From 53b6896e48aaa6ee2b414bd36275263c57b490f8 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Wed, 14 Feb 2024 08:39:58 +0200
Subject: Avoid duplicate fsdir{} in inject_fsdir(), match_prerequisite*() call
 sequences

---
 libbuild2/algorithm.cxx | 31 +++++++++++++++++++++++++++----
 libbuild2/algorithm.hxx |  9 +++++++--
 libbuild2/algorithm.ixx |  3 +++
 3 files changed, 37 insertions(+), 6 deletions(-)

diff --git a/libbuild2/algorithm.cxx b/libbuild2/algorithm.cxx
index 87217f7..26fad23 100644
--- a/libbuild2/algorithm.cxx
+++ b/libbuild2/algorithm.cxx
@@ -1723,12 +1723,25 @@ namespace build2
   {
     auto& pts (t.prerequisite_targets[a]);
 
+    size_t i (pts.size ()); // Index of the first to be added.
+
+    // Avoid duplicating fsdir{} that may have already been injected by
+    // inject_fsdir() (in which case it is expected to be first).
+    //
+    const target* dir (nullptr);
+    if (i != 0)
+    {
+      const prerequisite_target& pt (pts.front ());
+
+      if (pt.target != nullptr && pt.adhoc () && pt.target->is_a<fsdir> ())
+        dir = pt.target;
+    }
+
     // Start asynchronous matching of prerequisites. Wait with unlocked phase
     // to allow phase switching.
     //
     wait_guard wg (t.ctx, t.ctx.count_busy (), t[a].task_count, true);
 
-    size_t i (pts.size ()); // Index of the first to be added.
     for (auto&& p: forward<R> (r))
     {
       // Ignore excluded.
@@ -1742,7 +1755,9 @@ namespace build2
                               ? ms (a, t, p, pi)
                               : prerequisite_target (&search (t, p), pi));
 
-      if (pt.target == nullptr || (s != nullptr && !pt.target->in (*s)))
+      if (pt.target == nullptr ||
+          pt.target == dir     ||
+          (s != nullptr && !pt.target->in (*s)))
         continue;
 
       match_async (a, *pt.target, t.ctx.count_busy (), t[a].task_count);
@@ -1905,6 +1920,10 @@ namespace build2
   const fsdir*
   inject_fsdir (action a, target& t, bool match, bool prereq, bool parent)
   {
+    auto& pts (t.prerequisite_targets[a]);
+
+    assert (!prereq || pts.empty ()); // This prerequisite target must be first.
+
     const fsdir* r (inject_fsdir_impl (t, prereq, parent));
 
     if (r != nullptr)
@@ -1915,7 +1934,7 @@ namespace build2
       // Make it ad hoc so that it doesn't end up in prerequisite_targets
       // after execution.
       //
-      t.prerequisite_targets[a].emplace_back (r, include_type::adhoc);
+      pts.emplace_back (r, include_type::adhoc);
     }
 
     return r;
@@ -1924,12 +1943,16 @@ namespace build2
   const fsdir*
   inject_fsdir_direct (action a, target& t, bool prereq, bool parent)
   {
+    auto& pts (t.prerequisite_targets[a]);
+
+    assert (!prereq || pts.empty ()); // This prerequisite target must be first.
+
     const fsdir* r (inject_fsdir_impl (t, prereq, parent));
 
     if (r != nullptr)
     {
       match_direct_sync (a, *r);
-      t.prerequisite_targets[a].emplace_back (r, include_type::adhoc);
+      pts.emplace_back (r, include_type::adhoc);
     }
 
     return r;
diff --git a/libbuild2/algorithm.hxx b/libbuild2/algorithm.hxx
index 64660a9..1c5b717 100644
--- a/libbuild2/algorithm.hxx
+++ b/libbuild2/algorithm.hxx
@@ -659,8 +659,13 @@ namespace build2
   //
   // As an extension, unless prereq is false, this function will also search
   // for an existing fsdir{} prerequisite for the directory and if one exists,
-  // return that (even if the target is in src tree). This can be used, for
-  // example, to place output into an otherwise non-existent directory.
+  // return that (even if the target is in the src tree). In this case, the
+  // injected fsdir{} (if any) must be the first prerequisite in this target's
+  // prerequisite_targets, which is relied upon by the match_prerequisite*()
+  // family of functons to suppress the duplicate addition.
+  //
+  // Note that the explicit fsdir{} prerquiste is used to place output into an
+  // otherwise non-existent (in src) directory.
   //
   LIBBUILD2_SYMEXPORT const fsdir*
   inject_fsdir (action, target&,
diff --git a/libbuild2/algorithm.ixx b/libbuild2/algorithm.ixx
index 2fbec3f..de175eb 100644
--- a/libbuild2/algorithm.ixx
+++ b/libbuild2/algorithm.ixx
@@ -776,6 +776,9 @@ namespace build2
       // groups since the group target should clean eveything up. A bit of an
       // optimization.
       //
+      // @@ TMP: I wonder if this still holds for the new group semantics
+      //         we have in Qt automoc?
+      //
       match_search ms (
         msm
         ? [&msm] (action a,
-- 
cgit v1.1