gdb: use intrusive list for step-over chain

The threads that need a step-over are currently linked using an
hand-written intrusive doubly-linked list, so that seems a very good
candidate for intrusive_list, convert it.

For this, we have a use case of appending a list to another one (in
start_step_over).  Based on the std::list and Boost APIs, add a splice
method.  However, only support splicing the other list at the end of the
`this` list, since that's all we need.

Add explicit default assignment operators to
reference_to_pointer_iterator, which are otherwise implicitly deleted.
This is needed because to define thread_step_over_list_safe_iterator, we
wrap reference_to_pointer_iterator inside a basic_safe_iterator, and
basic_safe_iterator needs to be able to copy-assign the wrapped
iterator.  The move-assignment operator is therefore not needed, only
the copy-assignment operator is.  But for completeness, add both.

Change-Id: I31b2ff67c7b78251314646b31887ef1dfebe510c

2 files changed, 30 insertions, 0 deletions

diff --git a/gdbsupport/intrusive_list.h b/gdbsupport/intrusive_list.h
index 8d49ce4..58aa68f 100644
--- a/gdbsupport/intrusive_list.h
+++ b/gdbsupport/intrusive_list.h
@@ -350,6 +350,33 @@ public:
     pos_node->prev = &elem;
   }
 
+  /* Move elements from LIST at the end of the current list.  */
+  void splice (intrusive_list &&other)
+  {
+    if (other.empty ())
+      return;
+
+    if (this->empty ())
+      {
+	*this = std::move (other);
+	return;
+      }
+
+    /* [A ... B] + [C ... D] */
+    T *b_elem = m_back;
+    node_type *b_node = as_node (b_elem);
+    T *c_elem = other.m_front;
+    node_type *c_node = as_node (c_elem);
+    T *d_elem = other.m_back;
+
+    b_node->next = c_elem;
+    c_node->prev = b_elem;
+    m_back = d_elem;
+
+    other.m_front = nullptr;
+    other.m_back = nullptr;
+  }
+
   void pop_front ()
   {
     gdb_assert (!this->empty ());
diff --git a/gdbsupport/reference-to-pointer-iterator.h b/gdbsupport/reference-to-pointer-iterator.h
index 7303fa4..9210426 100644
--- a/gdbsupport/reference-to-pointer-iterator.h
+++ b/gdbsupport/reference-to-pointer-iterator.h
@@ -56,6 +56,9 @@ struct reference_to_pointer_iterator
   reference_to_pointer_iterator (const reference_to_pointer_iterator &) = default;
   reference_to_pointer_iterator (reference_to_pointer_iterator &&) = default;
 
+  reference_to_pointer_iterator &operator= (const reference_to_pointer_iterator &) = default;
+  reference_to_pointer_iterator &operator= (reference_to_pointer_iterator &&) = default;
+
   value_type operator* () const
   { return &*m_it; }