Don't copy FdTable::vms when cloning it, because the new FdTable will…

… have an entirely new TaskSet

Also, tweak the code so we die if we discover an empty AddressSpace in `vms`.

Resolves #3475

src/AddressSpace.cc

@@ -1695,6 +1695,7 @@ AddressSpace::AddressSpace(Task* t, const string& exe, uint32_t exec_count)
   }
 }
 
+// Does not copy the task set; the new AddressSpace will be for new tasks.
 AddressSpace::AddressSpace(Session* session, const AddressSpace& o,
                            pid_t leader_tid, uint32_t leader_serial,
                            uint32_t exec_count)

src/FdTable.cc

@@ -179,9 +179,14 @@ void FdTable::update_syscallbuf_fds_disabled(int fd) {
   // But tasks with the same VM might have different fd tables...
   for (auto address_space : vms) {
     RecordTask* rt = nullptr;
+    if (address_space.first->task_set().empty()) {
+      FATAL() << "Address space must have at least one task";
+    }
     for (Task* t : address_space.first->task_set()) {
       if (!t->session().is_recording()) {
-        return;
+        // We could return but we want to check that all our
+        // AddressSpaces have tasks (i.e. aren't dead/dangling)
+        break;
       }
       rt = static_cast<RecordTask*>(t);
       if (!rt->already_exited()) {

src/FdTable.h

@@ -80,8 +80,10 @@ class FdTable final : public HasTaskSet {
 
 private:
   FdTable() : fd_count_beyond_limit(0), last_free_fd_(0) {}
+  // Does not call the base-class copy constructor because
+  // we don't want to copy the task set; the new FdTable will
+  // be for new tasks.
   FdTable(const FdTable& other) : fds(other.fds),
-    vms(other.vms),
     fd_count_beyond_limit(other.fd_count_beyond_limit),
     last_free_fd_(other.last_free_fd_) {}