Update/cleanup comment documentation for inner/outer operation semantics

6 files changed, 49 insertions, 30 deletions

diff --git a/build2/algorithm.cxx b/build2/algorithm.cxx
index 58ddb24..d39711b 100644
--- a/build2/algorithm.cxx
+++ b/build2/algorithm.cxx
@@ -222,10 +222,10 @@ namespace build2
   const rule_match*
   match_impl (action a, target& t, const rule* skip, bool try_match)
   {
-    // If this is an outer operation (Y_for_X), then we look for rules
+    // If this is an outer operation (Y-for-X), then we look for rules
     // registered for the outer id (X). Note that we still pass the original
     // action to the rule's match() function so that it can distinguish
-    // between a pre/post operation (Y_for_X) and the actual operation (X).
+    // between a pre/post operation (Y-for-X) and the actual operation (X).
     //
     meta_operation_id mo (a.meta_operation ());
     operation_id o (a.inner () ? a.operation () : a.outer_operation ());
diff --git a/build2/cc/link-rule.cxx b/build2/cc/link-rule.cxx
index b6b707d..cd355c6 100644
--- a/build2/cc/link-rule.cxx
+++ b/build2/cc/link-rule.cxx
@@ -290,7 +290,7 @@ namespace build2
 
       file& t (xt.as<file> ());
 
-      // Note that for_install is signalled by install_rule and therefore
+      // Note that for-install is signalled by install_rule and therefore
       // can only be relied upon during execute.
       //
       match_data& md (t.data (match_data ()));
@@ -449,7 +449,7 @@ namespace build2
           //
           // Note that we do it here regardless of whether we are installing
           // or not for two reasons. Firstly, it is not easy to detect this
-          // situation in apply() since the for_install hasn't yet been
+          // situation in apply() since the for-install hasn't yet been
           // communicated by install_rule. Secondly, always having the member
           // takes care of cleanup automagically. The actual generation
           // happens in perform_update() below.
diff --git a/build2/install/rule.cxx b/build2/install/rule.cxx
index 8bf58aa..00166c2 100644
--- a/build2/install/rule.cxx
+++ b/build2/install/rule.cxx
@@ -45,9 +45,9 @@ namespace build2
     {
       // We always match.
       //
-      // Note that we are called both as the outer part during the "update for
-      // un/install" pre-operation and as the inner part during the
-      // un/install operation itself.
+      // Note that we are called both as the outer part during the update-for-
+      // un/install pre-operation and as the inner part during the un/install
+      // operation itself.
       //
       return true;
     }
@@ -234,12 +234,12 @@ namespace build2
     {
       tracer trace ("install::file_rule::apply");
 
-      // Note that we are called both as the outer part during the "update for
-      // un/install" pre-operation and as the inner part during the
-      // un/install operation itself.
+      // Note that we are called both as the outer part during the update-for-
+      // un/install pre-operation and as the inner part during the un/install
+      // operation itself.
       //
       // In both cases we first determine if the target is installable and
-      // return noop if it's not. Otherwise, in the first case (update for
+      // return noop if it's not. Otherwise, in the first case (update-for-
       // un/install) we delegate to the normal update and in the second
       // (un/install) -- perform the test.
       //
diff --git a/build2/operation.hxx b/build2/operation.hxx
index 7f59783..3f812d5 100644
--- a/build2/operation.hxx
+++ b/build2/operation.hxx
@@ -34,19 +34,38 @@ namespace build2
   using operation_id = uint8_t;
   using action_id = uint8_t;
 
-  // Meta-operations and operations are not the end of the story. We
-  // also have operation nesting (currently only one level deep) which
-  // is used to implement pre/post operations (currently, but may be
-  // useful for other things). Here is the idea: the test operation
-  // needs to make sure that the targets that it needs to test are
-  // up-to-date. So it runs update as its pre-operation. It is almost
-  // like an ordinary update except that it has test as its outer
-  // operation (the meta-operations are always the same). This way a
-  // rule can recognize that this is "update for test" and do something
-  // differently. For example, if an executable is not a test, then
-  // there is no use updating it. At the same time, most rules will
-  // ignore the fact that this is a nested update and for them it is
-  // "update as usual".
+  // Meta-operations and operations are not the end of the story. We also have
+  // operation nesting (currently only one level deep) which is used to
+  // implement pre/post operations (currently, but may be useful for other
+  // things). Here is the idea: the test operation needs to make sure that the
+  // targets that it needs to test are up-to-date. So it runs update as its
+  // pre-operation. It is almost like an ordinary update except that it has
+  // test as its outer operation (the meta-operations are always the same).
+  // This way a rule can recognize that this is "update for test" and do
+  // something differently. For example, if an executable is not a test, then
+  // there is no use updating it. At the same time, most rules will ignore the
+  // fact that this is a nested update and for them it is "update as usual".
+  //
+  // This inner/outer operation support is implemented by maintaining two
+  // independent "target states" (see target::state; initially we tried to do
+  // it via rule/recipe override but that didn't end up well, to put it
+  // mildly). While the outer operation normally "directs" the inner, inner
+  // rules can still be matched/executed directly, without outer's involvement
+  // (e.g., because of other inner rules). A typical implementation of an
+  // outer rule either returns noop or delegates to the inner rule. In
+  // particular, it should not replace or override the inner's logic.
+  //
+  // While most of the relevant target state is duplicated, certain things are
+  // shared among the inner/outer rules, such as the target data pad and the
+  // group state. In particular, it is assumed the group state is always
+  // determined by the inner rule (see resolve_group_members()).
+  //
+  // Normally, an outer rule will be responsible for any additional, outer
+  // operation-specific work. Sometimes, however, the inner rule needs to
+  // customize its behavior. In this case the outer and inner rules must
+  // communicate this explicitly (normally via the target's data pad) and
+  // there is a number of restrictions to this approach. See
+  // cc::{link,install}_rule for details.
   //
   struct action
   {
diff --git a/build2/target.hxx b/build2/target.hxx
index 405bf4f..23213a2 100644
--- a/build2/target.hxx
+++ b/build2/target.hxx
@@ -441,7 +441,7 @@ namespace build2
     static size_t count_executed () {return offset_executed + count_base ();}
     static size_t count_busy     () {return offset_busy     + count_base ();}
 
-    // Inner/outer operation state.
+    // Inner/outer operation state. See operation.hxx for details.
     //
     struct opstate
     {
diff --git a/build2/test/rule.cxx b/build2/test/rule.cxx
index a42ceca..7f6a133 100644
--- a/build2/test/rule.cxx
+++ b/build2/test/rule.cxx
@@ -35,8 +35,8 @@ namespace build2
     recipe rule::
     apply (action a, target& t) const
     {
-      // Note that we are called both as the outer part during the "update for
-      // test" pre-operation and as the inner part during the test operation
+      // Note that we are called both as the outer part during the update-for-
+      // test pre-operation and as the inner part during the test operation
       // itself.
       //
       // In both cases we first determine if the target is testable and return
@@ -52,7 +52,7 @@ namespace build2
       //
       //                         test'able      |  pass'able  |  neither
       //                                        |             |
-      // update for test     delegate (& pass)  |    pass     |   noop
+      // update-for-test     delegate (& pass)  |    pass     |   noop
       // ---------------------------------------+-------------+---------
       //            test     test     (& pass)  |    pass     |   noop
       //
@@ -150,7 +150,7 @@ namespace build2
             // execute them relying on update to assign their paths.
             //
             // Causing update for test inputs/scripts is tricky: we cannot
-            // match for update_for_install because this same rule will match
+            // match for update-for-install because this same rule will match
             // and since the target is not testable, it will return the noop
             // recipe.
             //
@@ -160,7 +160,7 @@ namespace build2
             // iffy, it does make sense: the outer rule we would have matched
             // would have simply delegated to the inner so we might as well
             // take a shortcut. The only potential drawback of this approach
-            // is that we won't be able to provide any for_test customizations
+            // is that we won't be able to provide any for-test customizations
             // when updating test inputs/scripts. But such a need seems rather
             // far fetched.
             //