Start work on a very simple permissions system.

First step for issue opencog#1855

opencog/atomspace/AtomSpace.cc

@@ -61,7 +61,7 @@ using namespace opencog;
 AtomSpace::AtomSpace(AtomSpace* parent, bool transient) :
     _atom_table(parent? &parent->_atom_table : nullptr, this, transient),
     _backing_store(nullptr),
-    _transient(transient)
+    _read_only(false)
 {
 }
 
@@ -79,6 +79,17 @@ void AtomSpace::clear_transient()
     _atom_table.clear_transient();
 }
 
+// An extremely primitive permissions system.
+void AtomSpace::set_read_only(void)
+{
+    _read_only = true;
+}
+
+void AtomSpace::set_read_write(void)
+{
+    _read_only = false;
+}
+
 bool AtomSpace::compare_atomspaces(const AtomSpace& space_first,
                                    const AtomSpace& space_second,
                                    bool check_truth_values,

opencog/atomspace/AtomSpace.h

@@ -76,7 +76,7 @@ class AtomSpace
 
     AtomTable& get_atomtable(void) { return _atom_table; }
 
-    bool _transient;
+    bool _read_only;
 protected:
 
     /**
@@ -92,9 +92,16 @@ class AtomSpace
     AtomSpace(AtomSpace* parent=nullptr, bool transient=false);
     ~AtomSpace();
 
+    // Transient atomspaces are lighter-weight, faster, but are missing
+    // some features. They are used during pattern matching, to hold
+    // temporary results.
     void ready_transient(AtomSpace* parent);
     void clear_transient();
 
+    void set_read_only(void);
+    void set_read_write(void);
+    bool get_read_only(void) { return _read_only; }
+
     /// Get the environment that this atomspace was created in.
     AtomSpace* get_environ() const {
         AtomTable* env = _atom_table.get_environ();