Optimise readng large number --server options at start up.

When re-reading upstream servers from /etc/resolv.conf or other
sources that can change dnsmasq tries to avoid memory fragmentation by
re-using existing records that are being re-read unchanged. This
involves seaching all the server records for each new one installed.
During startup this search is pointless, and can cause long start
times with thousands of --server options because the work needed is
O(n^2). Handle this case more intelligently.  Thanks to Ye Zhou for
spotting the problem and an initial patch.

3 files changed, 49 insertions, 21 deletions

diff --git a/CHANGELOG b/CHANGELOG
index 13cedd6..0aa6511 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -49,6 +49,16 @@ version 2.88
 	in the domain it attests to be returned as unvalidated, and not
 	as a validation error.
 
+	Optimise reading large numbers of --server options. When re-reading
+	upstream servers from /etc/resolv.conf or other sources that
+	can change dnsmasq tries to avoid memory fragmentation by re-using
+	existing records that are being re-read unchanged. This involves
+	seaching all the server records for each new one installed.
+	During startup this search is pointless, and can cause long
+	start times with thousands of --server options because the work
+	needed is O(n^2). Handle this case more intelligently.
+	Thanks to Ye Zhou for spotting the problem and an initial patch.
+	
 	
 version 2.87
         Allow arbitrary prefix lengths in --rev-server and
diff --git a/src/domain-match.c b/src/domain-match.c
index 76a1109..bef460a 100644
--- a/src/domain-match.c
+++ b/src/domain-match.c
@@ -546,11 +546,23 @@ static int order_qsort(const void *a, const void *b)
   return rc;
 }
 
+
+/* When loading large numbers of server=.... lines during startup,
+   there's no possibility that there will be server records that can be reused, but
+   searching a long list for each server added grows as O(n^2) and slows things down.
+   This flag is set only if is known there may be free server records that can be reused.
+   There's a call to mark_servers(0) in read_opts() to reset the flag before
+   main config read. */
+
+static int maybe_free_servers = 0;
+
 /* Must be called before  add_update_server() to set daemon->servers_tail */
 void mark_servers(int flag)
 {
   struct server *serv, **up;
 
+  maybe_free_servers = !!flag;
+  
   daemon->servers_tail = NULL;
   
   /* mark everything with argument flag */
@@ -667,27 +679,30 @@ int add_update_server(int flags,
 	 and move to the end of the list, for order. The entry found may already
 	 be at the end. */
       struct server **up, *tmp;
-      
-      for (serv = daemon->servers, up = &daemon->servers; serv; serv = tmp)
-	{
-	  tmp = serv->next;
-	  if ((serv->flags & SERV_MARK) &&
-	      hostname_isequal(alloc_domain, serv->domain))
-	    {
-	      /* Need to move down? */
-	      if (serv->next)
-		{
-		  *up = serv->next;
-		  daemon->servers_tail->next = serv;
-		  daemon->servers_tail = serv;
-		  serv->next = NULL;
-		}
-	      break;
-	    }
-	  else
-	    up = &serv->next;
-	}
 
+      serv = NULL;
+      
+      if (maybe_free_servers)
+	for (serv = daemon->servers, up = &daemon->servers; serv; serv = tmp)
+	  {
+	    tmp = serv->next;
+	    if ((serv->flags & SERV_MARK) &&
+		hostname_isequal(alloc_domain, serv->domain))
+	      {
+		/* Need to move down? */
+		if (serv->next)
+		  {
+		    *up = serv->next;
+		    daemon->servers_tail->next = serv;
+		    daemon->servers_tail = serv;
+		    serv->next = NULL;
+		  }
+		break;
+	      }
+	    else
+	      up = &serv->next;
+	  }
+      
       if (serv)
 	{
 	  free(alloc_domain);
diff --git a/src/option.c b/src/option.c
index 371a32d..6a63268 100644
--- a/src/option.c
+++ b/src/option.c
@@ -5753,7 +5753,10 @@ void read_opts(int argc, char **argv, char *compile_opts)
 #endif
   add_txt("servers.bind", NULL, TXT_STAT_SERVERS);
 #endif
-
+  
+  /* See comment above make_servers(). Optimises server-read code. */
+  mark_servers(0);
+  
   while (1) 
     {
 #ifdef HAVE_GETOPT_LONG