Fix clamd hang when number of multiscan commands == MaxThreads (bb #1700).

Each multiscan command needs 1 control thread, and 1 (possibly shared) scanner
thread.
If the number of multiscan control threads would be equal to MaxThreads no
progress could be made, since each one would be waiting for 1 free scanner
thread (but none can be spawned since maxthreads was already reached).
As long as there is 1 thread free for scanning, we can have N-1 multiscan
commands active, with N MaxThreads.

clamd/session.c

@@ -220,7 +220,27 @@ int command(client_conn_t *conn, int *virus)
 	    thrmgr_setactivetask(NULL, "CONTSCAN");
 	    type = TYPE_CONTSCAN;
 	    break;
-	case COMMAND_MULTISCAN:
+	case COMMAND_MULTISCAN: {
+	    int multiscan, max, alive;
+	    pthread_mutex_lock(&conn->thrpool->pool_mutex);
+	    multiscan = conn->thrpool->thr_multiscan;
+	    max = conn->thrpool->thr_max;
+	    if (multiscan+1 < max)
+		conn->thrpool->thr_multiscan = multiscan+1;
+	    else {
+		alive = conn->thrpool->thr_alive;
+		ret = -1;
+	    }
+	    pthread_mutex_unlock(&conn->thrpool->pool_mutex);
+	    if (ret) {
+		// multiscan has 1 control thread, so there needs to be at least
+		// 1 threads that is a non-multiscan controlthread to scan and
+		// make progress.
+		logg("^Not enough threads for multiscan. Max: %d, Alive: %d, Multiscan: %d+1\n",
+		     max, alive, multiscan);
+		conn_reply(conn, conn->filename, "Not enough threads for multiscan. Increase MaxThreads.","ERROR");
+		return 1;
+	    }
 	    flags &= ~CLI_FTW_NEED_STAT;
 	    thrmgr_setactivetask(NULL, "MULTISCAN");
 	    type = TYPE_MULTISCAN;
@@ -228,6 +248,7 @@ int command(client_conn_t *conn, int *virus)
 	    if (!group)
 		return CL_EMEM;
 	    break;
+	    }
 	case COMMAND_MULTISCANFILE:
 	    thrmgr_setactivetask(NULL, "MULTISCANFILE");
 	    scandata.group = NULL;
@@ -324,6 +345,9 @@ int command(client_conn_t *conn, int *virus)
 	    return -1;
     if (scandata.group && conn->cmdtype == COMMAND_MULTISCAN) {
 	thrmgr_group_waitforall(group, &ok, &error, &total);
+	pthread_mutex_lock(&conn->thrpool->pool_mutex);
+	conn->thrpool->thr_multiscan--;
+	pthread_mutex_unlock(&conn->thrpool->pool_mutex);
     } else {
 	error = scandata.errors;
 	total = scandata.total;

clamd/thrmgr.c

@@ -387,6 +387,7 @@ threadpool_t *thrmgr_new(int max_threads, int idle_timeout, int max_queue, void
 	threadpool->thr_max = max_threads;
 	threadpool->thr_alive = 0;
 	threadpool->thr_idle = 0;
+	threadpool->thr_multiscan = 0;
 	threadpool->idle_timeout = idle_timeout;
 	threadpool->handler = handler;
 	threadpool->tasks = NULL;

clamd/thrmgr.h

@@ -69,6 +69,7 @@ typedef struct threadpool_tag {
 	int queue_max;
 	int thr_alive;
 	int thr_idle;
+	int thr_multiscan;
 	int idle_timeout;
 	struct task_desc *tasks;