LinuxProcessList_recurseProcTree: open dirfd first

A process can die between reading the directory listing and opening the
directory FD (if HAVE_OPENAT) or /proc files (otherwise) for reading the
process data. This race would cause LinuxProcessList_recurseProcTree to
remove it from the list immediately, which is unexpected in the
"highlight dying processes" mode and can break the tree structure.
This patch closes this race in the HAVE_OPENAT case by only accessing
the process entry after the directory FD has been opened.

linux/LinuxProcessList.c

@@ -1446,22 +1446,22 @@ static bool LinuxProcessList_recurseProcTree(LinuxProcessList* this, openat_arg_
       if (parent && pid == parent->pid)
          continue;
 
-      bool preExisting;
-      Process* proc = ProcessList_getProcess(pl, pid, &preExisting, LinuxProcess_new);
-      LinuxProcess* lp = (LinuxProcess*) proc;
-
-      proc->tgid = parent ? parent->pid : pid;
-      proc->isUserlandThread = proc->pid != proc->tgid;
-
 #ifdef HAVE_OPENAT
       int procFd = openat(dirFd, entry->d_name, O_RDONLY | O_DIRECTORY | O_NOFOLLOW);
       if (procFd < 0)
-         goto errorReadingProcess;
+         continue;
 #else
       char procFd[4096];
       xSnprintf(procFd, sizeof(procFd), "%s/%s", dirFd, entry->d_name);
 #endif
 
+      bool preExisting;
+      Process* proc = ProcessList_getProcess(pl, pid, &preExisting, LinuxProcess_new);
+      LinuxProcess* lp = (LinuxProcess*) proc;
+
+      proc->tgid = parent ? parent->pid : pid;
+      proc->isUserlandThread = proc->pid != proc->tgid;
+
       LinuxProcessList_recurseProcTree(this, procFd, "task", proc, period);
 
       /*