From d0d4486702b045852dca36008746afeb8754ae85 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Wed, 1 Nov 2023 11:50:08 +0200
Subject: Tighten/optimize cleaning of fsdir{} during match

---
 libbuild2/algorithm.cxx | 11 ++++++++++-
 libbuild2/algorithm.hxx |  5 +++++
 libbuild2/algorithm.ixx |  2 +-
 libbuild2/rule.cxx      | 29 +++++++++++++++++++++--------
 4 files changed, 37 insertions(+), 10 deletions(-)

diff --git a/libbuild2/algorithm.cxx b/libbuild2/algorithm.cxx
index 4ec4db5..1f2d88b 100644
--- a/libbuild2/algorithm.cxx
+++ b/libbuild2/algorithm.cxx
@@ -3137,7 +3137,14 @@ namespace build2
     // Let's keep this as close to update_during_match() semantically as
     // possible until we see a clear reason to deviate.
 
-    assert (a == perform_clean_id);
+    // We have a problem with fsdir{}: if the directory is not empty because
+    // there are other targets that depend on it and we execute it here and
+    // now, it will not remove the directory (because it's not yet empty) but
+    // will cause the target to be in the executed state, which means that
+    // when other targets try to execute it, it will be a noop and the
+    // directory will be left behind.
+
+    assert (a == perform_clean_id && !t.is_a<fsdir> ());
 
     target_state os (t.matched_state (a));
 
@@ -3194,6 +3201,8 @@ namespace build2
         {
           const target& pt (*p.target);
 
+          assert (!pt.is_a<fsdir> ()); // See above.
+
           target_state os (pt.matched_state (a));
 
           if (os != target_state::unchanged)
diff --git a/libbuild2/algorithm.hxx b/libbuild2/algorithm.hxx
index 19f7db2..11e655b 100644
--- a/libbuild2/algorithm.hxx
+++ b/libbuild2/algorithm.hxx
@@ -740,6 +740,11 @@ namespace build2
   // cleaning to normal execute and these functions should only be used in
   // special cases where this is not possible.
   //
+  // Note also that neither function should be called on fsdir{} since it's
+  // hard to guarantee such an execution won't be too early (see the
+  // implementation for details). If you do need to clean fsdir{} during
+  // match, use fsdir_rule::perform_clean_direct() instead.
+  //
   LIBBUILD2_SYMEXPORT bool
   clean_during_match (tracer&,
                       action, const target&);
diff --git a/libbuild2/algorithm.ixx b/libbuild2/algorithm.ixx
index 09fc6d9..bae9151 100644
--- a/libbuild2/algorithm.ixx
+++ b/libbuild2/algorithm.ixx
@@ -482,7 +482,7 @@ namespace build2
         // cannot change their mind).
         //
         if ((s == target_state::unchanged && t.group == nullptr) ||
-            t[a].dependents.load (memory_order_consume) != 0)
+            t[a].dependents.load (memory_order_relaxed) != 0)
           return make_pair (true, s);
 
         break;
diff --git a/libbuild2/rule.cxx b/libbuild2/rule.cxx
index 79ccb86..9f90919 100644
--- a/libbuild2/rule.cxx
+++ b/libbuild2/rule.cxx
@@ -354,6 +354,8 @@ namespace build2
   void fsdir_rule::
   perform_update_direct (action a, const fsdir& t)
   {
+    assert (t.ctx.phase == run_phase::match);
+
     // First create the parent directory. If present, it is always first.
     //
     if (const target* p = (t.prerequisite_targets[a].empty ()
@@ -397,18 +399,29 @@ namespace build2
   void fsdir_rule::
   perform_clean_direct (action a, const fsdir& t)
   {
+    assert (t.ctx.phase == run_phase::match);
+
     // The same code as in perform_clean() above.
     //
-    rmdir (t.dir, t, t.ctx.current_diag_noise ? 1 : 2);
-
-    // Then clean the parent directory. If present, it is always first.
+    // Except that if there are other dependens of this fsdir{} then this will
+    // likely be a noop (because the directory won't be empty) and it makes
+    // sense to just defer cleaning to such other dependents. See
+    // clean_during_match() for backgound. This is similar logic as in
+    // unmatch::safe.
     //
-    if (const target* p = (t.prerequisite_targets[a].empty ()
-                           ? nullptr
-                           : t.prerequisite_targets[a][0]))
+    if (t[a].dependents.load (memory_order_relaxed) == 0)
     {
-      if (const fsdir* fp = p->is_a<fsdir> ())
-        perform_clean_direct (a, *fp);
+      rmdir (t.dir, t, t.ctx.current_diag_noise ? 1 : 2);
+
+      // Then clean the parent directory. If present, it is always first.
+      //
+      if (const target* p = (t.prerequisite_targets[a].empty ()
+                             ? nullptr
+                             : t.prerequisite_targets[a][0]))
+      {
+        if (const fsdir* fp = p->is_a<fsdir> ())
+          perform_clean_direct (a, *fp);
+      }
     }
   }
 
-- 
cgit v1.1