diff --git a/src/mainconf.c b/src/mainconf.c
index be25534..7817d8c 100644
--- a/src/mainconf.c
+++ b/src/mainconf.c
@@ -632,7 +632,11 @@ static int srvprepare(struct srvstart *s, int argc, char **argv){
 	if(srv->i_nsfd >= 0 && (setns(srv->i_nsfd, CLONE_NEWNET) || setns(mainnsfd, CLONE_NEWNET))) return 4;
 	if(srv->o_nsfd >= 0 && (setns(srv->o_nsfd, CLONE_NEWNET) || setns(mainnsfd, CLONE_NEWNET))) return 4;
  }
+ /* a thread which could not come back from the -ne namespace serves no
+    other client */
+ if(srv->o_nsfd >= 0) srv->threadperclient = 1;
 #endif
+ if(srv->logger && srv->logger->type && srv->logger->type->blocks) srv->threadperclient = 1;
 
 #ifndef STDMAIN
  if((error = copyfilter(conf->filters, srv))) return error;
diff --git a/src/plugins/FilePlugin/FilePlugin.c b/src/plugins/FilePlugin/FilePlugin.c
index 116c869..7562ace 100644
--- a/src/plugins/FilePlugin/FilePlugin.c
+++ b/src/plugins/FilePlugin/FilePlugin.c
@@ -767,7 +767,10 @@ static int fp_registercallback (int what, int max_size, int preview_size, struct
 }
 
 
+/* Its _prepoll writes the files and sends them: the service's clients each
+   have a thread. */
 static void * fp_open(void * idata, struct srvparam * param){
+	param->threadperclient = 1;
 	return idata;
 }
 
diff --git a/src/sql.c b/src/sql.c
index 0f0fd99..54500dd 100644
--- a/src/sql.c
+++ b/src/sql.c
@@ -132,7 +132,7 @@ static void odbcclose(struct logger *lg){
 }
 
 const struct logtype logtype_odbc = {
-    "odbc", odbcmatch, odbcinit, odbcformat, odbcwrite, odbcclose, NULL, LOGSTACKSIZE, 0, 0, 1
+    "odbc", odbcmatch, odbcinit, odbcformat, odbcwrite, odbcclose, NULL, LOGSTACKSIZE, 0, 0, 1, 1
 };
 
 #endif
diff --git a/src/structures.h b/src/structures.h
index 2ef8bf4..3289693 100644
--- a/src/structures.h
+++ b/src/structures.h
@@ -667,6 +667,7 @@ struct logtype {
 	int standard;	/* kept open for the life of the process */
 	int text;	/* writes a line of text: errors can go there */
 	int params;	/* the string after a comma is parameters */
+	int blocks;	/* waits for its writes other than in sockpoll() */
 };
 
 /* What a log's queue holds: a line (log.c's struct logslot, of which this
@@ -746,6 +747,9 @@ struct srvparam {
 	int family;
 	int resolvmode;		/* RESOLVE_* */
 	int stacksize;
+	/* a thread for each client: what serves them waits other than in
+	   sockpoll(), or must not be left by a thread to serve another */
+	int threadperclient;
 	struct extparam *config;	/* the configuration it was started with */
 	int timeouts[12];	/* conf.timeouts as they were when the service started */
 	int filtermaxsize, parentretries, maxseg;	/* and these */