LMDB returns them." (Not always, unless we walk carefully through which error codes are returned when first.) For a txn to pick up a DBI committed after the txn started, LMDB would need to mutex-protect DBI operations. Something like this: - Add an env+txn flag MDB_ADMIN_LOCKS. If it is set in the env, then mdb_dbi_open() fails if it is not also set in the txn. The txn flag wraps DBI handling in a new mutex in txn_begin(), dbi_open(), commit/abort() of a txn which opened a DBI. The env flag wraps dbi_close() in the mutex. - Add mdb_dbi_pickup(MDB_txn*, MDB_dbi of a named DB): Pick up a DBI which was committed after the txn began. It will lock the mutex, and maybe it'll require MDB_ADMIN_LOCKS in the txn. (I don't know if pickup(main DBI) would be safe.) Maybe that's more ACID-friendly - a separate function to step outside and look at the current env. If I've gotten it right... Of course, if the txn which calls dbi_pickup() began before that named DB was created, then the txn picks up a DBI which it cannot use. That's OK as far as LMDB is concerned, it does safely catch use of a non-existent named DB. But makes it less useful to users. So I wonder, will people optimize away the mutexing in the way you suggested above? Try to use a DBI in a txn without MDB_ADMIN_LOCKS, and if that fails call mdb_dbi_pickup(). If so, they reintroduce the heisenbug: The test for whether the DBI was OK, was not mutexed vs. mdb_txn_commit(txn which created the DBI). Or simpler, the new flag could just tell MDB to wrap a mutex around every single txn_begin and txn_commit. That won't tempt anyone to optimize away locks, since there are so much locking already. But then, why are they using LMDB? LMDB has made a lot of tradeoffs to avoid locking in read-only txns. See Caveats in lmdb.h. And it'd still be unsafe to allow overlapping txns to mdb_dbi_open() the same DB. txn1 creates the DBI, txn2 opens it too, txn1 aborts but must not close the DBI because txn2 is using it... So MDB_dbx would need a refcount of how many existing txns opened the DB during DBI creation. Also I suppose ADMIN_LOCKS would need to disable mdb_set_compare() & co. A new function mdb_dbi_open2() could init the entire MDB_dbx while holding the mutex, so other txns would not use the half-initialized DBX. Alternatively, dbi_open2(named DB) above can take a flag which says "make the DBI immediately available in the MDB_env". That'll be unsafe for txns where the user does not synchronize txn_begin against the dbi_open2(). And it has its own sets of issues with multiple txns opening a DB, nested txns, etc. Then there's another approach: "Transactions are isolated from each other, provided you use LMDB correctly. That's all. Sorry about the DBI inconveniences."

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