conncache: various concept cleanups

More connection cache accesses are protected by locks.

CONNCACHE_* is a beter prefix for the connection cache lock macros.

Curl_attach_connnection: now called as soon as there's a connection
struct available and before the connection is added to the connection
cache.

Curl_disconnect: now assumes that the connection is already removed from
the connection cache.

Ref: #4915
Closes #5009

10 files changed, 139 insertions, 127 deletions

diff --git a/lib/conncache.c b/lib/conncache.c
index cbd3bb1bb..95fcea6f3 100644
--- a/lib/conncache.c
+++ b/lib/conncache.c
@@ -49,53 +49,51 @@ static void conn_llist_dtor(void *user, void *element)
   conn->bundle = NULL;
 }
 
-static CURLcode bundle_create(struct Curl_easy *data,
-                              struct connectbundle **cb_ptr)
+static CURLcode bundle_create(struct connectbundle **bundlep)
 {
-  (void)data;
-  DEBUGASSERT(*cb_ptr == NULL);
-  *cb_ptr = malloc(sizeof(struct connectbundle));
-  if(!*cb_ptr)
+  DEBUGASSERT(*bundlep == NULL);
+  *bundlep = malloc(sizeof(struct connectbundle));
+  if(!*bundlep)
     return CURLE_OUT_OF_MEMORY;
 
-  (*cb_ptr)->num_connections = 0;
-  (*cb_ptr)->multiuse = BUNDLE_UNKNOWN;
+  (*bundlep)->num_connections = 0;
+  (*bundlep)->multiuse = BUNDLE_UNKNOWN;
 
-  Curl_llist_init(&(*cb_ptr)->conn_list, (curl_llist_dtor) conn_llist_dtor);
+  Curl_llist_init(&(*bundlep)->conn_list, (curl_llist_dtor) conn_llist_dtor);
   return CURLE_OK;
 }
 
-static void bundle_destroy(struct connectbundle *cb_ptr)
+static void bundle_destroy(struct connectbundle *bundle)
 {
-  if(!cb_ptr)
+  if(!bundle)
     return;
 
-  Curl_llist_destroy(&cb_ptr->conn_list, NULL);
+  Curl_llist_destroy(&bundle->conn_list, NULL);
 
-  free(cb_ptr);
+  free(bundle);
 }
 
 /* Add a connection to a bundle */
-static void bundle_add_conn(struct connectbundle *cb_ptr,
+static void bundle_add_conn(struct connectbundle *bundle,
                             struct connectdata *conn)
 {
-  Curl_llist_insert_next(&cb_ptr->conn_list, cb_ptr->conn_list.tail, conn,
+  Curl_llist_insert_next(&bundle->conn_list, bundle->conn_list.tail, conn,
                          &conn->bundle_node);
-  conn->bundle = cb_ptr;
-  cb_ptr->num_connections++;
+  conn->bundle = bundle;
+  bundle->num_connections++;
 }
 
 /* Remove a connection from a bundle */
-static int bundle_remove_conn(struct connectbundle *cb_ptr,
+static int bundle_remove_conn(struct connectbundle *bundle,
                               struct connectdata *conn)
 {
   struct curl_llist_element *curr;
 
-  curr = cb_ptr->conn_list.head;
+  curr = bundle->conn_list.head;
   while(curr) {
     if(curr->ptr == conn) {
-      Curl_llist_remove(&cb_ptr->conn_list, curr, NULL);
-      cb_ptr->num_connections--;
+      Curl_llist_remove(&bundle->conn_list, curr, NULL);
+      bundle->num_connections--;
       conn->bundle = NULL;
       return 1; /* we removed a handle */
     }
@@ -162,20 +160,15 @@ static void hashkey(struct connectdata *conn, char *buf,
   msnprintf(buf, len, "%ld%s", port, hostname);
 }
 
-void Curl_conncache_unlock(struct Curl_easy *data)
-{
-  CONN_UNLOCK(data);
-}
-
 /* Returns number of connections currently held in the connection cache.
    Locks/unlocks the cache itself!
 */
 size_t Curl_conncache_size(struct Curl_easy *data)
 {
   size_t num;
-  CONN_LOCK(data);
+  CONNCACHE_LOCK(data);
   num = data->state.conn_cache->num_conn;
-  CONN_UNLOCK(data);
+  CONNCACHE_UNLOCK(data);
   return num;
 }
 
@@ -188,7 +181,7 @@ struct connectbundle *Curl_conncache_find_bundle(struct connectdata *conn,
                                                  const char **hostp)
 {
   struct connectbundle *bundle = NULL;
-  CONN_LOCK(conn->data);
+  CONNCACHE_LOCK(conn->data);
   if(connc) {
     char key[HASHKEY_SIZE];
     hashkey(conn, key, sizeof(key), hostp);
@@ -235,8 +228,7 @@ CURLcode Curl_conncache_add_conn(struct conncache *connc,
                                  struct connectdata *conn)
 {
   CURLcode result = CURLE_OK;
-  struct connectbundle *bundle;
-  struct connectbundle *new_bundle = NULL;
+  struct connectbundle *bundle = NULL;
   struct Curl_easy *data = conn->data;
 
   /* *find_bundle() locks the connection cache */
@@ -245,20 +237,19 @@ CURLcode Curl_conncache_add_conn(struct conncache *connc,
     int rc;
     char key[HASHKEY_SIZE];
 
-    result = bundle_create(data, &new_bundle);
+    result = bundle_create(&bundle);
     if(result) {
       goto unlock;
     }
 
     hashkey(conn, key, sizeof(key), NULL);
-    rc = conncache_add_bundle(data->state.conn_cache, key, new_bundle);
+    rc = conncache_add_bundle(data->state.conn_cache, key, bundle);
 
     if(!rc) {
-      bundle_destroy(new_bundle);
+      bundle_destroy(bundle);
       result = CURLE_OUT_OF_MEMORY;
       goto unlock;
     }
-    bundle = new_bundle;
   }
 
   bundle_add_conn(bundle, conn);
@@ -270,15 +261,17 @@ CURLcode Curl_conncache_add_conn(struct conncache *connc,
                conn->connection_id, connc->num_conn));
 
   unlock:
-  CONN_UNLOCK(data);
+  CONNCACHE_UNLOCK(data);
 
   return result;
 }
 
 /*
- * Removes the connectdata object from the connection cache *and* clears the
- * ->data pointer association. Pass TRUE/FALSE in the 'lock' argument
- * depending on if the parent function already holds the lock or not.
+ * Removes the connectdata object from the connection cache, but does *not*
+ * clear the conn->data association. The transfer still owns this connection.
+ *
+ * Pass TRUE/FALSE in the 'lock' argument depending on if the parent function
+ * already holds the lock or not.
  */
 void Curl_conncache_remove_conn(struct Curl_easy *data,
                                 struct connectdata *conn, bool lock)
@@ -290,7 +283,7 @@ void Curl_conncache_remove_conn(struct Curl_easy *data,
      due to a failed connection attempt, before being added to a bundle */
   if(bundle) {
     if(lock) {
-      CONN_LOCK(data);
+      CONNCACHE_LOCK(data);
     }
     bundle_remove_conn(bundle, conn);
     if(bundle->num_connections == 0)
@@ -301,9 +294,8 @@ void Curl_conncache_remove_conn(struct Curl_easy *data,
       DEBUGF(infof(data, "The cache now contains %zu members\n",
                    connc->num_conn));
     }
-    conn->data = NULL; /* clear the association */
     if(lock) {
-      CONN_UNLOCK(data);
+      CONNCACHE_UNLOCK(data);
     }
   }
 }
@@ -332,7 +324,7 @@ bool Curl_conncache_foreach(struct Curl_easy *data,
   if(!connc)
     return FALSE;
 
-  CONN_LOCK(data);
+  CONNCACHE_LOCK(data);
   Curl_hash_start_iterate(&connc->hash, &iter);
 
   he = Curl_hash_next_element(&iter);
@@ -350,12 +342,12 @@ bool Curl_conncache_foreach(struct Curl_easy *data,
       curr = curr->next;
 
       if(1 == func(conn, param)) {
-        CONN_UNLOCK(data);
+        CONNCACHE_UNLOCK(data);
         return TRUE;
       }
     }
   }
-  CONN_UNLOCK(data);
+  CONNCACHE_UNLOCK(data);
   return FALSE;
 }
 
@@ -494,7 +486,7 @@ Curl_conncache_extract_oldest(struct Curl_easy *data)
 
   now = Curl_now();
 
-  CONN_LOCK(data);
+  CONNCACHE_LOCK(data);
   Curl_hash_start_iterate(&connc->hash, &iter);
 
   he = Curl_hash_next_element(&iter);
@@ -531,7 +523,7 @@ Curl_conncache_extract_oldest(struct Curl_easy *data)
                  connc->num_conn));
     conn_candidate->data = data; /* associate! */
   }
-  CONN_UNLOCK(data);
+  CONNCACHE_UNLOCK(data);
 
   return conn_candidate;
 }
@@ -548,6 +540,7 @@ void Curl_conncache_close_all_connections(struct conncache *connc)
     sigpipe_ignore(conn->data, &pipe_st);
     /* This will remove the connection from the cache */
     connclose(conn, "kill all");
+    Curl_conncache_remove_conn(conn->data, conn, TRUE);
     (void)Curl_disconnect(connc->closure_handle, conn, FALSE);
     sigpipe_restore(&pipe_st);
 
diff --git a/lib/conncache.h b/lib/conncache.h
index e3e4c9c28..3dda21cdc 100644
--- a/lib/conncache.h
+++ b/lib/conncache.h
@@ -45,21 +45,21 @@ struct conncache {
 #ifdef CURLDEBUG
 /* the debug versions of these macros make extra certain that the lock is
    never doubly locked or unlocked */
-#define CONN_LOCK(x) if((x)->share) {                                   \
+#define CONNCACHE_LOCK(x) if((x)->share) {                              \
     Curl_share_lock((x), CURL_LOCK_DATA_CONNECT, CURL_LOCK_ACCESS_SINGLE); \
     DEBUGASSERT(!(x)->state.conncache_lock);                            \
     (x)->state.conncache_lock = TRUE;                                   \
   }
 
-#define CONN_UNLOCK(x) if((x)->share) {                                 \
+#define CONNCACHE_UNLOCK(x) if((x)->share) {                            \
     DEBUGASSERT((x)->state.conncache_lock);                             \
     (x)->state.conncache_lock = FALSE;                                  \
     Curl_share_unlock((x), CURL_LOCK_DATA_CONNECT);                     \
   }
 #else
-#define CONN_LOCK(x) if((x)->share)                                     \
+#define CONNCACHE_LOCK(x) if((x)->share)                                \
     Curl_share_lock((x), CURL_LOCK_DATA_CONNECT, CURL_LOCK_ACCESS_SINGLE)
-#define CONN_UNLOCK(x) if((x)->share)                   \
+#define CONNCACHE_UNLOCK(x) if((x)->share)              \
     Curl_share_unlock((x), CURL_LOCK_DATA_CONNECT)
 #endif
 
@@ -77,7 +77,6 @@ void Curl_conncache_destroy(struct conncache *connc);
 struct connectbundle *Curl_conncache_find_bundle(struct connectdata *conn,
                                                  struct conncache *connc,
                                                  const char **hostp);
-void Curl_conncache_unlock(struct Curl_easy *data);
 /* returns number of connections currently held in the connection cache */
 size_t Curl_conncache_size(struct Curl_easy *data);
 
diff --git a/lib/hostip.c b/lib/hostip.c
index c0feb79fb..f5bb6348a 100644
--- a/lib/hostip.c
+++ b/lib/hostip.c
@@ -1085,10 +1085,12 @@ CURLcode Curl_once_resolved(struct connectdata *conn,
 
   result = Curl_setup_conn(conn, protocol_done);
 
-  if(result)
-    /* We're not allowed to return failure with memory left allocated
-       in the connectdata struct, free those here */
-    Curl_disconnect(conn->data, conn, TRUE); /* close the connection */
-
+  if(result) {
+    struct Curl_easy *data = conn->data;
+    DEBUGASSERT(data);
+    Curl_detach_connnection(data);
+    Curl_conncache_remove_conn(data, conn, TRUE);
+    Curl_disconnect(data, conn, TRUE);
+  }
   return result;
 }
diff --git a/lib/http_negotiate.h b/lib/http_negotiate.h
index 4f0ac1686..a737f6f78 100644
--- a/lib/http_negotiate.h
+++ b/lib/http_negotiate.h
@@ -7,7 +7,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -33,6 +33,8 @@ CURLcode Curl_output_negotiate(struct connectdata *conn, bool proxy);
 
 void Curl_http_auth_cleanup_negotiate(struct connectdata *conn);
 
-#endif /* !CURL_DISABLE_HTTP && USE_SPNEGO */
+#else /* !CURL_DISABLE_HTTP && USE_SPNEGO */
+#define Curl_http_auth_cleanup_negotiate(x)
+#endif
 
 #endif /* HEADER_CURL_HTTP_NEGOTIATE_H */
diff --git a/lib/http_ntlm.h b/lib/http_ntlm.h
index 003714dbd..3ebdf979e 100644
--- a/lib/http_ntlm.h
+++ b/lib/http_ntlm.h
@@ -7,7 +7,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -35,6 +35,8 @@ CURLcode Curl_output_ntlm(struct connectdata *conn, bool proxy);
 
 void Curl_http_auth_cleanup_ntlm(struct connectdata *conn);
 
-#endif /* !CURL_DISABLE_HTTP && USE_NTLM */
+#else /* !CURL_DISABLE_HTTP && USE_NTLM */
+#define Curl_http_auth_cleanup_ntlm(x)
+#endif
 
 #endif /* HEADER_CURL_HTTP_NTLM_H */
diff --git a/lib/multi.c b/lib/multi.c
index d4f031873..b77a9a0ee 100644
--- a/lib/multi.c
+++ b/lib/multi.c
@@ -79,7 +79,6 @@ static CURLMcode add_next_timeout(struct curltime now,
 static CURLMcode multi_timeout(struct Curl_multi *multi,
                                long *timeout_ms);
 static void process_pending_handles(struct Curl_multi *multi);
-static void detach_connnection(struct Curl_easy *data);
 
 #ifdef DEBUGBUILD
 static const char * const statename[]={
@@ -112,7 +111,7 @@ static void Curl_init_completed(struct Curl_easy *data)
 
   /* Important: reset the conn pointer so that we don't point to memory
      that could be freed anytime */
-  detach_connnection(data);
+  Curl_detach_connnection(data);
   Curl_expire_clear(data); /* stop all timers */
 }
 
@@ -506,6 +505,7 @@ CURLMcode curl_multi_add_handle(struct Curl_multi *multi,
      easy handle is added */
   memset(&multi->timer_lastcall, 0, sizeof(multi->timer_lastcall));
 
+  CONNCACHE_LOCK(data);
   /* The closure handle only ever has default timeouts set. To improve the
      state somewhat we clone the timeouts from each added handle so that the
      closure handle always has the same timeouts as the most recently added
@@ -515,6 +515,7 @@ CURLMcode curl_multi_add_handle(struct Curl_multi *multi,
     data->set.server_response_timeout;
   data->state.conn_cache->closure_handle->set.no_signal =
     data->set.no_signal;
+  CONNCACHE_UNLOCK(data);
 
   Curl_update_timer(multi);
   return CURLM_OK;
@@ -589,14 +590,14 @@ static CURLcode multi_done(struct Curl_easy *data,
 
   process_pending_handles(data->multi); /* connection / multiplex */
 
-  CONN_LOCK(data);
-  detach_connnection(data);
+  CONNCACHE_LOCK(data);
+  Curl_detach_connnection(data);
   if(CONN_INUSE(conn)) {
     /* Stop if still used. */
     /* conn->data must not remain pointing to this transfer since it is going
        away! Find another to own it! */
     conn->data = conn->easyq.head->ptr;
-    CONN_UNLOCK(data);
+    CONNCACHE_UNLOCK(data);
     DEBUGF(infof(data, "Connection still in use %zu, "
                  "no more multi_done now!\n",
                  conn->easyq.size));
@@ -647,7 +648,8 @@ static CURLcode multi_done(struct Curl_easy *data,
        || (premature && !(conn->handler->flags & PROTOPT_STREAM))) {
     CURLcode res2;
     connclose(conn, "disconnecting");
-    CONN_UNLOCK(data);
+    Curl_conncache_remove_conn(data, conn, FALSE);
+    CONNCACHE_UNLOCK(data);
     res2 = Curl_disconnect(data, conn, premature);
 
     /* If we had an error already, make sure we return that one. But
@@ -666,7 +668,7 @@ static CURLcode multi_done(struct Curl_easy *data,
               conn->bits.conn_to_host ? conn->conn_to_host.dispname :
               conn->host.dispname);
     /* the connection is no longer in use by this transfer */
-    CONN_UNLOCK(data);
+    CONNCACHE_UNLOCK(data);
     if(Curl_conncache_return_conn(data, conn)) {
       /* remember the most recently used connection */
       data->state.lastconnect = conn;
@@ -778,8 +780,7 @@ CURLMcode curl_multi_remove_handle(struct Curl_multi *multi,
                                 vanish with this handle */
 
   /* Remove the association between the connection and the handle */
-  if(data->conn)
-    detach_connnection(data);
+  Curl_detach_connnection(data);
 
 #ifdef USE_LIBPSL
   /* Remove the PSL association. */
@@ -828,9 +829,13 @@ bool Curl_multiplex_wanted(const struct Curl_multi *multi)
   return (multi && (multi->multiplexing));
 }
 
-/* This is the only function that should clear data->conn. This will
-   occasionally be called with the pointer already cleared. */
-static void detach_connnection(struct Curl_easy *data)
+/*
+ * Curl_detach_connnection() removes the given transfer from the connection.
+ *
+ * This is the only function that should clear data->conn. This will
+ * occasionally be called with the data->conn pointer already cleared.
+ */
+void Curl_detach_connnection(struct Curl_easy *data)
 {
   struct connectdata *conn = data->conn;
   if(conn)
@@ -838,7 +843,11 @@ static void detach_connnection(struct Curl_easy *data)
   data->conn = NULL;
 }
 
-/* This is the only function that should assign data->conn */
+/*
+ * Curl_attach_connnection() attaches this transfer to this connection.
+ *
+ * This is the only function that should assign data->conn
+ */
 void Curl_attach_connnection(struct Curl_easy *data,
                              struct connectdata *conn)
 {
@@ -1540,19 +1549,6 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
     bool stream_error = FALSE;
     rc = CURLM_OK;
 
-    DEBUGASSERT((data->mstate <= CURLM_STATE_CONNECT) ||
-                (data->mstate >= CURLM_STATE_DONE) ||
-                data->conn);
-    if(!data->conn &&
-       data->mstate > CURLM_STATE_CONNECT &&
-       data->mstate < CURLM_STATE_DONE) {
-      /* In all these states, the code will blindly access 'data->conn'
-         so this is precaution that it isn't NULL. And it silences static
-         analyzers. */
-      failf(data, "In state %d with no conn, bail out!\n", data->mstate);
-      return CURLM_INTERNAL_ERROR;
-    }
-
     if(multi_ischanged(multi, TRUE)) {
       DEBUGF(infof(data, "multi changed, check CONNECT_PEND queue!\n"));
       process_pending_handles(multi); /* multiplexed */
@@ -2235,8 +2231,7 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
          * access free'd data, if the connection is free'd and the handle
          * removed before we perform the processing in CURLM_STATE_COMPLETED
          */
-        if(data->conn)
-          detach_connnection(data);
+        Curl_detach_connnection(data);
       }
 
 #ifndef CURL_DISABLE_FTP
@@ -2288,7 +2283,10 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
             /* This is where we make sure that the conn pointer is reset.
                We don't have to do this in every case block above where a
                failure is detected */
-            detach_connnection(data);
+            Curl_detach_connnection(data);
+
+            /* remove connection from cache */
+            Curl_conncache_remove_conn(data, conn, TRUE);
 
             /* disconnect properly */
             Curl_disconnect(data, conn, dead_connection);
diff --git a/lib/multiif.h b/lib/multiif.h
index bde755ee0..c07587baf 100644
--- a/lib/multiif.h
+++ b/lib/multiif.h
@@ -33,6 +33,7 @@ void Curl_expire_done(struct Curl_easy *data, expire_id id);
 void Curl_update_timer(struct Curl_multi *multi);
 void Curl_attach_connnection(struct Curl_easy *data,
                              struct connectdata *conn);
+void Curl_detach_connnection(struct Curl_easy *data);
 bool Curl_multiplex_wanted(const struct Curl_multi *multi);
 void Curl_set_in_callback(struct Curl_easy *data, bool value);
 bool Curl_is_in_callback(struct Curl_easy *easy);
diff --git a/lib/url.c b/lib/url.c
index 03c274438..1df3dfd3a 100644
--- a/lib/url.c
+++ b/lib/url.c
@@ -684,9 +684,7 @@ static void conn_reset_all_postponed_data(struct connectdata *conn)
 
 static void conn_shutdown(struct connectdata *conn)
 {
-  if(!conn)
-    return;
-
+  DEBUGASSERT(conn);
   infof(conn->data, "Closing connection %ld\n", conn->connection_id);
   DEBUGASSERT(conn->data);
 
@@ -707,16 +705,11 @@ static void conn_shutdown(struct connectdata *conn)
     Curl_closesocket(conn, conn->tempsock[0]);
   if(CURL_SOCKET_BAD != conn->tempsock[1])
     Curl_closesocket(conn, conn->tempsock[1]);
-
-  /* unlink ourselves. this should be called last since other shutdown
-     procedures need a valid conn->data and this may clear it. */
-  Curl_conncache_remove_conn(conn->data, conn, TRUE);
 }
 
 static void conn_free(struct connectdata *conn)
 {
-  if(!conn)
-    return;
+  DEBUGASSERT(conn);
 
   Curl_free_idnconverted_hostname(&conn->host);
   Curl_free_idnconverted_hostname(&conn->conn_to_host);
@@ -783,13 +776,17 @@ static void conn_free(struct connectdata *conn)
 CURLcode Curl_disconnect(struct Curl_easy *data,
                          struct connectdata *conn, bool dead_connection)
 {
-  if(!conn)
-    return CURLE_OK; /* this is closed and fine already */
+  /* there must be a connection to close */
+  DEBUGASSERT(conn);
 
-  if(!data) {
-    DEBUGF(infof(data, "DISCONNECT without easy handle, ignoring\n"));
-    return CURLE_OK;
-  }
+  /* it must be removed from the connection cache */
+  DEBUGASSERT(!conn->bundle);
+
+  /* there must be an associated transfer */
+  DEBUGASSERT(data);
+
+  /* the transfer must be detached from the connection */
+  DEBUGASSERT(!data->conn);
 
   /*
    * If this connection isn't marked to force-close, leave it open if there
@@ -805,16 +802,11 @@ CURLcode Curl_disconnect(struct Curl_easy *data,
     conn->dns_entry = NULL;
   }
 
-  Curl_hostcache_prune(data); /* kill old DNS cache entries */
-
-#if !defined(CURL_DISABLE_HTTP) && defined(USE_NTLM)
   /* Cleanup NTLM connection-related data */
   Curl_http_auth_cleanup_ntlm(conn);
-#endif
-#if !defined(CURL_DISABLE_HTTP) && defined(USE_SPNEGO)
+
   /* Cleanup NEGOTIATE connection-related data */
   Curl_http_auth_cleanup_negotiate(conn);
-#endif
 
   /* the protocol specific disconnect handler and conn_shutdown need a transfer
      for the connection! */
@@ -1011,8 +1003,12 @@ static int call_extract_if_dead(struct connectdata *conn, void *param)
 static void prune_dead_connections(struct Curl_easy *data)
 {
   struct curltime now = Curl_now();
-  timediff_t elapsed =
+  timediff_t elapsed;
+
+  CONNCACHE_LOCK(data);
+  elapsed =
     Curl_timediff(now, data->state.conn_cache->last_cleanup);
+  CONNCACHE_UNLOCK(data);
 
   if(elapsed >= 1000L) {
     struct prunedead prune;
@@ -1020,10 +1016,17 @@ static void prune_dead_connections(struct Curl_easy *data)
     prune.extracted = NULL;
     while(Curl_conncache_foreach(data, data->state.conn_cache, &prune,
                                  call_extract_if_dead)) {
+      /* unlocked */
+
+      /* remove connection from cache */
+      Curl_conncache_remove_conn(data, prune.extracted, TRUE);
+
       /* disconnect it */
       (void)Curl_disconnect(data, prune.extracted, /* dead_connection */TRUE);
     }
+    CONNCACHE_LOCK(data);
     data->state.conn_cache->last_cleanup = now;
+    CONNCACHE_UNLOCK(data);
   }
 }
 
@@ -1083,7 +1086,7 @@ ConnectionExists(struct Curl_easy *data,
         if(data->set.pipewait) {
           infof(data, "Server doesn't support multiplex yet, wait\n");
           *waitpipe = TRUE;
-          Curl_conncache_unlock(data);
+          CONNCACHE_UNLOCK(data);
           return FALSE; /* no re-use */
         }
 
@@ -1407,11 +1410,12 @@ ConnectionExists(struct Curl_easy *data,
   if(chosen) {
     /* mark it as used before releasing the lock */
     chosen->data = data; /* own it! */
-    Curl_conncache_unlock(data);
+    Curl_attach_connnection(data, chosen);
+    CONNCACHE_UNLOCK(data);
     *usethis = chosen;
     return TRUE; /* yes, we found one to use! */
   }
-  Curl_conncache_unlock(data);
+  CONNCACHE_UNLOCK(data);
 
   if(foundPendingCandidate && data->set.pipewait) {
     infof(data,
@@ -3529,6 +3533,7 @@ static CURLcode create_conn(struct Curl_easy *data,
     if(!result) {
       conn->bits.tcpconnect[FIRSTSOCKET] = TRUE; /* we are "connected */
 
+      Curl_attach_connnection(data, conn);
       result = Curl_conncache_add_conn(data->state.conn_cache, conn);
       if(result)
         goto out;
@@ -3543,7 +3548,6 @@ static CURLcode create_conn(struct Curl_easy *data,
         (void)conn->handler->done(conn, result, FALSE);
         goto out;
       }
-      Curl_attach_connnection(data, conn);
       Curl_setup_transfer(data, -1, -1, FALSE, -1);
     }
 
@@ -3693,7 +3697,7 @@ static CURLcode create_conn(struct Curl_easy *data,
 
         /* The bundle is full. Extract the oldest connection. */
         conn_candidate = Curl_conncache_extract_bundle(data, bundle);
-        Curl_conncache_unlock(data);
+        CONNCACHE_UNLOCK(data);
 
         if(conn_candidate)
           (void)Curl_disconnect(data, conn_candidate,
@@ -3705,7 +3709,7 @@ static CURLcode create_conn(struct Curl_easy *data,
         }
       }
       else
-        Curl_conncache_unlock(data);
+        CONNCACHE_UNLOCK(data);
 
     }
 
@@ -3739,6 +3743,8 @@ static CURLcode create_conn(struct Curl_easy *data,
        * This is a brand new connection, so let's store it in the connection
        * cache of ours!
        */
+      Curl_attach_connnection(data, conn);
+
       result = Curl_conncache_add_conn(data->state.conn_cache, conn);
       if(result)
         goto out;
@@ -3893,7 +3899,7 @@ CURLcode Curl_connect(struct Curl_easy *data,
   result = create_conn(data, &conn, asyncp);
 
   if(!result) {
-    if(CONN_INUSE(conn))
+    if(CONN_INUSE(conn) > 1)
       /* multiplexed */
       *protocol_done = TRUE;
     else if(!*asyncp) {
@@ -3910,11 +3916,10 @@ CURLcode Curl_connect(struct Curl_easy *data,
   else if(result && conn) {
     /* We're not allowed to return failure with memory left allocated in the
        connectdata struct, free those here */
+    Curl_detach_connnection(data);
+    Curl_conncache_remove_conn(data, conn, TRUE);
     Curl_disconnect(data, conn, TRUE);
   }
-  else if(!result && !data->conn)
-    /* FILE: transfers already have the connection attached */
-    Curl_attach_connnection(data, conn);
 
   return result;
 }
diff --git a/tests/data/test1554 b/tests/data/test1554
index 06f189724..d3926d916 100644
--- a/tests/data/test1554
+++ b/tests/data/test1554
@@ -29,6 +29,12 @@ run 1: foobar and so on fun!
 <- Mutex unlock
 -> Mutex lock
 <- Mutex unlock
+-> Mutex lock
+<- Mutex unlock
+-> Mutex lock
+<- Mutex unlock
+-> Mutex lock
+<- Mutex unlock
 run 1: foobar and so on fun!
 -> Mutex lock
 <- Mutex unlock
@@ -40,6 +46,10 @@ run 1: foobar and so on fun!
 <- Mutex unlock
 -> Mutex lock
 <- Mutex unlock
+-> Mutex lock
+<- Mutex unlock
+-> Mutex lock
+<- Mutex unlock
 run 1: foobar and so on fun!
 -> Mutex lock
 <- Mutex unlock
@@ -51,6 +61,10 @@ run 1: foobar and so on fun!
 <- Mutex unlock
 -> Mutex lock
 <- Mutex unlock
+-> Mutex lock
+<- Mutex unlock
+-> Mutex lock
+<- Mutex unlock
 run 1: foobar and so on fun!
 -> Mutex lock
 <- Mutex unlock
diff --git a/tests/unit/unit1620.c b/tests/unit/unit1620.c
index 6e572c648..b23e5b912 100644
--- a/tests/unit/unit1620.c
+++ b/tests/unit/unit1620.c
@@ -5,7 +5,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -73,10 +73,6 @@ UNITTEST_START
   fail_unless(rc == CURLE_OK,
               "Curl_parse_login_details() failed");
 
-  rc = Curl_disconnect(empty, empty->conn, FALSE);
-  fail_unless(rc == CURLE_OK,
-              "Curl_disconnect() with dead_connection set FALSE failed");
-
   Curl_freeset(empty);
   for(i = (enum dupstring)0; i < STRING_LAST; i++) {
     fail_unless(empty->set.str[i] == NULL,