ITS#7971 mdb_txn_renew0(): Fix new readers slots.

commit 9a8eb95674c7b500cfe5f44d03493ff76c9fc0c1 (mdb_txn_renew0)
plus b2ab9910dd5f13ca2d92eeb4c39a90b922df8dfe (reader allocation)
from mdb.master.

diff --git a/libraries/liblmdb/mdb.c b/libraries/liblmdb/mdb.c
index 2ae60436b5bec1848216ee10340d01262e4189b9..67e0e19165bc9cf8d004acca903e1b7d33b81824 100644 (file)
--- a/libraries/liblmdb/mdb.c
+++ b/libraries/liblmdb/mdb.c
@@ -1121,7 +1121,8 @@ struct MDB_env {
        unsigned int    me_psize;       /**< DB page size, inited from me_os_psize */
        unsigned int    me_os_psize;    /**< OS page size, from #GET_PAGESIZE */
        unsigned int    me_maxreaders;  /**< size of the reader table */
-       unsigned int    me_numreaders;  /**< max numreaders set by this env */
+       /** Max #MDB_txninfo.%mti_numreaders of interest to #mdb_env_close() */
+       volatile int    me_close_readers;
        MDB_dbi         me_numdbs;              /**< number of DBs opened */
        MDB_dbi         me_maxdbs;              /**< size of the DB table */
        MDB_PID_T       me_pid;         /**< process ID of this env */
@@ -2545,15 +2546,22 @@ mdb_txn_renew0(MDB_txn *txn)
                                        UNLOCK_MUTEX_R(env);
                                        return MDB_READERS_FULL;
                                }
-                               ti->mti_readers[i].mr_pid = pid;
-                               ti->mti_readers[i].mr_tid = tid;
+                               r = &ti->mti_readers[i];
+                               /* Claim the reader slot, carefully since other code
+                                * uses the reader table un-mutexed: First reset the
+                                * slot, next publish it in mti_numreaders.  After
+                                * that, it is safe for mdb_env_close() to touch it.
+                                * When it will be closed, we can finally claim it.
+                                */
+                               r->mr_pid = 0;
+                               r->mr_txnid = (txnid_t)-1;
+                               r->mr_tid = tid;
                                if (i == nr)
                                        ti->mti_numreaders = ++nr;
-                               /* Save numreaders for un-mutexed mdb_env_close() */
-                               env->me_numreaders = nr;
+                               env->me_close_readers = nr;
+                               r->mr_pid = pid;
                                UNLOCK_MUTEX_R(env);
 
-                               r = &ti->mti_readers[i];
                                new_notls = (env->me_flags & MDB_NOTLS);
                                if (!new_notls && (rc=pthread_setspecific(env->me_txkey, r))) {
                                        r->mr_pid = 0;
@@ -4747,8 +4755,12 @@ mdb_env_close0(MDB_env *env, int excl)
                MDB_PID_T pid = env->me_pid;
                /* Clearing readers is done in this function because
                 * me_txkey with its destructor must be disabled first.
+                *
+                * We skip the the reader mutex, so we touch only
+                * data owned by this process (me_close_readers and
+                * our readers), and clear each reader atomically.
                 */
-               for (i = env->me_numreaders; --i >= 0; )
+               for (i = env->me_close_readers; --i >= 0; )
                        if (env->me_txns->mti_readers[i].mr_pid == pid)
                                env->me_txns->mti_readers[i].mr_pid = 0;
 #ifdef _WIN32
@@ -9181,11 +9193,7 @@ mdb_env_info(MDB_env *env, MDB_envinfo *arg)
        arg->me_mapaddr = env->me_metas[toggle]->mm_address;
        arg->me_mapsize = env->me_mapsize;
        arg->me_maxreaders = env->me_maxreaders;
-
-       /* me_numreaders may be zero if this process never used any readers. Use
-        * the shared numreader count if it exists.
-        */
-       arg->me_numreaders = env->me_txns ? env->me_txns->mti_numreaders : env->me_numreaders;
+       arg->me_numreaders = env->me_txns ? env->me_txns->mti_numreaders : 0;
 
        arg->me_last_pgno = env->me_metas[toggle]->mm_last_pg;
        arg->me_last_txnid = env->me_metas[toggle]->mm_txnid;