multi: fix queueing of pending easy handles

Multi handles repeatedly invert the queue of pending easy handles when
used with CURLMOPT_MAX_TOTAL_CONNECTIONS. This is caused by a multistep
process involving Curl_splaygetbest and violates the FIFO property of
the multi handle.
This patch fixes this issue by redefining the "best" node in the
context of timeouts as the "smallest not larger than now", and
implementing the necessary data structure modifications to do this
effectively, namely:
 - splay nodes with the same key are now stored in a doubly-linked
   circular list instead of a non-circular one to enable O(1)
   insertion to the tail of the list
 - Curl_splayinsert inserts nodes with the same key to the tail of
   the same list
 - in case of multiple nodes with the same key, the one on the head of
   the list gets selected

lib/splay.c

@@ -110,22 +110,17 @@ struct Curl_tree *Curl_splayinsert(struct timeval i,
     t = Curl_splay(i, t);
     if(compare(i, t->key)==0) {
       /* There already exists a node in the tree with the very same key. Build
-         a linked list of nodes. We make the new 'node' struct the new master
-         node and make the previous node the first one in the 'same' list. */
+         a doubly-linked circular list of nodes. We add the new 'node' struct
+         to the end of this list. */
 
-      node->same = t;
-      node->key = i;
-      node->smaller = t->smaller;
-      node->larger = t->larger;
-
-      t->smaller = node; /* in the sub node for this same key, we use the
-                            smaller pointer to point back to the master
-                            node */
-
-      t->key = KEY_NOTUSED; /* and we set the key in the sub node to NOTUSED
+      node->key = KEY_NOTUSED; /* we set the key in the sub node to NOTUSED
                                to quickly identify this node as a subnode */
+      node->samen = t;
+      node->samep = t->samep;
+      t->samep->samen = node;
+      t->samep = node;
 
-      return node; /* new root node */
+      return t; /* the root node always stays the same */
     }
   }
 
@@ -145,16 +140,20 @@ struct Curl_tree *Curl_splayinsert(struct timeval i,
   }
   node->key = i;
 
-  node->same = NULL; /* no identical node (yet) */
+  /* no identical nodes (yet), we are the only one in the list of nodes */
+  node->samen = node;
+  node->samep = node;
   return node;
 }
 
 /* Finds and deletes the best-fit node from the tree. Return a pointer to the
-   resulting tree.  best-fit means the node with the given or lower key */
+   resulting tree.  best-fit means the smallest node if it is not larger than
+   the key */
 struct Curl_tree *Curl_splaygetbest(struct timeval i,
-                                    struct Curl_tree *t,
-                                    struct Curl_tree **removed)
+                                       struct Curl_tree *t,
+                                       struct Curl_tree **removed)
 {
+  static struct timeval tv_zero = {0, 0};
   struct Curl_tree *x;
 
   if(!t) {
@@ -162,47 +161,36 @@ struct Curl_tree *Curl_splaygetbest(struct timeval i,
     return NULL;
   }
 
-  t = Curl_splay(i, t);
+  /* find smallest */
+  t = Curl_splay(tv_zero, t);
   if(compare(i, t->key) < 0) {
-    /* too big node, try the smaller chain */
-    if(t->smaller)
-      t=Curl_splay(t->smaller->key, t);
-    else {
-      /* fail */
-      *removed = NULL;
-      return t;
-    }
+    /* even the smallest is too big */
+    *removed = NULL;
+    return t;
   }
 
-  if(compare(i, t->key) >= 0) {               /* found it */
-    /* FIRST! Check if there is a list with identical keys */
-    x = t->same;
-    if(x) {
-      /* there is, pick one from the list */
+  /* FIRST! Check if there is a list with identical keys */
+  x = t->samen;
+  if(x != t) {
+    /* there is, pick one from the list */
 
-      /* 'x' is the new root node */
+    /* 'x' is the new root node */
 
-      x->key = t->key;
-      x->larger = t->larger;
-      x->smaller = t->smaller;
+    x->key = t->key;
+    x->larger = t->larger;
+    x->smaller = t->smaller;
+    x->samep = t->samep;
+    t->samep->samen = x;
 
-      *removed = t;
-      return x; /* new root */
-    }
-
-    if(t->smaller == NULL) {
-      x = t->larger;
-    }
-    else {
-      x = Curl_splay(i, t->smaller);
-      x->larger = t->larger;
-    }
     *removed = t;
-
-    return x;
+    return x; /* new root */
   }
-  *removed = NULL; /* no match */
-  return t;        /* It wasn't there */
+
+  /* we splayed the tree to the smallest element, there is no smaller */
+  x = t->larger;
+  *removed = t;
+
+  return x;
 }
 
 
@@ -229,19 +217,17 @@ int Curl_splayremovebyaddr(struct Curl_tree *t,
 
   if(compare(KEY_NOTUSED, removenode->key) == 0) {
     /* Key set to NOTUSED means it is a subnode within a 'same' linked list
-       and thus we can unlink it easily. The 'smaller' link of a subnode
-       links to the parent node. */
-    if(removenode->smaller == NULL)
+       and thus we can unlink it easily. */
+    if(removenode->samen == removenode)
+      /* A non-subnode should never be set to KEY_NOTUSED */
       return 3;
 
-    removenode->smaller->same = removenode->same;
-    if(removenode->same)
-      removenode->same->smaller = removenode->smaller;
+    removenode->samep->samen = removenode->samen;
+    removenode->samen->samep = removenode->samep;
 
     /* Ensures that double-remove gets caught. */
-    removenode->smaller = NULL;
+    removenode->samen = removenode;
 
-    /* voila, we're done! */
     *newroot = t; /* return the same root */
     return 0;
   }
@@ -260,14 +246,16 @@ int Curl_splayremovebyaddr(struct Curl_tree *t,
 
   /* Check if there is a list with identical sizes, as then we're trying to
      remove the root node of a list of nodes with identical keys. */
-  x = t->same;
-  if(x) {
+  x = t->samen;
+  if(x != t) {
     /* 'x' is the new root node, we just make it use the root node's
        smaller/larger links */
 
     x->key = t->key;
     x->larger = t->larger;
     x->smaller = t->smaller;
+    x->samep = t->samep;
+    t->samep->samen = x;
   }
   else {
     /* Remove the root node */

lib/splay.h

@@ -26,7 +26,8 @@
 struct Curl_tree {
   struct Curl_tree *smaller; /* smaller node */
   struct Curl_tree *larger;  /* larger node */
-  struct Curl_tree *same;    /* points to a node with identical key */
+  struct Curl_tree *samen;   /* points to the next node with identical key */
+  struct Curl_tree *samep;   /* points to the prev node with identical key */
   struct timeval key;        /* this node's "sort" key */
   void *payload;             /* data the splay code doesn't care about */
 };

tests/unit/unit1309.c

@@ -52,7 +52,7 @@ static void splayprint(struct Curl_tree * t, int d, char output)
            (long)t->key.tv_usec, i);
   }
 
-  for(count=0, node = t->same; node; node = node->same, count++)
+  for(count=0, node = t->samen; node != t; node = node->samen, count++)
     ;
 
   if(output) {