[flang] Safer hermetic module file reading (#121002)

When a hermetic module file is read, use a new scope to hold its
dependent modules so that they don't conflict with any modules in the
global scope.

1 files changed, 19 insertions, 0 deletions

diff --git a/flang/lib/Semantics/mod-file.cpp b/flang/lib/Semantics/mod-file.cpp
index 51ff70c..4367dd1 100644
--- a/flang/lib/Semantics/mod-file.cpp
+++ b/flang/lib/Semantics/mod-file.cpp
@@ -1366,6 +1366,12 @@ Scope *ModFileReader::Read(SourceName name, std::optional<bool> isIntrinsic,
       name.ToString(), isIntrinsic.value_or(false))};
   if (!isIntrinsic.value_or(false) && !ancestor) {
     // Already present in the symbol table as a usable non-intrinsic module?
+    if (Scope * hermeticScope{context_.currentHermeticModuleFileScope()}) {
+      auto it{hermeticScope->find(name)};
+      if (it != hermeticScope->end()) {
+        return it->second->scope();
+      }
+    }
     auto it{context_.globalScope().find(name)};
     if (it != context_.globalScope().end()) {
       Scope *scope{it->second->scope()};
@@ -1544,9 +1550,22 @@ Scope *ModFileReader::Read(SourceName name, std::optional<bool> isIntrinsic,
   // Process declarations from the module file
   auto wasModuleFileName{context_.foldingContext().moduleFileName()};
   context_.foldingContext().set_moduleFileName(name);
+  // Are there multiple modules in the module file due to it having been
+  // created under -fhermetic-module-files?  If so, process them first in
+  // their own nested scope that will be visible only to USE statements
+  // within the module file.
+  if (parseTree.v.size() > 1) {
+    parser::Program hermeticModules{std::move(parseTree.v)};
+    parseTree.v.emplace_back(std::move(hermeticModules.v.front()));
+    hermeticModules.v.pop_front();
+    Scope &hermeticScope{topScope.MakeScope(Scope::Kind::Global)};
+    context_.set_currentHermeticModuleFileScope(&hermeticScope);
+    ResolveNames(context_, hermeticModules, hermeticScope);
+  }
   GetModuleDependences(context_.moduleDependences(), sourceFile->content());
   ResolveNames(context_, parseTree, topScope);
   context_.foldingContext().set_moduleFileName(wasModuleFileName);
+  context_.set_currentHermeticModuleFileScope(nullptr);
   if (!moduleSymbol) {
     // Submodule symbols' storage are owned by their parents' scopes,
     // but their names are not in their parents' dictionaries -- we