[C++20][Modules] Prevent premature calls to PassInterestingDeclsToConsumer() within FinishedDeserializing(). (#129982)

`ASTReader::FinishedDeserializing` uses `NumCurrentElementsDeserializing` to keep track of nested `Deserializing` RAII actions. The `FinishedDeserializing` only performs actions if it is the top-level `Deserializing` layer. This works fine in general, but there is a problematic edge case.

If a call to `redecls()` in `FinishedDeserializing` performs deserialization, we re-enter `FinishedDeserializing` while in the middle of the previous `FinishedDeserializing` call.

The known problematic part of this is that this inner `FinishedDeserializing` can go all the way to `PassInterestingDeclsToConsumer`, which operates on `PotentiallyInterestingDecls` data structure which contain decls that should be handled by the previous `FinishedDeserializing` stage.

The other shared data structures are also somewhat concerning at a high-level in that the inner `FinishedDeserializing` would be handling pending actions that are not "within its scope", but this part is not known to be problematic.

We already have a guard within `PassInterestingDeclsToConsumer` because we can end up with recursive deserialization within `PassInterestingDeclsToConsumer`. The implemented solution is to apply this guard to the portion of `FinishedDeserializing` that performs further deserialization as well. This ensures that recursive deserialization does not trigger `PassInterestingDeclsToConsumer` which may operate on entries that are not ready to be passed.

clang/docs/ReleaseNotes.rst

@@ -278,6 +278,9 @@ Bug Fixes in This Version
   considered an error in C23 mode and are allowed as an extension in earlier language modes.
 
 - Remove the ``static`` specifier for the value of ``_FUNCTION_`` for static functions, in MSVC compatibility mode.
+- Fixed a modules crash where exception specifications were not propagated properly (#GH121245, relanded in #GH129982)
+- Fixed a problematic case with recursive deserialization within ``FinishedDeserializing()`` where
+  ``PassInterestingDeclsToConsumer()`` was called before the declarations were safe to be passed. (#GH129982)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Serialization/ASTReader.h

@@ -1176,11 +1176,11 @@ private:
   /// Number of Decl/types that are currently deserializing.
   unsigned NumCurrentElementsDeserializing = 0;
 
-  /// Set true while we are in the process of passing deserialized
-  /// "interesting" decls to consumer inside FinishedDeserializing().
-  /// This is used as a guard to avoid recursively repeating the process of
+  /// Set false while we are in a state where we cannot safely pass deserialized
+  /// "interesting" decls to the consumer inside FinishedDeserializing().
+  /// This is used as a guard to avoid recursively entering the process of
   /// passing decls to consumer.
-  bool PassingDeclsToConsumer = false;
+  bool CanPassDeclsToConsumer = true;
 
   /// The set of identifiers that were read while the AST reader was
   /// (recursively) loading declarations.

clang/lib/Serialization/ASTReader.cpp

@@ -10182,12 +10182,12 @@ void ASTReader::visitTopLevelModuleMaps(
 }
 
 void ASTReader::finishPendingActions() {
-  while (
-      !PendingIdentifierInfos.empty() || !PendingDeducedFunctionTypes.empty() ||
-      !PendingDeducedVarTypes.empty() || !PendingIncompleteDeclChains.empty() ||
-      !PendingDeclChains.empty() || !PendingMacroIDs.empty() ||
-      !PendingDeclContextInfos.empty() || !PendingUpdateRecords.empty() ||
-      !PendingObjCExtensionIvarRedeclarations.empty()) {
+  while (!PendingIdentifierInfos.empty() ||
+         !PendingDeducedFunctionTypes.empty() ||
+         !PendingDeducedVarTypes.empty() || !PendingDeclChains.empty() ||
+         !PendingMacroIDs.empty() || !PendingDeclContextInfos.empty() ||
+         !PendingUpdateRecords.empty() ||
+         !PendingObjCExtensionIvarRedeclarations.empty()) {
     // If any identifiers with corresponding top-level declarations have
     // been loaded, load those declarations now.
     using TopLevelDeclsMap =
@@ -10235,13 +10235,6 @@ void ASTReader::finishPendingActions() {
     }
     PendingDeducedVarTypes.clear();
 
-    // For each decl chain that we wanted to complete while deserializing, mark
-    // it as "still needs to be completed".
-    for (unsigned I = 0; I != PendingIncompleteDeclChains.size(); ++I) {
-      markIncompleteDeclChain(PendingIncompleteDeclChains[I]);
-    }
-    PendingIncompleteDeclChains.clear();
-
     // Load pending declaration chains.
     for (unsigned I = 0; I != PendingDeclChains.size(); ++I)
       loadPendingDeclChain(PendingDeclChains[I].first,
@@ -10479,6 +10472,43 @@ void ASTReader::finishPendingActions() {
   for (auto *ND : PendingMergedDefinitionsToDeduplicate)
     getContext().deduplicateMergedDefinitonsFor(ND);
   PendingMergedDefinitionsToDeduplicate.clear();
+
+  // For each decl chain that we wanted to complete while deserializing, mark
+  // it as "still needs to be completed".
+  for (Decl *D : PendingIncompleteDeclChains)
+    markIncompleteDeclChain(D);
+  PendingIncompleteDeclChains.clear();
+
+  assert(PendingIdentifierInfos.empty() &&
+         "Should be empty at the end of finishPendingActions");
+  assert(PendingDeducedFunctionTypes.empty() &&
+         "Should be empty at the end of finishPendingActions");
+  assert(PendingDeducedVarTypes.empty() &&
+         "Should be empty at the end of finishPendingActions");
+  assert(PendingDeclChains.empty() &&
+         "Should be empty at the end of finishPendingActions");
+  assert(PendingMacroIDs.empty() &&
+         "Should be empty at the end of finishPendingActions");
+  assert(PendingDeclContextInfos.empty() &&
+         "Should be empty at the end of finishPendingActions");
+  assert(PendingUpdateRecords.empty() &&
+         "Should be empty at the end of finishPendingActions");
+  assert(PendingObjCExtensionIvarRedeclarations.empty() &&
+         "Should be empty at the end of finishPendingActions");
+  assert(PendingFakeDefinitionData.empty() &&
+         "Should be empty at the end of finishPendingActions");
+  assert(PendingDefinitions.empty() &&
+         "Should be empty at the end of finishPendingActions");
+  assert(PendingWarningForDuplicatedDefsInModuleUnits.empty() &&
+         "Should be empty at the end of finishPendingActions");
+  assert(PendingBodies.empty() &&
+         "Should be empty at the end of finishPendingActions");
+  assert(PendingAddedClassMembers.empty() &&
+         "Should be empty at the end of finishPendingActions");
+  assert(PendingMergedDefinitionsToDeduplicate.empty() &&
+         "Should be empty at the end of finishPendingActions");
+  assert(PendingIncompleteDeclChains.empty() &&
+         "Should be empty at the end of finishPendingActions");
 }
 
 void ASTReader::diagnoseOdrViolations() {
@@ -10792,48 +10822,55 @@ void ASTReader::FinishedDeserializing() {
   --NumCurrentElementsDeserializing;
 
   if (NumCurrentElementsDeserializing == 0) {
-    // Propagate exception specification and deduced type updates along
-    // redeclaration chains.
-    //
-    // We do this now rather than in finishPendingActions because we want to
-    // be able to walk the complete redeclaration chains of the updated decls.
-    while (!PendingExceptionSpecUpdates.empty() ||
-           !PendingDeducedTypeUpdates.empty() ||
-           !PendingUndeducedFunctionDecls.empty()) {
-      auto ESUpdates = std::move(PendingExceptionSpecUpdates);
-      PendingExceptionSpecUpdates.clear();
-      for (auto Update : ESUpdates) {
-        ProcessingUpdatesRAIIObj ProcessingUpdates(*this);
-        auto *FPT = Update.second->getType()->castAs<FunctionProtoType>();
-        auto ESI = FPT->getExtProtoInfo().ExceptionSpec;
-        if (auto *Listener = getContext().getASTMutationListener())
-          Listener->ResolvedExceptionSpec(cast<FunctionDecl>(Update.second));
-        for (auto *Redecl : Update.second->redecls())
-          getContext().adjustExceptionSpec(cast<FunctionDecl>(Redecl), ESI);
+    {
+      // Guard variable to avoid recursively entering the process of passing
+      // decls to consumer.
+      SaveAndRestore GuardPassingDeclsToConsumer(CanPassDeclsToConsumer, false);
+
+      // Propagate exception specification and deduced type updates along
+      // redeclaration chains.
+      //
+      // We do this now rather than in finishPendingActions because we want to
+      // be able to walk the complete redeclaration chains of the updated decls.
+      while (!PendingExceptionSpecUpdates.empty() ||
+             !PendingDeducedTypeUpdates.empty() ||
+             !PendingUndeducedFunctionDecls.empty()) {
+        auto ESUpdates = std::move(PendingExceptionSpecUpdates);
+        PendingExceptionSpecUpdates.clear();
+        for (auto Update : ESUpdates) {
+          ProcessingUpdatesRAIIObj ProcessingUpdates(*this);
+          auto *FPT = Update.second->getType()->castAs<FunctionProtoType>();
+          auto ESI = FPT->getExtProtoInfo().ExceptionSpec;
+          if (auto *Listener = getContext().getASTMutationListener())
+            Listener->ResolvedExceptionSpec(cast<FunctionDecl>(Update.second));
+          for (auto *Redecl : Update.second->redecls())
+            getContext().adjustExceptionSpec(cast<FunctionDecl>(Redecl), ESI);
+        }
+
+        auto DTUpdates = std::move(PendingDeducedTypeUpdates);
+        PendingDeducedTypeUpdates.clear();
+        for (auto Update : DTUpdates) {
+          ProcessingUpdatesRAIIObj ProcessingUpdates(*this);
+          // FIXME: If the return type is already deduced, check that it
+          // matches.
+          getContext().adjustDeducedFunctionResultType(Update.first,
+                                                       Update.second);
+        }
+
+        auto UDTUpdates = std::move(PendingUndeducedFunctionDecls);
+        PendingUndeducedFunctionDecls.clear();
+        // We hope we can find the deduced type for the functions by iterating
+        // redeclarations in other modules.
+        for (FunctionDecl *UndeducedFD : UDTUpdates)
+          (void)UndeducedFD->getMostRecentDecl();
       }
 
-      auto DTUpdates = std::move(PendingDeducedTypeUpdates);
-      PendingDeducedTypeUpdates.clear();
-      for (auto Update : DTUpdates) {
-        ProcessingUpdatesRAIIObj ProcessingUpdates(*this);
-        // FIXME: If the return type is already deduced, check that it matches.
-        getContext().adjustDeducedFunctionResultType(Update.first,
-                                                     Update.second);
-      }
+      if (ReadTimer)
+        ReadTimer->stopTimer();
 
-      auto UDTUpdates = std::move(PendingUndeducedFunctionDecls);
-      PendingUndeducedFunctionDecls.clear();
-      // We hope we can find the deduced type for the functions by iterating
-      // redeclarations in other modules.
-      for (FunctionDecl *UndeducedFD : UDTUpdates)
-        (void)UndeducedFD->getMostRecentDecl();
+      diagnoseOdrViolations();
     }
 
-    if (ReadTimer)
-      ReadTimer->stopTimer();
-
-    diagnoseOdrViolations();
-
     // We are not in recursive loading, so it's safe to pass the "interesting"
     // decls to the consumer.
     if (Consumer)

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -4309,12 +4309,12 @@ Decl *ASTReader::ReadDeclRecord(GlobalDeclID ID) {
 void ASTReader::PassInterestingDeclsToConsumer() {
   assert(Consumer);
 
-  if (PassingDeclsToConsumer)
+  if (!CanPassDeclsToConsumer)
     return;
 
   // Guard variable to avoid recursively redoing the process of passing
   // decls to consumer.
-  SaveAndRestore GuardPassingDeclsToConsumer(PassingDeclsToConsumer, true);
+  SaveAndRestore GuardPassingDeclsToConsumer(CanPassDeclsToConsumer, false);
 
   // Ensure that we've loaded all potentially-interesting declarations
   // that need to be eagerly loaded.

clang/test/Modules/pr121245.cpp

@@ -0,0 +1,93 @@
+// If this test fails, it should be investigated under Debug builds.
+// Before the PR, this test was encountering an `llvm_unreachable()`.
+
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+// RUN: cd %t
+
+// RUN: %clang_cc1 -std=c++20 -emit-header-unit -xc++-user-header %t/hu-01.h \
+// RUN:  -fcxx-exceptions -o %t/hu-01.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-header-unit -xc++-user-header %t/hu-02.h \
+// RUN:  -Wno-experimental-header-units -fcxx-exceptions \
+// RUN:  -fmodule-file=%t/hu-01.pcm -o %t/hu-02.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-header-unit -xc++-user-header %t/hu-03.h \
+// RUN:  -Wno-experimental-header-units -fcxx-exceptions \
+// RUN:  -fmodule-file=%t/hu-01.pcm -o %t/hu-03.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-header-unit -xc++-user-header %t/hu-04.h \
+// RUN:  -Wno-experimental-header-units -fcxx-exceptions \
+// RUN:  -fmodule-file=%t/hu-01.pcm -o %t/hu-04.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-header-unit -xc++-user-header %t/hu-05.h \
+// RUN:  -Wno-experimental-header-units -fcxx-exceptions \
+// RUN:  -fmodule-file=%t/hu-03.pcm -fmodule-file=%t/hu-04.pcm \
+// RUN:  -fmodule-file=%t/hu-01.pcm -o %t/hu-05.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-obj %t/main.cpp \
+// RUN:  -Wno-experimental-header-units -fcxx-exceptions \
+// RUN:  -fmodule-file=%t/hu-02.pcm -fmodule-file=%t/hu-05.pcm \
+// RUN:  -fmodule-file=%t/hu-04.pcm -fmodule-file=%t/hu-03.pcm \
+// RUN:  -fmodule-file=%t/hu-01.pcm
+
+//--- hu-01.h
+template <typename T>
+struct A {
+  A() {}
+  ~A() {}
+};
+
+template <typename T>
+struct EBO : T {
+  EBO() = default;
+};
+
+template <typename T>
+struct HT : EBO<A<T>> {};
+
+//--- hu-02.h
+import "hu-01.h";
+
+inline void f() {
+  HT<int>();
+}
+
+//--- hu-03.h
+import "hu-01.h";
+
+struct C {
+  C();
+
+  HT<long> _;
+};
+
+//--- hu-04.h
+import "hu-01.h";
+
+void g(HT<long> = {});
+
+//--- hu-05.h
+import "hu-03.h";
+import "hu-04.h";
+import "hu-01.h";
+
+struct B {
+  virtual ~B() = default;
+
+  virtual void f() {
+    HT<long>();
+  }
+};
+
+//--- main.cpp
+import "hu-02.h";
+import "hu-05.h";
+import "hu-03.h";
+
+int main() {
+  f();
+  C();
+  B();
+}

clang/test/Modules/pr129982.cpp

@@ -0,0 +1,107 @@
+// If this test fails, it should be investigated under Debug builds.
+// Before the PR, this test was violating an assertion.
+
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -std=c++20 -emit-obj -fmodules \
+// RUN:  -fmodule-map-file=%t/module.modulemap \
+// RUN:  -fmodules-cache-path=%t %t/a.cpp
+
+//--- module.modulemap
+module ebo {
+  header "ebo.h"
+}
+
+module fwd {
+  header "fwd.h"
+}
+
+module s {
+  header "s.h"
+  export *
+}
+
+module mod {
+  header "a.h"
+  header "b.h"
+}
+
+//--- ebo.h
+#pragma once
+
+namespace N { inline namespace __1 {
+
+template <typename T>
+struct EBO : T {
+  EBO() = default;
+};
+
+}}
+
+//--- fwd.h
+#pragma once
+
+namespace N { inline namespace __1 {
+
+template <typename T>
+struct Empty;
+
+template <typename T>
+struct BS;
+
+using S = BS<Empty<char>>;
+
+}}
+
+//--- s.h
+#pragma once
+
+#include "fwd.h"
+#include "ebo.h"
+
+namespace N { inline namespace __1 {
+
+template <typename T>
+struct Empty {};
+
+template <typename T>
+struct BS {
+    EBO<T> _;
+    void f();
+};
+
+extern template void BS<Empty<char>>::f();
+
+}}
+
+//--- b.h
+#pragma once
+
+#include "s.h"
+
+struct B {
+  void f() {
+    N::S{}.f();
+  }
+};
+
+//--- a.h
+#pragma once
+
+#include "s.h"
+
+struct A {
+  void f(int) {}
+  void f(const N::S &) {}
+
+  void g();
+};
+
+//--- a.cpp
+#include "a.h"
+
+void A::g() { f(0); }
+
+// expected-no-diagnostics