built-in "install" rule: verify well-formedness of resulting stage

The install target, like any other target, has to have artifacts
and runfiles being proper stages, i.e., in such a way that the
keys can be interpreted as names in the file system without causing
conflicts. This property used to be unchecked, thus allowing users
to define mal-formed targets that, when used as inputs to actions,
would result in unspecified layout of the action directory. Fix
this by adding an appropriate check enforcing well-formedness of
the resulting stage.

(cherry picked from f66db4bb21efeead140a453f91280bb9471f24a9)

2 files changed, 10 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 27c95c44..f0220b16 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,11 @@
 
 Bug fixes on top of release `1.2.3`.
 
+### Fixes
+
+- The built-in rule `"install"` now properly enforces that the
+  resulting stage is well-formed, i.e., without tree conflicts.
+
 ## Release `1.2.3` (2023-11-15)
 
 Bug fixes on top of release `1.2.2`.
diff --git a/src/buildtool/build_engine/target_map/built_in_rules.cpp b/src/buildtool/build_engine/target_map/built_in_rules.cpp
index 29eba7bf..6d55784f 100644
--- a/src/buildtool/build_engine/target_map/built_in_rules.cpp
+++ b/src/buildtool/build_engine/target_map/built_in_rules.cpp
@@ -665,6 +665,11 @@ void InstallRuleWithDeps(
         stage = ExpressionPtr{Expression::map_t{stage, to_stage}};
     }
 
+    auto conflict = BuildMaps::Target::Utils::tree_conflict(stage);
+    if (conflict) {
+        (*logger)(fmt::format("TREE conflict on subtree {}", *conflict), true);
+        return;
+    }
     auto const& empty_map = Expression::kEmptyMap;
     auto result = std::make_shared<AnalysedTarget const>(
         TargetResult{