Hi, I have an issue where it is not possible to open multiple reader at the same time

Open multiple reader simultanously about rust-kv HOT 2 CLOSED

zshipko commented on August 11, 2024

Open multiple reader simultanously

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Comments (2)

bytesnake commented on August 11, 2024 1

the default setting won't allow multiple txn_reader at the same time. A note in mdb_txn_begin (http://www.lmdb.tech/doc/group__mdb.html#gad7ea55da06b77513609efebd44b26920) clarifies this:

A transaction and its cursors must only be used by a single thread, and a thread may only have a single transaction at a time. If MDB_NOTLS is in use, this does not apply to read-only transactions.

I totally missed this, with CHECK(rc = mdb_env_open(env, "./testing", MDB_NOTLS, 0664)); everything works fine :)

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zshipko commented on August 11, 2024

Thanks for reporting this!

I assumed this would work too, but after looking into this it seems to be a bit more complicated than I thought.

Even this minimal C reproduction exhibits the same behavior as your example:

#include <lmdb.h>
#include <stdio.h>
#include <stdlib.h>

#define CHECK(x)                                                  \
  {                                                                             \
    int y = x;                                                               \
    if (y != 0) {                                                            \
      printf("FAIL: %d: %d\n", __LINE__, y);              \
      abort();                                                               \
    };                                                                           \
  }

int main(void) {
  int rc;
  MDB_env *env;
  MDB_txn *txn, *txn2;

  CHECK(rc = mdb_env_create(&env));

  mdb_env_set_maxreaders(env, 1024);
  CHECK(rc = mdb_env_open(env, "./testing", 0, 0664));
  CHECK(rc = mdb_txn_begin(env, NULL, MDB_RDONLY, &txn));
  CHECK(rc = mdb_txn_begin(env, NULL, MDB_RDONLY, &txn2));

  mdb_txn_abort(txn);
  mdb_txn_abort(txn2);
  mdb_env_close(env);
  return 0;
}