Re: Segmentation fault when overwriting a k/v with MDB_RESERVE ?

Simon Majou wrote:

In your previous mail you said they were incompatible, but you provided a logical reason why (you can't sort the data). Now I found a case where the reason doesn't apply (you can append without doing the comparison).

No, you can't.

http://symas.com/mdb/doc/group__mdb.html#ga4fa8573d9236d54687c61827ebf8cac0

"Loading unsorted keys with this flag will cause a MDB_KEYEXIST error."

The key is always checked, because not checking it would allow the DB to become corrupted. Again, with RESERVE, you're not providing anything that can be checked.

So I thought maybe there was something else I missed. But ok I understand this is not supported.

For exactly the same reason as I already told you.

On 21 August 2015 at 21:21, Howard Chu <hyc@symas.com mailto:hyc@symas.com> wrote:

Simon Majou wrote:

    Correction, with the flags MDB_RESERVE|MDB_APPENDDUP


What part of "DUPSORT and RESERVE are incompatible" do you not understand?


    Here is the stack from gdb :

    Program received signal SIGSEGV, Segmentation fault.
    mdb_node_del (mc=mc@entry=0x7fffffffdb10, ksize=ksize@entry=0) at
    mdb.c:7183
    7183                mp->mp_ptrs[j] = mp->mp_ptrs[i];
    (gdb) bt
    #0  mdb_node_del (mc=mc@entry=0x7fffffffdb10, ksize=ksize@entry=0)
          at mdb.c:7183
    #1  0x00007ffff7bd06e2 in mdb_cursor_put (mc=0x7fffffffdb10,
          key=0x7fffffffd8d0, data=0x7fffffffd8c0, flags=<optimized out>)
          at mdb.c:6698
    #2  0x00007ffff7bd0f8e in mdb_cursor_put (mc=0x7fffffffd980,
          key=0x7fffffffdd70, data=0x7fffffffdda0, flags=<optimized out>)
          at mdb.c:6757
    #3  0x00007ffff7bd29b2 in mdb_put (txn=<optimized out>, dbi=<optimized
    out>,
          key=0x7fffffffdd70, data=0x7fffffffdda0, flags=327680) at mdb.c:8611
    #4  0x0000000000400dea in main (argc=1, argv=0x7fffffffded8)
          at test_lmdb_appenddup.c:59



    On 21 August 2015 at 19:25, Simon Majou <simon@majou.org
    <mailto:simon@majou.org>
    <mailto:simon@majou.org <mailto:simon@majou.org>>> wrote:

         And in the case where I want to append data in a DUPSORT database, is
         there a way to write directly in the persisted page ?
         I tried with the flags MDB_RESERVE|MDB_APPEND but it fails.

         On 18 August 2015 at 15:41, Simon Majou <simon@majou.org
    <mailto:simon@majou.org>
         <mailto:simon@majou.org <mailto:simon@majou.org>>> wrote:

             Oh ok it makes sense now. Thank you for the heads up

             On 18 August 2015 at 15:36, Howard Chu <hyc@symas.com
    <mailto:hyc@symas.com>
             <mailto:hyc@symas.com <mailto:hyc@symas.com>>> wrote:

                 Simon Majou wrote:

                     This is version 0.9.16

                     I used sample-bdb.txt for instance, with the DUP_SORT
    option,
                     and the mdb_put
                     with the MDB_RESERVED flags, and execute it 2 times.
    The first
                     run (creation)
                     is ok, the second (update) gets the seg fault.


                 DUPSORT and RESERVED are incompatible. RESERVED means
    you're not
                 providing a data value, only a length. DUPSORT sorts
    *values* -
                 since you didn't provide a valid value, it probably chased a
                 pointer off into space.


                     I ran it in gdb and got the stack :
                     #0  mdb_cursor_set (mc=mc@entry=0x7fffffffdb40,
                     key=key@entry=0x7fffffffdda0,
                           data=data@entry=0x7fffffffd800,
    op=op@entry=MDB_SET,
                           exactp=exactp@entry=0x7fffffffd7f0) at mdb.c:5867
                     #1  0x00007ffff7bd024c in mdb_cursor_put
    (mc=0x7fffffffdb40,
                           key=0x7fffffffdda0, data=0x7fffffffd8f0,
    flags=32768) at
                     mdb.c:6391
                     #2  0x00007ffff7bd095f in mdb_cursor_put
    (mc=0x7fffffffd9b0,
                           key=0x7fffffffdd90, data=0x7fffffffdda0,
                     flags=<optimized out>)
                           at mdb.c:6780
                     #3  0x00007ffff7bd29b2 in mdb_put (txn=<optimized out>,
                     dbi=<optimized out>,
                           key=0x7fffffffdd90, data=0x7fffffffdda0,
    flags=65536) at
                     mdb.c:8611
                     #4  0x0000000000400ccf in main (argc=1,
    argv=0x7fffffffded8)
                     at sample-mdb.c:43

                     It seems the mp_lower value of the page is set to 0
    instead of
                     a value inside
                     the page.

                     On 18 August 2015 at 13:58, Howard Chu <hyc@symas.com
    <mailto:hyc@symas.com>
                     <mailto:hyc@symas.com <mailto:hyc@symas.com>>
    <mailto:hyc@symas.com <mailto:hyc@symas.com>

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

                          Simon Majou wrote:

                              Hello,

                              I wonder, why do I get a Segmentation fault
    when I
                     use mdb_put with
                              MDB_RESERVE on a key that already exists ?


                          Have no idea. Provide some more details and
    maybe we can
                     guess.

                          LMDB version? stack trace from SEGV? What's the
    size of
                     the existing
                          record, what's the size of the new record? Show
    sample
                     code that
                          demonstrates the problem?

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