multi: remove PENDING + MSGSENT handles from the main linked list

As they are not driving transfers or any socket activity, the main loop
does not need to iterate over these handles. A performance improvement.

They are instead only held in their own separate lists.

Assisted-by: Stefan Eissing
Ref: #10743
Closes #10762

lib/multi.c

@@ -383,12 +383,10 @@ static void sh_init(struct Curl_hash *hash, int hashsize)
  * Called when a transfer is completed. Adds the given msg pointer to
  * the list kept in the multi handle.
  */
-static CURLMcode multi_addmsg(struct Curl_multi *multi,
-                              struct Curl_message *msg)
+static void multi_addmsg(struct Curl_multi *multi, struct Curl_message *msg)
 {
   Curl_llist_insert_next(&multi->msglist, multi->msglist.tail, msg,
                          &msg->list);
-  return CURLM_OK;
 }
 
 struct Curl_multi *Curl_multi_handle(int hashsize, /* socket hash */
@@ -411,6 +409,7 @@ struct Curl_multi *Curl_multi_handle(int hashsize, /* socket hash */
 
   Curl_llist_init(&multi->msglist, NULL);
   Curl_llist_init(&multi->pending, NULL);
+  Curl_llist_init(&multi->msgsent, NULL);
 
   multi->multiplexing = TRUE;
 
@@ -456,6 +455,14 @@ struct Curl_multi *curl_multi_init(void)
                            CURL_DNS_HASH_SIZE);
 }
 
+/* returns TRUE if the easy handle is supposed to be present in the main link
+   list */
+static bool in_main_list(struct Curl_easy *data)
+{
+  return ((data->mstate != MSTATE_PENDING) &&
+          (data->mstate != MSTATE_MSGSENT));
+}
+
 static void link_easy(struct Curl_multi *multi,
                       struct Curl_easy *data)
 {
@@ -489,6 +496,8 @@ static void unlink_easy(struct Curl_multi *multi,
     data->next->prev = data->prev;
   else
     multi->easylp = data->prev; /* point to last node */
+
+  data->prev = data->next = NULL;
 }
 
 
@@ -848,10 +857,16 @@ CURLMcode curl_multi_remove_handle(struct Curl_multi *multi,
      called. Do it after multi_done() in case that sets another time! */
   Curl_expire_clear(data);
 
-  if(data->connect_queue.ptr)
-    /* the handle was in the pending list waiting for an available connection,
-       so go ahead and remove it */
-    Curl_llist_remove(&multi->pending, &data->connect_queue, NULL);
+  if(data->connect_queue.ptr) {
+    /* the handle is in the pending or msgsent lists, so go ahead and remove
+       it */
+    if(data->mstate == MSTATE_PENDING)
+      Curl_llist_remove(&multi->pending, &data->connect_queue, NULL);
+    else
+      Curl_llist_remove(&multi->msgsent, &data->connect_queue, NULL);
+  }
+  if(in_main_list(data))
+    unlink_easy(multi, data);
 
   if(data->dns.hostcachetype == HCACHE_MULTI) {
     /* stop using the multi handle's DNS cache, *after* the possible
@@ -912,7 +927,6 @@ CURLMcode curl_multi_remove_handle(struct Curl_multi *multi,
 
   /* make sure there's no pending message in the queue sent from this easy
      handle */
-
   for(e = multi->msglist.head; e; e = e->next) {
     struct Curl_message *msg = e->ptr;
 
@@ -923,19 +937,6 @@ CURLMcode curl_multi_remove_handle(struct Curl_multi *multi,
     }
   }
 
-  /* Remove from the pending list if it is there. Otherwise this will
-     remain on the pending list forever due to the state change. */
-  for(e = multi->pending.head; e; e = e->next) {
-    struct Curl_easy *curr_data = e->ptr;
-
-    if(curr_data == data) {
-      Curl_llist_remove(&multi->pending, e, NULL);
-      break;
-    }
-  }
-
-  unlink_easy(multi, data);
-
   /* NOTE NOTE NOTE
      We do not touch the easy handle here! */
   multi->num_easy--; /* one less to care about now */
@@ -1943,11 +1944,6 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
       }
       break;
 
-    case MSTATE_PENDING:
-      /* We will stay here until there is a connection available. Then
-         we try again in the MSTATE_CONNECT state. */
-      break;
-
     case MSTATE_CONNECT:
       /* Connect. We want to get a connection identifier filled in. */
       /* init this transfer. */
@@ -1971,6 +1967,8 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
         /* add this handle to the list of connect-pending handles */
         Curl_llist_insert_next(&multi->pending, multi->pending.tail, data,
                                &data->connect_queue);
+        /* unlink from the main list */
+        unlink_easy(multi, data);
         result = CURLE_OK;
         break;
       }
@@ -2597,9 +2595,11 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
     case MSTATE_COMPLETED:
       break;
 
+    case MSTATE_PENDING:
     case MSTATE_MSGSENT:
-      data->result = result;
-      return CURLM_OK; /* do nothing */
+      /* handles in these states should NOT be in this list */
+      DEBUGASSERT(0);
+      break;
 
     default:
       return CURLM_INTERNAL_ERROR;
@@ -2687,10 +2687,17 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
         msg->extmsg.easy_handle = data;
         msg->extmsg.data.result = result;
 
-        rc = multi_addmsg(multi, msg);
+        multi_addmsg(multi, msg);
         DEBUGASSERT(!data->conn);
       }
       multistate(data, MSTATE_MSGSENT);
+
+      /* add this handle to the list of msgsent handles */
+      Curl_llist_insert_next(&multi->msgsent, multi->msgsent.tail, data,
+                             &data->connect_queue);
+      /* unlink from the main list */
+      unlink_easy(multi, data);
+      return CURLM_OK;
     }
   } while((rc == CURLM_CALL_MULTI_PERFORM) || multi_ischanged(multi, FALSE));
 
@@ -2721,6 +2728,9 @@ CURLMcode curl_multi_perform(struct Curl_multi *multi, int *running_handles)
     /* Do the loop and only alter the signal ignore state if the next handle
        has a different NO_SIGNAL state than the previous */
     do {
+      /* the current node might be unlinked in multi_runsingle(), get the next
+         pointer now */
+      struct Curl_easy *datanext = data->next;
       if(data->set.no_signal != nosig) {
         sigpipe_restore(&pipe_st);
         sigpipe_ignore(data, &pipe_st);
@@ -2729,7 +2739,7 @@ CURLMcode curl_multi_perform(struct Curl_multi *multi, int *running_handles)
       result = multi_runsingle(multi, &now, data);
       if(result)
         returncode = result;
-      data = data->next; /* operate on next handle */
+      data = datanext; /* operate on next handle */
     } while(data);
     sigpipe_restore(&pipe_st);
   }
@@ -3710,6 +3720,9 @@ static void process_pending_handles(struct Curl_multi *multi)
 
     DEBUGASSERT(data->mstate == MSTATE_PENDING);
 
+    /* put it back into the main list */
+    link_easy(multi, data);
+
     multistate(data, MSTATE_CONNECT);
 
     /* Remove this node from the list */

lib/multihandle.h

@@ -101,6 +101,8 @@ struct Curl_multi {
 
   struct Curl_llist pending; /* Curl_easys that are in the
                                 MSTATE_PENDING state */
+  struct Curl_llist msgsent; /* Curl_easys that are in the
+                                MSTATE_MSGSENT state */
 
   /* callback function and user data pointer for the *socket() API */
   curl_socket_callback socket_cb;

lib/urldata.h

@@ -1894,7 +1894,8 @@ struct Curl_easy {
   struct Curl_easy *prev;
 
   struct connectdata *conn;
-  struct Curl_llist_element connect_queue;
+  struct Curl_llist_element connect_queue; /* for the pending and msgsent
+                                              lists */
   struct Curl_llist_element conn_queue; /* list per connectdata */
 
   CURLMstate mstate;  /* the handle's state */