From 1821a11704143a89581b26e285d968230b86a8ac Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Fri, 10 Feb 2017 17:27:55 +0200
Subject: Track variable value modifications

---
 build2/file.cxx               |  2 +-
 build2/scope.cxx              |  2 +-
 build2/target.cxx             |  2 +-
 build2/test/script/script.cxx |  2 +-
 build2/variable               | 18 ++++++++++++++++--
 build2/variable.cxx           | 22 +++++++++-------------
 6 files changed, 29 insertions(+), 19 deletions(-)

diff --git a/build2/file.cxx b/build2/file.cxx
index c403eea..06910dc 100644
--- a/build2/file.cxx
+++ b/build2/file.cxx
@@ -339,7 +339,7 @@ namespace build2
       temp_scope tmp (s.global ());
       p.parse_variable (lex, tmp, var, tt);
 
-      value* v (tmp.vars.find (var));
+      value* v (tmp.vars.find_to_modify (var));
       assert (v != nullptr);
 
       // Steal the value, the scope is going away.
diff --git a/build2/scope.cxx b/build2/scope.cxx
index 0b76647..aaa0077 100644
--- a/build2/scope.cxx
+++ b/build2/scope.cxx
@@ -507,7 +507,7 @@ namespace build2
     lookup l (find_original (var).first);
 
     if (l.defined () && l.belongs (*this)) // Existing var in this scope.
-      return const_cast<value&> (*l); // Ok since this is original.
+      return vars.modify (l); // Ok since this is original.
 
     value& r (assign (var)); // NULL.
 
diff --git a/build2/target.cxx b/build2/target.cxx
index a6f7ef4..5389ec9 100644
--- a/build2/target.cxx
+++ b/build2/target.cxx
@@ -223,7 +223,7 @@ namespace build2
     lookup l (find_original (var).first);
 
     if (l.defined () && l.belongs (*this)) // Existing var in this target.
-      return const_cast<value&> (*l); // Ok since this is original.
+      return vars.modify (l); // Ok since this is original.
 
     value& r (assign (var)); // NULL.
 
diff --git a/build2/test/script/script.cxx b/build2/test/script/script.cxx
index bc816d7..0586377 100644
--- a/build2/test/script/script.cxx
+++ b/build2/test/script/script.cxx
@@ -659,7 +659,7 @@ namespace build2
         lookup l (find (var));
 
         if (l.defined () && l.belongs (*this)) // Existing var in this scope.
-          return const_cast<value&> (*l);
+          return vars.modify (l);
 
         value& r (assign (var)); // NULL.
 
diff --git a/build2/variable b/build2/variable
index e78b074..40470fa 100644
--- a/build2/variable
+++ b/build2/variable
@@ -1047,7 +1047,8 @@ namespace build2
     {
       using value::value;
 
-      size_t generation; // load_generation of this value (global only).
+      size_t version = 0; // Incremented on each modification (override cache).
+      size_t generation;  // load_generation of this value (global state only).
     };
 
     using map_type = butl::prefix_map<reference_wrapper<const variable>,
@@ -1107,7 +1108,20 @@ namespace build2
     find (const variable&, bool typed = true) const;
 
     value_data*
-    find (const variable&, bool typed = true);
+    find_to_modify (const variable&, bool typed = true);
+
+    // Convert a lookup pointing to a value belonging to this variable map
+    // to its non-const version. Note that this is only safe on the original
+    // values (see find_original()).
+    //
+    value&
+    modify (const lookup& l)
+    {
+      assert (l.vars == this);
+      value& r (const_cast<value&> (*l.value));
+      static_cast<value_data&> (r).version++;
+      return r;
+    }
 
     // Return a value suitable for assignment. See scope for details.
     //
diff --git a/build2/variable.cxx b/build2/variable.cxx
index 48f3345..7e105e6 100644
--- a/build2/variable.cxx
+++ b/build2/variable.cxx
@@ -1146,6 +1146,8 @@ namespace build2
             load_generation == 0 ||
             v.generation == load_generation);
 
+    // We assume typification is not modification.
+    //
     build2::typify (v, *var.type, &var);
   }
 
@@ -1169,22 +1171,14 @@ namespace build2
   }
 
   auto variable_map::
-  find (const variable& var, bool typed) -> value_data*
+  find_to_modify (const variable& var, bool typed) -> value_data*
   {
-    auto i (m_.find (var));
-    value_data* r (i != m_.end () ? &i->second : nullptr);
+    auto* r (const_cast<value_data*> (find (var, typed)));
 
-    // First access after being assigned a type?
-    //
-    if (r != nullptr && typed && var.type != nullptr && r->type != var.type)
-    {
-      // All values shall be typed during load.
-      //
-      assert (!global_ || phase == run_phase::load);
-      typify (*r, var);
-    }
+    if (r != nullptr)
+      r->version++;
 
-    return  r;
+    return r;
   }
 
   pair<reference_wrapper<value>, bool> variable_map::
@@ -1205,6 +1199,8 @@ namespace build2
         typify (r, var);
     }
 
+    r.version++;
+
     return make_pair (reference_wrapper<value> (r), p.second);
   }
 
-- 
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