- SCP and SFTP with the multi interface had the same flaw: the 'DONE'

operation didn't complete properly if the EAGAIN equivalent was returned but
  libcurl would simply continue with a half-completed close operation
  performed. This ruined persistent connection re-use and cause some
  SSH-protocol errors in general. The correction is unfortunately adding a
  blocking function - doing it entirely non-blocking should be considered for
  a better fix.

lib/ssh.c

@@ -2309,30 +2309,24 @@ static CURLcode scp_disconnect(struct connectdata *conn)
 static CURLcode ssh_done(struct connectdata *conn, CURLcode status)
 {
   CURLcode result = CURLE_OK;
-  bool done = FALSE;
+  struct SSHPROTO *sftp_scp = conn->data->state.proto.ssh;
 
   if(status == CURLE_OK) {
-    /* run the state-machine */
-    if(conn->data->state.used_interface == Curl_if_multi) {
-      result = ssh_multi_statemach(conn, &done);
-    }
-    else {
-      result = ssh_easy_statemach(conn);
-      done = TRUE;
-    }
+    /* run the state-machine
+
+       TODO: when the multi interface this _really_ should be using the
+       ssh_multi_statemach function but we have no general support for
+       non-blocking DONE operations, not in the multi state machine and with
+       Curl_done() invokes on several places in the code!
+    */
+    result = ssh_easy_statemach(conn);
   }
-  else {
+  else
     result = status;
-    done = TRUE;
-  }
 
-  if(done) {
-    struct SSHPROTO *sftp_scp = conn->data->state.proto.ssh;
-
-    Curl_safefree(sftp_scp->path);
-    sftp_scp->path = NULL;
-    Curl_pgrsDone(conn);
-  }
+  Curl_safefree(sftp_scp->path);
+  sftp_scp->path = NULL;
+  Curl_pgrsDone(conn);
 
   return result;
 }