Bryan Matsuo wrote:

openldap-technical,

I am working on some Go (golang) bindings[1] for the LMDB library and I have some interest in exposing the functionality of mdb_set_compare (and mdb_set_dupsort). But it is proving difficult and I have a question about the function(s).

Calling mdb_set_compare from the Go runtime is challenging. Using C APIs with callbacks comes with restrictions[2][3]. I believe it impossible to bind these functions way that is flexible, as one would expect. A potential change to LMDB that would make binding drastically easier is having MDB_cmp_func to take a third "context" argument with type void*. Then a binding could safely use an arbitrary Go function for comparisons.

Is it possible for future versions of LMDB to add a third argument to the MDB_cmp_func signature? Otherwise would it be acceptable for a variant API to be added using a different function type, one accepting three arguments?

Thanks for the consideration.

Cheers,

• Bryan

[1] Go bindings -- https://github.com/bmatsuo/lmdb-go [2] Cgo pointer restrictions -- https://github.com/golang/proposal/blob/master/design/12416-cgo-pointers.md [3] Cgo documentation -- https://golang.org/cmd/cgo/

I see nothing in these restrictions that requires extra information to be passed from Go to C or from C to Go.

There is a vague mention in [2]

"A particular unsafe area is C code that wants to hold on to Go func and pointer values for future callbacks from C to Go. This works today but is not permitted by the invariant. It is hard to detect. One safe approach is: Go code that wants to preserve funcs/pointers stores them into a map indexed by an int. Go code calls the C code, passing the int, which the C code may store freely. When the C code wants to call into Go, it passes the int to a Go function that looks in the map and makes the call."

But it's nonsense in this case - you want to pass a Go function pointer to C, but the only way for C to use it is to call some *other* Go function? Sorry but there is no other Go function for the mdb_cmp() function to call, the only one it knows about is the function pointer that you pass.

If this is what you're referring to, adding a context pointer doesn't achieve anything. If this isn't what you're referring to, then please explain exactly what you hope to achieve with this context pointer.

Juerg,

That is is interesting proposal. As an alternative to letting users hook up arbitrary Go function for comparison, I have also thought about the possibility of providing a small set of static C functions usable for comparison. A flexible compound key comparison function like this could fit well into that idea.

Howard,

Sorry I did not find the issues mentioned in previous searches.

I understand the concern about such a hot code path. I'm not sure that Go would see acceptable performance.

But, Go is not an interpreted language (though there is glue). And while I'm not positive about the performance of Go in this area you seem to dismiss comparison functions in any other language. Is it unreasonable to think that comparison functions written in other compiled languages like Rust, Nim, or ML variants would also be impractically slow?

I also believe you have misunderstood the practical problems of passing Go function pointers to C. But to be fair, I think the wording of that quoted paragraph could be better.

Sorry but there is no other Go function for the mdb_cmp() function to

call, the only one it knows about is the function pointer that you pass.

It may be of benefit to see how the I've used the context argument in a binding being developed for the mdb_reader_list function.

https://github.com/bmatsuo/lmdb-go/blob/bmatsuo/reader-list-context-fix/lmdb...

The callback passed to mdb_reader_list is always the same static function because correctly calling a Go function from C requires an annotated static Go function. The context argument allows dispatch to the correct Go function that was configured at runtime. I believe that is the "other" Go function you mentioned.

The implementation would be similar for mdb_set_compare. The callback would always be the same static function which handles the dynamic dispatch.

Cheers, - Bryan

On Thu, Oct 29, 2015 at 3:12 AM Jürg Bircher juerg.bircher@helmedica.com wrote:

Actually I’m not commenting on binding Go but I’m voting for a context passed to the compare function.

I fully agree that the compare function is part of the critical path. But as I need to define custom indexes with compound keys the compare functions varies and it would be impractical to predefine for any compound key combination a c function.

The compare context would be stored on the struct MDB_dbx.

typedef struct MDB_dbx { MDB_val md_name; /**< name of the database */ MDB_cmp_func *md_cmp; /**< function for comparing keys */ void *md_cmpctx; /** user-provided context for md_cmp **/ MDB_cmp_func *md_dcmp; /**< function for comparing data items */ void *md_dcmpctx;/** user-provided context for md_dcmp **/ MDB_rel_func *md_rel; /**< user relocate function */ void *md_relctx; /**< user-provided context for md_rel */ } MDB_dbx;

The following is a draft (not tested yet) of a generic compare function. The context contains a compare specification which is a null terminated list of <type><order> pairs.

// compareSpec <type><order>...<null> int key_comp_generic(const MDB_val *a, const MDB_val *b, char *compareSpec) { int result = 0; char *pa = a->mv_data; char *pb = b->mv_data;

while (1) {
    switch (*compareSpec++) {
       case 0:
            break;
       case INT32_KEY :
       {
            unsigned int va = *(unsigned int *)pa;
            unsigned int vb = *(unsigned int *)pb;

            if (*compareSpec++ == ASCENDING_ORDER) {
                result = (va < vb) ? -1 : va > vb;
            }
            else {
                result = (va > vb) ? -1 : va < vb;
            }

            if (result != 0) {
                break;
            }
            else {
                pa += 4;
                pb += 4;
            }
        }
        case INT64_KEY :
        {
            unsigned long long va = *(unsigned long long *)pa;
            unsigned long long vb = *(unsigned long long *)pb;

            if (*compareSpec++ == ASCENDING_ORDER) {
                result = (va < vb) ? -1 : va > vb;
            }
            else {
                result = (va > vb) ? -1 : va < vb;
            }

            if (result != 0) {
                break;
            }
            else {
                pa += 8;
                pb += 8;
            }
        }
        case STRING_KEY :
        {
            unsigned int la = *(unsigned int *)pa;
            unsigned int lb = *(unsigned int *)pb;

            pa += 4;
            pb += 4;

            if (*compareSpec++ == ASCENDING_ORDER) {
                result = strncmp(pa, pb, (la < lb) ? la : lb);
                if (result != 0) {
                    break;
                }
                else {
                    result = (la < lb) ? -1 : la > lb;
                }
            }
            else {
                result = strncmp(pb, pa, (la < lb) ? la : lb);
                if (result != 0) {
                    break;
                }
                else {
                    result = (la > lb) ? -1 : la < lb;
                }
            }

            if (result != 0) {
                break;
            }
            else {
                pa += la;
                pb += lb;
            }
        }
    }
}

return result;

}

Regards Juerg

On 29/10/15 10:40, "openldap-technical on behalf of Howard Chu" < openldap-technical-bounces@openldap.org on behalf of hyc@symas.com> wrote:

...

Bryan Matsuo wrote:

...

openldap-technical,

I am working on some Go (golang) bindings[1] for the LMDB library and I

have

...

...

some interest in exposing the functionality of mdb_set_compare (and mdb_set_dupsort). But it is proving difficult and I have a question

about the

...

...

function(s).

Calling mdb_set_compare from the Go runtime is challenging. Using C

APIs with

...

...

callbacks comes with restrictions[2][3]. I believe it impossible to

bind these

...

...

functions way that is flexible, as one would expect. A potential change

to

...

...

LMDB that would make binding drastically easier is having MDB_cmp_func

to take

...

...

a third "context" argument with type void*. Then a binding could safely

use an

...

...

arbitrary Go function for comparisons.

Is it possible for future versions of LMDB to add a third argument to

the

...

...

MDB_cmp_func signature? Otherwise would it be acceptable for a variant

API to

...

...

be added using a different function type, one accepting three arguments?

Thanks for the consideration.

Cheers,

• Bryan

[1] Go bindings -- https://github.com/bmatsuo/lmdb-go [2] Cgo pointer restrictions --

https://github.com/golang/proposal/blob/master/design/12416-cgo-pointers.md

...

...

[3] Cgo documentation -- https://golang.org/cmd/cgo/

I see nothing in these restrictions that requires extra information to be passed from Go to C or from C to Go.

There is a vague mention in [2]

"A particular unsafe area is C code that wants to hold on to Go func and pointer values for future callbacks from C to Go. This works today but is

not

...

permitted by the invariant. It is hard to detect. One safe approach is: Go code that wants to preserve funcs/pointers stores them into a map indexed

by

...

an int. Go code calls the C code, passing the int, which the C code may

store

...

freely. When the C code wants to call into Go, it passes the int to a Go function that looks in the map and makes the call."

But it's nonsense in this case - you want to pass a Go function pointer

to C,

...

but the only way for C to use it is to call some *other* Go function?

Sorry

...

but there is no other Go function for the mdb_cmp() function to call, the

only

...

one it knows about is the function pointer that you pass.

If this is what you're referring to, adding a context pointer doesn't

achieve

...

anything. If this isn't what you're referring to, then please explain

exactly

...

what you hope to achieve with this context pointer.

-- -- Howard Chu CTO, Symas Corp. http://www.symas.com Director, Highland Sun http://highlandsun.com/hyc/ Chief Architect, OpenLDAP http://www.openldap.org/project/

From the peanut gallery: Small set of static C functions is probably the

way to go. If I understand correctly, which I probablay don't, the mismatch between green threads and OS threads means there's a lot of expensive stack-switching involved in go->C->go execution.

On Thu, Oct 29, 2015 at 5:28 PM, Bryan Matsuo bryan.matsuo@gmail.com wrote:

Juerg,

That is is interesting proposal. As an alternative to letting users hook up arbitrary Go function for comparison, I have also thought about the possibility of providing a small set of static C functions usable for comparison. A flexible compound key comparison function like this could fit well into that idea.

Howard,

Sorry I did not find the issues mentioned in previous searches.

I understand the concern about such a hot code path. I'm not sure that Go would see acceptable performance.

But, Go is not an interpreted language (though there is glue). And while I'm not positive about the performance of Go in this area you seem to dismiss comparison functions in any other language. Is it unreasonable to think that comparison functions written in other compiled languages like Rust, Nim, or ML variants would also be impractically slow?

I also believe you have misunderstood the practical problems of passing Go function pointers to C. But to be fair, I think the wording of that quoted paragraph could be better.

...

Sorry but there is no other Go function for the mdb_cmp() function to

call, the only one it knows about is the function pointer that you pass.

It may be of benefit to see how the I've used the context argument in a binding being developed for the mdb_reader_list function.

https://github.com/bmatsuo/lmdb-go/blob/bmatsuo/reader-list-context-fix/lmdb...

The callback passed to mdb_reader_list is always the same static function because correctly calling a Go function from C requires an annotated static Go function. The context argument allows dispatch to the correct Go function that was configured at runtime. I believe that is the "other" Go function you mentioned.

The implementation would be similar for mdb_set_compare. The callback would always be the same static function which handles the dynamic dispatch.

Cheers,

• Bryan

On Thu, Oct 29, 2015 at 3:12 AM Jürg Bircher juerg.bircher@helmedica.com wrote:

...

Actually I’m not commenting on binding Go but I’m voting for a context passed to the compare function.

I fully agree that the compare function is part of the critical path. But as I need to define custom indexes with compound keys the compare functions varies and it would be impractical to predefine for any compound key combination a c function.

The compare context would be stored on the struct MDB_dbx.

typedef struct MDB_dbx { MDB_val md_name; /**< name of the database */ MDB_cmp_func *md_cmp; /**< function for comparing keys */ void *md_cmpctx; /** user-provided context for md_cmp **/ MDB_cmp_func *md_dcmp; /**< function for comparing data items */ void *md_dcmpctx;/** user-provided context for md_dcmp **/ MDB_rel_func *md_rel; /**< user relocate function */ void *md_relctx; /**< user-provided context for md_rel */ } MDB_dbx;

The following is a draft (not tested yet) of a generic compare function. The context contains a compare specification which is a null terminated list of <type><order> pairs.

// compareSpec <type><order>...<null> int key_comp_generic(const MDB_val *a, const MDB_val *b, char *compareSpec) { int result = 0; char *pa = a->mv_data; char *pb = b->mv_data;

while (1) {
    switch (*compareSpec++) {
       case 0:
            break;
       case INT32_KEY :
       {
            unsigned int va = *(unsigned int *)pa;
            unsigned int vb = *(unsigned int *)pb;

            if (*compareSpec++ == ASCENDING_ORDER) {
                result = (va < vb) ? -1 : va > vb;
            }
            else {
                result = (va > vb) ? -1 : va < vb;
            }

            if (result != 0) {
                break;
            }
            else {
                pa += 4;
                pb += 4;
            }
        }
        case INT64_KEY :
        {
            unsigned long long va = *(unsigned long long *)pa;
            unsigned long long vb = *(unsigned long long *)pb;

            if (*compareSpec++ == ASCENDING_ORDER) {
                result = (va < vb) ? -1 : va > vb;
            }
            else {
                result = (va > vb) ? -1 : va < vb;
            }

            if (result != 0) {
                break;
            }
            else {
                pa += 8;
                pb += 8;
            }
        }
        case STRING_KEY :
        {
            unsigned int la = *(unsigned int *)pa;
            unsigned int lb = *(unsigned int *)pb;

            pa += 4;
            pb += 4;

            if (*compareSpec++ == ASCENDING_ORDER) {
                result = strncmp(pa, pb, (la < lb) ? la : lb);
                if (result != 0) {
                    break;
                }
                else {
                    result = (la < lb) ? -1 : la > lb;
                }
            }
            else {
                result = strncmp(pb, pa, (la < lb) ? la : lb);
                if (result != 0) {
                    break;
                }
                else {
                    result = (la > lb) ? -1 : la < lb;
                }
            }

            if (result != 0) {
                break;
            }
            else {
                pa += la;
                pb += lb;
            }
        }
    }
}

return result;

}

Regards Juerg

On 29/10/15 10:40, "openldap-technical on behalf of Howard Chu" < openldap-technical-bounces@openldap.org on behalf of hyc@symas.com> wrote:

...

Bryan Matsuo wrote:

...

openldap-technical,

I am working on some Go (golang) bindings[1] for the LMDB library and

I have

...

...

some interest in exposing the functionality of mdb_set_compare (and mdb_set_dupsort). But it is proving difficult and I have a question

about the

...

...

function(s).

Calling mdb_set_compare from the Go runtime is challenging. Using C

APIs with

...

...

callbacks comes with restrictions[2][3]. I believe it impossible to

bind these

...

...

functions way that is flexible, as one would expect. A potential

change to

...

...

LMDB that would make binding drastically easier is having MDB_cmp_func

to take

...

...

a third "context" argument with type void*. Then a binding could

safely use an

...

...

arbitrary Go function for comparisons.

Is it possible for future versions of LMDB to add a third argument to

the

...

...

MDB_cmp_func signature? Otherwise would it be acceptable for a variant

API to

...

...

be added using a different function type, one accepting three

arguments?

...

...

Thanks for the consideration.

Cheers,

• Bryan

[1] Go bindings -- https://github.com/bmatsuo/lmdb-go [2] Cgo pointer restrictions --

https://github.com/golang/proposal/blob/master/design/12416-cgo-pointers.md

...

...

[3] Cgo documentation -- https://golang.org/cmd/cgo/

I see nothing in these restrictions that requires extra information to be passed from Go to C or from C to Go.

There is a vague mention in [2]

"A particular unsafe area is C code that wants to hold on to Go func and pointer values for future callbacks from C to Go. This works today but

is not

...

permitted by the invariant. It is hard to detect. One safe approach is:

Go

...

code that wants to preserve funcs/pointers stores them into a map

indexed by

...

an int. Go code calls the C code, passing the int, which the C code may

store

...

freely. When the C code wants to call into Go, it passes the int to a Go function that looks in the map and makes the call."

But it's nonsense in this case - you want to pass a Go function pointer

to C,

...

but the only way for C to use it is to call some *other* Go function?

Sorry

...

but there is no other Go function for the mdb_cmp() function to call,

the only

...

one it knows about is the function pointer that you pass.

If this is what you're referring to, adding a context pointer doesn't

achieve

...

anything. If this isn't what you're referring to, then please explain

exactly

...

what you hope to achieve with this context pointer.

-- -- Howard Chu CTO, Symas Corp. http://www.symas.com Director, Highland Sun http://highlandsun.com/hyc/ Chief Architect, OpenLDAP http://www.openldap.org/project/

Bryan Matsuo wrote:

After digging it seems that it will continue to be possible for users to safely pass static Go function references. It is quite a burden on the user. But I will continue to think about it.

Jay, noted. I am open to exploring that direction. Though as was pointed out earlier a library of static functions can be made more useful (if somewhat slower) when a context object can configure their behavior. Before reading that suggestion I was uncertain how much useful functionality could be exposed as a library. I am writing general purpose bindings, so I would prefer a function library be fairly generic.

Howard, do you have thoughts on the proposal from Juerg regarding a compound-key comparison function implemented using a context value?

I remain unconvinced. key-comparison is still per-DB; a comparison specifier saves some space but at the expense of time - more compare ops, more branching per key compare. Dedicated functions are still the better way to go. Plus, naive constructs like in the emailed example are easy to get wrong - his example will never terminate because he only breaks from the switch statement, nothing breaks from the while(1) loop. It is attempting to be too clever, when a more straightforward approach will be faster and obviously bug-free.

On Thu, Oct 29, 2015 at 8:49 PM Jay Booth <jaybooth@gmail.com mailto:jaybooth@gmail.com> wrote:

 From the peanut gallery:  Small set of static C functions is probably the
way to go.  If I understand correctly, which I probablay don't, the
mismatch between green threads and OS threads means there's a lot of
expensive stack-switching involved in go->C->go execution.

On Thu, Oct 29, 2015 at 5:28 PM, Bryan Matsuo <bryan.matsuo@gmail.com
<mailto:bryan.matsuo@gmail.com>> wrote:

    Juerg,

    That is is interesting proposal. As an alternative to letting users
    hook up arbitrary Go function for comparison, I have also thought
    about the possibility of providing a small set of static C functions
    usable for comparison. A flexible compound key comparison function
    like this could fit well into that idea.

    Howard,

    Sorry I did not find the issues mentioned in previous searches.

    I understand the concern about such a hot code path. I'm not sure that
    Go would see acceptable performance.

    But, Go is not an interpreted language (though there is glue). And
    while I'm not positive about the performance of Go in this area you
    seem to dismiss comparison functions in any other language. Is it
    unreasonable to think that comparison functions written in other
    compiled languages like Rust, Nim, or ML variants would also be
    impractically slow?

    I also believe you have misunderstood the practical problems of
    passing Go function pointers to C. But to be fair, I think the wording
    of that quoted paragraph could be better.

    >Sorry but there is no other Go function for the mdb_cmp() function to
    call, the only one it knows about is the function pointer that you pass.

    It may be of benefit to see how the I've used the context argument in
    a binding being developed for the mdb_reader_list function.

    https://github.com/bmatsuo/lmdb-go/blob/bmatsuo/reader-list-context-fix/lmdb/lmdbgo.c

    The callback passed to mdb_reader_list is always the same static
    function because correctly calling a Go function from C requires an
    annotated static Go function. The context argument allows dispatch to
    the correct Go function that was configured at runtime. I believe that
    is the "other" Go function you mentioned.

    The implementation would be similar for mdb_set_compare. The callback
    would always be the same static function which handles the dynamic
    dispatch.

    Cheers,
    - Bryan

    On Thu, Oct 29, 2015 at 3:12 AM Jürg Bircher
    <juerg.bircher@helmedica.com <mailto:juerg.bircher@helmedica.com>> wrote:

        Actually I’m not commenting on binding Go but I’m voting for a
        context passed to the compare function.

        I fully agree that the compare function is part of the critical
        path. But as I need to define custom indexes with compound keys
        the compare functions varies and it would be impractical to
        predefine for any compound key combination a c function.

        The compare context would be stored on the struct MDB_dbx.

        typedef struct MDB_dbx {
                 MDB_val         md_name;                /**< name of the
        database */
                 MDB_cmp_func    *md_cmp;        /**< function for
        comparing keys */
                 void            *md_cmpctx; /** user-provided context for
        md_cmp **/
                 MDB_cmp_func    *md_dcmp;       /**< function for
        comparing data items */
                 void            *md_dcmpctx;/** user-provided context for
        md_dcmp **/
                 MDB_rel_func    *md_rel;        /**< user relocate
        function */
                 void            *md_relctx;             /**<
        user-provided context for md_rel */
        } MDB_dbx;

        The following is a draft (not tested yet) of a generic compare
        function. The context contains a compare specification which is a
        null terminated list of <type><order> pairs.

        // compareSpec <type><order>...<null>
        int key_comp_generic(const MDB_val *a, const MDB_val *b, char
        *compareSpec) {
             int result = 0;
             char *pa = a->mv_data;
             char *pb = b->mv_data;

             while (1) {
                 switch (*compareSpec++) {
                    case 0:
                         break;
                    case INT32_KEY :
                    {
                         unsigned int va = *(unsigned int *)pa;
                         unsigned int vb = *(unsigned int *)pb;

                         if (*compareSpec++ == ASCENDING_ORDER) {
                             result = (va < vb) ? -1 : va > vb;
                         }
                         else {
                             result = (va > vb) ? -1 : va < vb;
                         }

                         if (result != 0) {
                             break;
                         }
                         else {
                             pa += 4;
                             pb += 4;
                         }
                     }
                     case INT64_KEY :
                     {
                         unsigned long long va = *(unsigned long long *)pa;
                         unsigned long long vb = *(unsigned long long *)pb;

                         if (*compareSpec++ == ASCENDING_ORDER) {
                             result = (va < vb) ? -1 : va > vb;
                         }
                         else {
                             result = (va > vb) ? -1 : va < vb;
                         }

                         if (result != 0) {
                             break;
                         }
                         else {
                             pa += 8;
                             pb += 8;
                         }
                     }
                     case STRING_KEY :
                     {
                         unsigned int la = *(unsigned int *)pa;
                         unsigned int lb = *(unsigned int *)pb;

                         pa += 4;
                         pb += 4;

                         if (*compareSpec++ == ASCENDING_ORDER) {
                             result = strncmp(pa, pb, (la < lb) ? la : lb);
                             if (result != 0) {
                                 break;
                             }
                             else {
                                 result = (la < lb) ? -1 : la > lb;
                             }
                         }
                         else {
                             result = strncmp(pb, pa, (la < lb) ? la : lb);
                             if (result != 0) {
                                 break;
                             }
                             else {
                                 result = (la > lb) ? -1 : la < lb;
                             }
                         }

                         if (result != 0) {
                             break;
                         }
                         else {
                             pa += la;
                             pb += lb;
                         }
                     }
                 }
             }

             return result;
        }

        Regards
        Juerg


        On 29/10/15 10:40, "openldap-technical on behalf of Howard Chu"
        <openldap-technical-bounces@openldap.org
        <mailto:openldap-technical-bounces@openldap.org> on behalf of
        hyc@symas.com <mailto:hyc@symas.com>> wrote:

         >Bryan Matsuo wrote:
         >> openldap-technical,
         >>
         >> I am working on some Go (golang) bindings[1] for the LMDB
        library and I have
         >> some interest in exposing the functionality of mdb_set_compare
        (and
         >> mdb_set_dupsort). But it is proving difficult and I have a
        question about the
         >> function(s).
         >>
         >> Calling mdb_set_compare from the Go runtime is challenging.
        Using C APIs with
         >> callbacks comes with restrictions[2][3]. I believe it
        impossible to bind these
         >> functions way that is flexible, as one would expect. A
        potential change to
         >> LMDB that would make binding drastically easier is having
        MDB_cmp_func to take
         >> a third "context" argument with type void*. Then a binding
        could safely use an
         >> arbitrary Go function for comparisons.
         >>
         >> Is it possible for future versions of LMDB to add a third
        argument to the
         >> MDB_cmp_func signature? Otherwise would it be acceptable for a
        variant API to
         >> be added using a different function type, one accepting three
        arguments?
         >>
         >> Thanks for the consideration.
         >>
         >> Cheers,
         >> - Bryan
         >>
         >> [1] Go bindings -- https://github.com/bmatsuo/lmdb-go
         >> [2] Cgo pointer restrictions --
         >>
        https://github.com/golang/proposal/blob/master/design/12416-cgo-pointers.md
         >> [3] Cgo documentation -- https://golang.org/cmd/cgo/
         >
         >I see nothing in these restrictions that requires extra
        information to be
         >passed from Go to C or from C to Go.
         >
         >There is a vague mention in [2]
         >
         >"A particular unsafe area is C code that wants to hold on to Go
        func and
         >pointer values for future callbacks from C to Go. This works
        today but is not
         >permitted by the invariant. It is hard to detect. One safe
        approach is: Go
         >code that wants to preserve funcs/pointers stores them into a
        map indexed by
         >an int. Go code calls the C code, passing the int, which the C
        code may store
         >freely. When the C code wants to call into Go, it passes the int
        to a Go
         >function that looks in the map and makes the call."
         >
         >But it's nonsense in this case - you want to pass a Go function
        pointer to C,
         >but the only way for C to use it is to call some *other* Go
        function? Sorry
         >but there is no other Go function for the mdb_cmp() function to
        call, the only
         >one it knows about is the function pointer that you pass.
         >
         >If this is what you're referring to, adding a context pointer
        doesn't achieve
         >anything. If this isn't what you're referring to, then please
        explain exactly
         >what you hope to achieve with this context pointer.
         >
         >--
         >   -- Howard Chu
         >   CTO, Symas Corp. http://www.symas.com
         >   Director, Highland Sun http://highlandsun.com/hyc/
         >   Chief Architect, OpenLDAP http://www.openldap.org/project/

I think arguments about how error prone a given function is are moot in this case. Tests can be written. And maintaining the number of functions Juerg is talking about is error prone in itself (and would potentially be harder to test). I also believe that asking everyone else to roll there own "simple" compound index functions involving strings will result in its own set of errors.

Simply put, putting the mdb_set_compare and mdb_set_dupsort functions in lmdb.h is letting people to shoot themselves in the foot. Allowing functions to be reused more easily helps reduce that overall.

On Fri, Oct 30, 2015 at 1:29 AM Jürg Bircher juerg.bircher@helmedica.com wrote:

The example is just a draft. It is a suggestion to support compound keys in a generic way with the intention to keep the extra coast as low as possible. I need a generic way as I have to support user-defined compound indices.

Could we not compromise on a #define so those who need the context could compile with SUPPORT_CMP_CONTEXT. And if not needed there is no performance penalty. Therefore the code basis would remain the same.

Thanks for considering…

On 30/10/15 08:43, "Howard Chu" hyc@symas.com wrote:

...

Bryan Matsuo wrote:

...

After digging it seems that it will continue to be possible for users to safely pass static Go function references. It is quite a burden on the

user.

...

...

But I will continue to think about it.

Jay, noted. I am open to exploring that direction. Though as was

pointed out

...

...

earlier a library of static functions can be made more useful (if

somewhat

...

...

slower) when a context object can configure their behavior. Before

reading

...

...

that suggestion I was uncertain how much useful functionality could be

exposed

...

...

as a library. I am writing general purpose bindings, so I would prefer a function library be fairly generic.

Howard, do you have thoughts on the proposal from Juerg regarding a compound-key comparison function implemented using a context value?

I remain unconvinced. key-comparison is still per-DB; a comparison

specifier

...

saves some space but at the expense of time - more compare ops, more

branching

...

per key compare. Dedicated functions are still the better way to go. Plus, naive constructs like in the emailed example are easy to get wrong - his example will never terminate because he only breaks from the switch

statement,

...

nothing breaks from the while(1) loop. It is attempting to be too clever,

when

...

a more straightforward approach will be faster and obviously bug-free.

...

On Thu, Oct 29, 2015 at 8:49 PM Jay Booth <jaybooth@gmail.com mailto:jaybooth@gmail.com> wrote:

 From the peanut gallery:  Small set of static C functions is

probably the

...

...

way to go.  If I understand correctly, which I probablay don't, the
mismatch between green threads and OS threads means there's a lot of
expensive stack-switching involved in go->C->go execution.

On Thu, Oct 29, 2015 at 5:28 PM, Bryan Matsuo <

bryan.matsuo@gmail.com

...

...

<mailto:bryan.matsuo@gmail.com>> wrote:

    Juerg,

    That is is interesting proposal. As an alternative to letting

users

...

...

    hook up arbitrary Go function for comparison, I have also

thought

...

...

    about the possibility of providing a small set of static C

functions

...

...

    usable for comparison. A flexible compound key comparison

function

...

...

    like this could fit well into that idea.

    Howard,

    Sorry I did not find the issues mentioned in previous searches.

    I understand the concern about such a hot code path. I'm not

sure that

...

...

    Go would see acceptable performance.

    But, Go is not an interpreted language (though there is glue).

And

...

...

    while I'm not positive about the performance of Go in this area

you

...

...

    seem to dismiss comparison functions in any other language. Is

it

...

...

    unreasonable to think that comparison functions written in other
    compiled languages like Rust, Nim, or ML variants would also be
    impractically slow?

    I also believe you have misunderstood the practical problems of
    passing Go function pointers to C. But to be fair, I think the

wording

...

...

    of that quoted paragraph could be better.

    >Sorry but there is no other Go function for the mdb_cmp()

function to

...

...

    call, the only one it knows about is the function pointer that

you pass.

...

...

    It may be of benefit to see how the I've used the context

argument in

...

...

    a binding being developed for the mdb_reader_list function.

https://github.com/bmatsuo/lmdb-go/blob/bmatsuo/reader-list-context-fix/lmdb...

...

...

    The callback passed to mdb_reader_list is always the same static
    function because correctly calling a Go function from C

requires an

...

...

    annotated static Go function. The context argument allows

dispatch to

...

...

    the correct Go function that was configured at runtime. I

believe that

...

...

    is the "other" Go function you mentioned.

    The implementation would be similar for mdb_set_compare. The

callback

...

...

    would always be the same static function which handles the

dynamic

...

...

    dispatch.

    Cheers,
    - Bryan

    On Thu, Oct 29, 2015 at 3:12 AM Jürg Bircher
    <juerg.bircher@helmedica.com <mailto:

juerg.bircher@helmedica.com>> wrote:

...

...

        Actually I’m not commenting on binding Go but I’m voting

for a

...

...

        context passed to the compare function.

        I fully agree that the compare function is part of the

critical

...

...

        path. But as I need to define custom indexes with compound

keys

...

...

        the compare functions varies and it would be impractical to
        predefine for any compound key combination a c function.

        The compare context would be stored on the struct MDB_dbx.

        typedef struct MDB_dbx {
                 MDB_val         md_name;                /**< name

of the

...

...

        database */
                 MDB_cmp_func    *md_cmp;        /**< function for
        comparing keys */
                 void            *md_cmpctx; /** user-provided

context for

...

...

        md_cmp **/
                 MDB_cmp_func    *md_dcmp;       /**< function for
        comparing data items */
                 void            *md_dcmpctx;/** user-provided

context for

...

...

        md_dcmp **/
                 MDB_rel_func    *md_rel;        /**< user relocate
        function */
                 void            *md_relctx;             /**<
        user-provided context for md_rel */
        } MDB_dbx;

        The following is a draft (not tested yet) of a generic

compare

...

...

        function. The context contains a compare specification

which is a

...

...

        null terminated list of <type><order> pairs.

        // compareSpec <type><order>...<null>
        int key_comp_generic(const MDB_val *a, const MDB_val *b,

char

...

...

        *compareSpec) {
             int result = 0;
             char *pa = a->mv_data;
             char *pb = b->mv_data;

             while (1) {
                 switch (*compareSpec++) {
                    case 0:
                         break;
                    case INT32_KEY :
                    {
                         unsigned int va = *(unsigned int *)pa;
                         unsigned int vb = *(unsigned int *)pb;

                         if (*compareSpec++ == ASCENDING_ORDER) {
                             result = (va < vb) ? -1 : va > vb;
                         }
                         else {
                             result = (va > vb) ? -1 : va < vb;
                         }

                         if (result != 0) {
                             break;
                         }
                         else {
                             pa += 4;
                             pb += 4;
                         }
                     }
                     case INT64_KEY :
                     {
                         unsigned long long va = *(unsigned long

long *)pa;

...

...

                         unsigned long long vb = *(unsigned long

long *)pb;

...

...

                         if (*compareSpec++ == ASCENDING_ORDER) {
                             result = (va < vb) ? -1 : va > vb;
                         }
                         else {
                             result = (va > vb) ? -1 : va < vb;
                         }

                         if (result != 0) {
                             break;
                         }
                         else {
                             pa += 8;
                             pb += 8;
                         }
                     }
                     case STRING_KEY :
                     {
                         unsigned int la = *(unsigned int *)pa;
                         unsigned int lb = *(unsigned int *)pb;

                         pa += 4;
                         pb += 4;

                         if (*compareSpec++ == ASCENDING_ORDER) {
                             result = strncmp(pa, pb, (la < lb) ?

la : lb);

...

...

                             if (result != 0) {
                                 break;
                             }
                             else {
                                 result = (la < lb) ? -1 : la > lb;
                             }
                         }
                         else {
                             result = strncmp(pb, pa, (la < lb) ?

la : lb);

...

...

                             if (result != 0) {
                                 break;
                             }
                             else {
                                 result = (la > lb) ? -1 : la < lb;
                             }
                         }

                         if (result != 0) {
                             break;
                         }
                         else {
                             pa += la;
                             pb += lb;
                         }
                     }
                 }
             }

             return result;
        }

        Regards
        Juerg


        On 29/10/15 10:40, "openldap-technical on behalf of Howard

Chu"

...

...

        <openldap-technical-bounces@openldap.org
        <mailto:openldap-technical-bounces@openldap.org> on behalf

of

...

...

        hyc@symas.com <mailto:hyc@symas.com>> wrote:

         >Bryan Matsuo wrote:
         >> openldap-technical,
         >>
         >> I am working on some Go (golang) bindings[1] for the

LMDB

...

...

        library and I have
         >> some interest in exposing the functionality of

mdb_set_compare

...

...

        (and
         >> mdb_set_dupsort). But it is proving difficult and I

have a

...

...

        question about the
         >> function(s).
         >>
         >> Calling mdb_set_compare from the Go runtime is

challenging.

...

...

        Using C APIs with
         >> callbacks comes with restrictions[2][3]. I believe it
        impossible to bind these
         >> functions way that is flexible, as one would expect. A
        potential change to
         >> LMDB that would make binding drastically easier is

having

...

...

        MDB_cmp_func to take
         >> a third "context" argument with type void*. Then a

binding

...

...

        could safely use an
         >> arbitrary Go function for comparisons.
         >>
         >> Is it possible for future versions of LMDB to add a

third

...

...

        argument to the
         >> MDB_cmp_func signature? Otherwise would it be

acceptable for a

...

...

        variant API to
         >> be added using a different function type, one accepting

three

...

...

        arguments?
         >>
         >> Thanks for the consideration.
         >>
         >> Cheers,
         >> - Bryan
         >>
         >> [1] Go bindings -- https://github.com/bmatsuo/lmdb-go
         >> [2] Cgo pointer restrictions --
         >>

https://github.com/golang/proposal/blob/master/design/12416-cgo-pointers.md

...

...

         >> [3] Cgo documentation -- https://golang.org/cmd/cgo/
         >
         >I see nothing in these restrictions that requires extra
        information to be
         >passed from Go to C or from C to Go.
         >
         >There is a vague mention in [2]
         >
         >"A particular unsafe area is C code that wants to hold on

to Go

...

...

        func and
         >pointer values for future callbacks from C to Go. This

works

...

...

        today but is not
         >permitted by the invariant. It is hard to detect. One safe
        approach is: Go
         >code that wants to preserve funcs/pointers stores them

into a

...

...

        map indexed by
         >an int. Go code calls the C code, passing the int, which

the C

...

...

        code may store
         >freely. When the C code wants to call into Go, it passes

the int

...

...

        to a Go
         >function that looks in the map and makes the call."
         >
         >But it's nonsense in this case - you want to pass a Go

function

...

...

        pointer to C,
         >but the only way for C to use it is to call some *other*

Go

...

...

        function? Sorry
         >but there is no other Go function for the mdb_cmp()

function to

...

...

        call, the only
         >one it knows about is the function pointer that you pass.
         >
         >If this is what you're referring to, adding a context

pointer

...

...

        doesn't achieve
         >anything. If this isn't what you're referring to, then

please

...

...

        explain exactly
         >what you hope to achieve with this context pointer.
         >
         >--
         >   -- Howard Chu
         >   CTO, Symas Corp. http://www.symas.com
         >   Director, Highland Sun http://highlandsun.com/hyc/
         >   Chief Architect, OpenLDAP

http://www.openldap.org/project/

...

...

-- -- Howard Chu CTO, Symas Corp. http://www.symas.com Director, Highland Sun http://highlandsun.com/hyc/ Chief Architect, OpenLDAP http://www.openldap.org/project/