Add update operation-specific variable with unmatch|match additional values

Note that the unmatch (match but do not update) and match (update during
match) values are only supported by certain rules (and potentially only for
certain prerequisite types).

Additionally:

- All operation-specific variables are now checked for false as an override
  for the prerequisite-specific include value. In particular, this can now be
  used to disable a prerequisite for update, for example:

  ./: exe{test}: update = false

- The cc::link_rule now supports the update=match value for headers and ad hoc
  prerequisites. In particular, this can be used to make sure all the library
  headers are updated before matching any of its (or dependent's) object
  files.

1 files changed, 20 insertions, 4 deletions

diff --git a/libbuild2/context.cxx b/libbuild2/context.cxx
index d78efba..d0b24e0 100644
--- a/libbuild2/context.cxx
+++ b/libbuild2/context.cxx
@@ -586,9 +586,10 @@ namespace build2
     var_export_metadata = &vp.insert ("export.metadata", v_t); // Untyped.
 
     var_extension = &vp.insert<string> ("extension", v_t);
-    var_clean     = &vp.insert<bool>   ("clean",    v_t);
-    var_backlink  = &vp.insert<string> ("backlink", v_t);
-    var_include   = &vp.insert<string> ("include",  v_q);
+    var_update    = &vp.insert<string> ("update",    v_q);
+    var_clean     = &vp.insert<bool>   ("clean",     v_t);
+    var_backlink  = &vp.insert<string> ("backlink",  v_t);
+    var_include   = &vp.insert<string> ("include",   v_q);
 
     // Backlink executables and (generated) documentation by default.
     //
@@ -696,13 +697,28 @@ namespace build2
                      const operation_info* outer_oif,
                      bool diag_noise)
   {
-    current_oname = (outer_oif == nullptr ? inner_oif : *outer_oif).name;
+    const auto& oif (outer_oif == nullptr ? inner_oif : *outer_oif);
+
+    current_oname = oif.name;
     current_inner_oif = &inner_oif;
     current_outer_oif = outer_oif;
     current_on++;
     current_mode = inner_oif.mode;
     current_diag_noise = diag_noise;
 
+    if (oif.var_name != nullptr)
+    {
+      current_ovar = var_pool.find (oif.var_name);
+
+      // The operation variable should have prerequisite or target visibility.
+      //
+      assert (current_ovar != nullptr &&
+              (current_ovar->visibility == variable_visibility::prereq ||
+               current_ovar->visibility == variable_visibility::target));
+    }
+    else
+      current_ovar = nullptr;
+
     // Reset counters (serial execution).
     //
     dependency_count.store (0, memory_order_relaxed);