Resolves: bug 218076

Bug Description: Server autoconf build crashes when stopped/started very quickly
Reviewed by: nhosoi (Thanks!)
Fix Description: The problem was that 3 of the database threads were being started and stopped before the 4th had a chance to start.  So the thread count would start at 3 and drop to 0, and the dblayer_pre_close code would think everything was fine.  The 4th thread is the checkpoint thread which was doing a db_checkpoint operation before incrementing the thread count.  For some reason, on x86_64 with the system provided libdb-4.2, the checkpoint operation was taking longer than it usually does with our locally built libdb-4.2, so this allowed the other 3 threads to stop and start before the checkpoint thread had a chance to increment the thread count.
The solution is to make sure the incrementing of the thread count occurs as early as possible in the thread function, before any executable code that might take any time.  This should ensure that all of the threads start up and increment the thread count before the shutdown occurs.
The second part of the solution is that, according to wtc, the NSPR maintainer, the PR_Atomic functions should not be used as a semaphore like this.  So, the code was rewritten to use locks and condition variables.  The code is not performance critical, so adding locking should not have any impact on performance.  In addition, the new code is much cleaner, more correct, and more obvious about what it's doing.
Platforms tested: RHEL4 x86_64
Flag Day: no
Doc impact: no

ldap/servers/slapd/back-ldbm/dblayer.c

@@ -168,6 +168,28 @@
 #endif
 #endif
 
+/* Use these macros to incr/decrement the thread count for the
+   database housekeeping threads.  This ensures that the
+   value is changed in a thread safe manner, and safely notifies
+   the main thread during cleanup. INCR_THREAD_COUNT should be
+   the first real statement in the thread function, before any
+   actual work is done, other than perhaps variable assignments.
+   DECR_THREAD_COUNT should be called as the next to last thing
+   in the thread function, just before the trace log message and
+   return.
+*/
+#define INCR_THREAD_COUNT(priv) \
+    PR_Lock(priv->thread_count_lock); \
+    ++priv->dblayer_thread_count; \
+    PR_Unlock(priv->thread_count_lock)
+
+#define DECR_THREAD_COUNT(priv) \
+    PR_Lock(priv->thread_count_lock); \
+    if (--priv->dblayer_thread_count == 0) { \
+        PR_NotifyCondVar(priv->thread_count_cv); \
+    } \
+    PR_Unlock(priv->thread_count_lock)
+
 #define NEWDIR_MODE 0755
 
 
@@ -513,6 +535,8 @@ int dblayer_init(struct ldbminfo *li)
         /* Memory allocation failed */
         return -1;
     }
+    priv->thread_count_lock = PR_NewLock();
+    priv->thread_count_cv = PR_NewCondVar(priv->thread_count_lock);
     li->li_dblayer_private = priv;
 
     /* For now, we call this to get debug printout */
@@ -539,9 +563,11 @@ int dblayer_terminate(struct ldbminfo *li)
         inst = (ldbm_instance *)object_get_data(inst_obj);
         if (NULL != inst->inst_db_mutex) {
             PR_DestroyLock(inst->inst_db_mutex);
+            inst->inst_db_mutex = NULL;
         }
         if (NULL != inst->inst_handle_list_mutex) {
             PR_DestroyLock(inst->inst_handle_list_mutex);
+            inst->inst_handle_list_mutex = NULL;
         }
     }
     
@@ -549,6 +575,11 @@ int dblayer_terminate(struct ldbminfo *li)
     /* no need to release dblayer_home_directory,
      * which is one of dblayer_data_directories */
     charray_free(priv->dblayer_data_directories); 
+    priv->dblayer_data_directories = NULL;
+    PR_DestroyCondVar(priv->thread_count_cv);
+    priv->thread_count_cv = NULL;
+    PR_DestroyLock(priv->thread_count_lock);
+    priv->thread_count_lock = NULL;
     slapi_ch_free((void**)&priv);
     li->li_dblayer_private = NULL;
 
@@ -2321,6 +2352,7 @@ int dblayer_instance_close(backend *be)
 void dblayer_pre_close(struct ldbminfo *li)
 {
     dblayer_private *priv = 0;
+    PRInt32 threadcount = 0;
 
     PR_ASSERT(NULL != li);
     priv = (dblayer_private*)li->li_dblayer_private;
@@ -2328,48 +2360,56 @@ void dblayer_pre_close(struct ldbminfo *li)
     if (priv->dblayer_stop_threads)    /* already stopped.  do nothing... */
         return;
 
-    /* Now stop the make sure they've stopped,
-     * the various threads we have running */
-    priv->dblayer_stop_threads = 1;
-    if (priv->dblayer_thread_count > 0) {
-        int sleep_counter = 0;
+    /* first, see if there are any housekeeping threads running */
+    PR_Lock(priv->thread_count_lock);
+    threadcount = priv->dblayer_thread_count;
+    PR_Unlock(priv->thread_count_lock);
+
+    if (threadcount) {
+        PRIntervalTime cvwaittime = PR_MillisecondsToInterval(DBLAYER_SLEEP_INTERVAL * 100);
+        int timedout = 0;
         /* Print handy-dandy log message */
         LDAPDebug(LDAP_DEBUG_ANY,"Waiting for %d database threads to stop\n",
-                  priv->dblayer_thread_count, 0,0);
-        while(priv->dblayer_thread_count > 0) {
-            /* We have no alternative here but to wait for them to finish,
-             * since if any thread activity,
-             * such as a checkpoint, wasn't finished when we shut down,
-             * the database would need recovery on startup */
-            PRIntervalTime    interval;
-
-            sleep_counter++;
-            if (0 == sleep_counter % 10) {
-                if (priv->dblayer_thread_count > 1) {
-                    LDAPDebug(LDAP_DEBUG_ANY,
-                              "Still waiting on %d database threads...\n",
-                              priv->dblayer_thread_count,0,0);
-                } else {
-                    LDAPDebug(LDAP_DEBUG_ANY,
-                              "Still waiting one database thread...\n",0,0,0);
+                  threadcount, 0,0);
+        PR_Lock(priv->thread_count_lock);
+        /* Tell them to stop - we wait until the last possible moment to invoke
+           this.  If we do this much sooner than this, we could find ourselves
+           in a situation where the threads see the stop_threads and exit before
+           we can issue the WaitCondVar below, which means the last thread to
+           exit will do a NotifyCondVar that has nothing waiting.  If we do this
+           inside the lock, we will ensure that the threads will block until we
+           issue the WaitCondVar below */
+        priv->dblayer_stop_threads = 1;
+        /* Wait for them to exit */
+        while (priv->dblayer_thread_count > 0) {
+            PRIntervalTime before = PR_IntervalNow();
+            /* There are 3 ways to wake up from this WaitCondVar:
+               1) The last database thread exits and calls NotifyCondVar - thread_count
+               should be 0 in this case
+               2) Timeout - in this case, thread_count will be > 0 - bad
+               3) A bad error occurs - bad - will be reported as a timeout
+            */
+            PR_WaitCondVar(priv->thread_count_cv, cvwaittime);
+            if (priv->dblayer_thread_count > 0) {
+                /* still at least 1 thread running - see if this is a timeout */
+                if ((PR_IntervalNow() - before) >= cvwaittime) {
+                    threadcount = priv->dblayer_thread_count;
+                    timedout = 1;
+                    break;
                 }
+                /* else just a spurious interrupt */
             }
-
-            interval = PR_MillisecondsToInterval(DBLAYER_SLEEP_INTERVAL);
-            DS_Sleep(interval);
-            if (sleep_counter >= 100) {
-                LDAPDebug(LDAP_DEBUG_ANY,"Timeout; leave %d thread(s)...\n",
-                          priv->dblayer_thread_count,0,0);
-                /*
-                 * The guardian file should not
-                 * get created under the timeout condition.
-                 */
-                priv->dblayer_bad_stuff_happened = 1;
-                goto timeout_escape;
-            }
         }
-        LDAPDebug(LDAP_DEBUG_ANY,"All database threads now stopped\n",0,0,0);
+        PR_Unlock(priv->thread_count_lock); 
+        if (timedout) {
+            LDAPDebug(LDAP_DEBUG_ANY,
+                      "Timeout after [%d] milliseconds; leave %d database thread(s)...\n",
+                      (DBLAYER_SLEEP_INTERVAL * 100), threadcount,0);
+            priv->dblayer_bad_stuff_happened = 1;
+            goto timeout_escape;
+        }
     }
+    LDAPDebug(LDAP_DEBUG_ANY,"All database threads now stopped\n",0,0,0);
 timeout_escape:
     return;
 }
@@ -3211,12 +3251,16 @@ static int perf_threadmain(void *param)
 
     priv = (dblayer_private*)li->li_dblayer_private;
     PR_ASSERT(NULL != priv);
-    PR_AtomicIncrement(&(priv->dblayer_thread_count));
+
+    INCR_THREAD_COUNT(priv);
+
     while (!priv->dblayer_stop_threads) {
         /* sleep for a while, updating perf counters if we need to */
         perfctrs_wait(1000,priv->perf_private,priv->dblayer_env->dblayer_DB_ENV);
     }
-    PR_AtomicDecrement(&(priv->dblayer_thread_count));
+	
+    DECR_THREAD_COUNT(priv);
+    LDAPDebug(LDAP_DEBUG_TRACE, "Leaving perf_threadmain\n", 0, 0, 0);
     return 0;
 }
 
@@ -3257,8 +3301,9 @@ static int deadlock_threadmain(void *param)
     priv = (dblayer_private*)li->li_dblayer_private;
     PR_ASSERT(NULL != priv);
 
+    INCR_THREAD_COUNT(priv);
+
     interval = PR_MillisecondsToInterval(100);
-    PR_AtomicIncrement(&(priv->dblayer_thread_count));
     while (!priv->dblayer_stop_threads)
     {
         if (priv->dblayer_enable_transactions) 
@@ -3278,7 +3323,9 @@ static int deadlock_threadmain(void *param)
         }
         DS_Sleep(interval);
     }
-    PR_AtomicDecrement(&(priv->dblayer_thread_count));
+
+    DECR_THREAD_COUNT(priv);
+    LDAPDebug(LDAP_DEBUG_TRACE, "Leaving deadlock_threadmain\n", 0, 0, 0);
     return 0;
 }
 
@@ -3325,13 +3372,13 @@ static int log_flush_threadmain(void *param)
 {
     dblayer_private *priv = NULL;
     PRIntervalTime    interval;
-        
 
     PR_ASSERT(NULL != param);
     priv = (dblayer_private *) param;
-    PR_ASSERT(NULL != priv);
+
+    INCR_THREAD_COUNT(priv);
+
     interval = PR_MillisecondsToInterval(300);
-    PR_AtomicIncrement(&(priv->dblayer_thread_count));
     while ((!priv->dblayer_stop_threads) && (log_flush_thread))
     {      
         if (priv->dblayer_enable_transactions) 
@@ -3347,7 +3394,9 @@ static int log_flush_threadmain(void *param)
         }
         DS_Sleep(interval);
     }
-    PR_AtomicDecrement(&(priv->dblayer_thread_count));
+
+    DECR_THREAD_COUNT(priv);
+    LDAPDebug(LDAP_DEBUG_TRACE, "Leaving log_flush_threadmain\n", 0, 0, 0);
     return 0;
 }
 
@@ -3377,7 +3426,7 @@ dblayer_start_checkpoint_thread(struct ldbminfo *li)
 static int checkpoint_threadmain(void *param)
 {
     time_t time_of_last_checkpoint_completion = 0;    /* seconds since epoch */
-    PRIntervalTime    interval;   /*NSPR timeout stuffy*/
+    PRIntervalTime    interval;
     int rval = -1;
     dblayer_private *priv = NULL;
     struct ldbminfo *li = NULL;
@@ -3388,24 +3437,25 @@ static int checkpoint_threadmain(void *param)
     PR_ASSERT(NULL != param);
     li = (struct ldbminfo*)param;
 
+    priv = (dblayer_private*)li->li_dblayer_private;
+    PR_ASSERT(NULL != priv);
+
+    INCR_THREAD_COUNT(priv);
+
+    interval = PR_MillisecondsToInterval(DBLAYER_SLEEP_INTERVAL);
     home_dir = dblayer_get_home_dir(li, NULL);
     if (NULL == home_dir)
     {
         LDAPDebug(LDAP_DEBUG_ANY, 
             "Checkpoint thread failed due to missing db home directory info\n",
             0, 0, 0);
-        return rval;
+        goto error_return;
     }
 
-    priv = (dblayer_private*)li->li_dblayer_private;
-    PR_ASSERT(NULL != priv);
-
     /* work around a problem with newly created environments */
     dblayer_force_checkpoint(li);
 
-    interval = PR_MillisecondsToInterval(DBLAYER_SLEEP_INTERVAL);
     debug_checkpointing = priv->db_debug_checkpointing;
-    PR_AtomicIncrement(&(priv->dblayer_thread_count));
     /* assumes dblayer_force_checkpoint worked */
     time_of_last_checkpoint_completion = current_time();
     while (!priv->dblayer_stop_threads)
@@ -3452,7 +3502,7 @@ static int checkpoint_threadmain(void *param)
                     "err=%d (%s)\n", rval, dblayer_strerror(rval), 0);
                 if (LDBM_OS_ERR_IS_DISKFULL(rval)) {
                     operation_out_of_disk_space();
-                    goto diskfull_return;
+                    goto error_return;
                 }
             } else {
                 time_of_last_checkpoint_completion = current_time();
@@ -3479,7 +3529,7 @@ static int checkpoint_threadmain(void *param)
                     "err=%d (%s)\n", rval, dblayer_strerror(rval), 0);
                 if (LDBM_OS_ERR_IS_DISKFULL(rval)) {
                     operation_out_of_disk_space();
-                    goto diskfull_return;
+                    goto error_return;
                 }
             } else {
                 time_of_last_checkpoint_completion = current_time();
@@ -3532,10 +3582,13 @@ static int checkpoint_threadmain(void *param)
             }
         }
     }
-    dblayer_force_checkpoint(li);
-diskfull_return: 
-    PR_AtomicDecrement(&(priv->dblayer_thread_count));
-    return 0;
+    LDAPDebug(LDAP_DEBUG_TRACE, "Leaving checkpoint_threadmain before checkpoint\n", 0, 0, 0);
+    rval = dblayer_force_checkpoint(li);
+error_return:
+
+    DECR_THREAD_COUNT(priv);
+    LDAPDebug(LDAP_DEBUG_TRACE, "Leaving checkpoint_threadmain\n", 0, 0, 0);
+    return rval;
 }
 
 /*
@@ -3574,9 +3627,10 @@ static int trickle_threadmain(void *param)
     priv = (dblayer_private*)li->li_dblayer_private;
     PR_ASSERT(NULL != priv);
 
+    INCR_THREAD_COUNT(priv);
+
     interval = PR_MillisecondsToInterval(DBLAYER_SLEEP_INTERVAL);
     debug_checkpointing = priv->db_debug_checkpointing;
-    PR_AtomicIncrement(&(priv->dblayer_thread_count));
     while (!priv->dblayer_stop_threads)
     {
         DS_Sleep(interval);   /* 622855: wait for other threads fully started */
@@ -3600,7 +3654,9 @@ static int trickle_threadmain(void *param)
             }
         }
     }
-    PR_AtomicDecrement(&(priv->dblayer_thread_count));
+
+    DECR_THREAD_COUNT(priv);
+    LDAPDebug(LDAP_DEBUG_TRACE, "Leaving trickle_threadmain priv\n", 0, 0, 0);
     return 0;
 }
 

ldap/servers/slapd/back-ldbm/dblayer.h

@@ -166,6 +166,8 @@ struct dblayer_private
     PRInt32 dblayer_thread_count;   /* Tells us how many threads are running,
                                      * used to figure out when they're all
                                      * stopped */
+    PRLock *thread_count_lock;      /* lock for thread_count_cv */
+    PRCondVar *thread_count_cv;     /* condition variable for housekeeping thread shutdown */
     int dblayer_lockdown;           /* use DB_LOCKDOWN */
     int dblayer_lock_config;
 };