[Lldb-commits] [lldb] [lldb][ClangASTImporter][NFC] Emit a log message when we break MapImported invariant (PR #112748)

[Michael Buch via lldb-commits]

https://github.com/Michael137 created https://github.com/llvm/llvm-project/pull/112748

This patch emits a warning into the expression log when we call `MapImported` on a decl which has already been imported, but with a new `to` destination decl. In asserts builds this would lead to triggering this [ASTImporter::MapImported assertion](https://github.com/llvm/llvm-project/blob/6d7712a70c163d2ae9e1dc928db31fcb45d9e404/clang/lib/AST/ASTImporter.cpp#L10493-L10494). In no-asserts builds we will likely crash, in potentially non-obvious ways. The hope is that the log message will help in diagnosing this type of issue in the field.

The underlying issue is discussed in more detail in: https://github.com/llvm/llvm-project/pull/112566.

In a non-asserts build, the last few expression log entries would look as follows:
```
     CompleteTagDecl on (ASTContext*)scratch ASTContext Completing (TagDecl*)0x00000001132d31d0 named Foo
       CTD Before:
CXXRecordDecl 0x1132d31d0 <<invalid sloc>> <invalid sloc> <undeserialized declarations> struct Foo

 [ClangASTImporter] WARNING: overwriting an already imported decl '0x000000014378fd80' ('Foo') from '0x0000000143790c00' with 0x00000001132d31d0. Likely due to a name conflict when importing 'Foo'.
     [ClangASTImporter] Imported (FieldDecl*)0x0000000143790220, named service (from (Decl*)0x0000000143791270), metadata 271
     [ClangASTImporter] Decl has no origin information in (ASTContext*)0x00000001132c8c00
 FindExternalLexicalDecls on (ASTContext*)0x0000000143c1f600 'scratch ASTContext' in 'Foo' (CXXRecordDecl*)0x000000014378FD80
   FELD Original decl (ASTContext*)0x00000001132c8c00 (Decl*)0x0000000143790c00:
CXXRecordDecl 0x143790c00 <<invalid sloc>> <invalid sloc> struct Foo definition
|-DefinitionData pass_in_registers aggregate standard_layout trivially_copyable pod trivial literal
| |-DefaultConstructor exists trivial needs_implicit
| |-CopyConstructor simple trivial has_const_param needs_implicit implicit_has_const_param
| |-MoveConstructor exists simple trivial needs_implicit
| |-CopyAssignment simple trivial has_const_param needs_implicit implicit_has_const_param
| |-MoveAssignment exists simple trivial needs_implicit
| `-Destructor simple irrelevant trivial needs_implicit
|-FieldDecl 0x143791270 <<invalid sloc>> <invalid sloc> service 'Service *'
`-FieldDecl 0x1437912c8 <<invalid sloc>> <invalid sloc> mach_endpoint 'int'

   FELD Adding [to CXXRecordDecl Foo] lexical FieldDecl FieldDecl 0x143791270 <<invalid sloc>> <invalid sloc> service 'Service *'

   FELD Adding [to CXXRecordDecl Foo] lexical FieldDecl FieldDecl 0x1437912c8 <<invalid sloc>> <invalid sloc> mach_endpoint 'int'

     [ClangASTImporter] Imported (FieldDecl*)0x0000000143790278, named mach_endpoint (from (Decl*)0x00000001437912c8), metadata 280
     [ClangASTImporter] Decl has no origin information in (ASTContext*)0x00000001132c8c00
```
Note how we start "completing" `Foo`. Then emit our new `WARNING`. Shortly after, we crash, and the log abruptly ends.

rdar://135551810

>From 247f2638be2decf0f806050e90c25c4b664b2f97 Mon Sep 17 00:00:00 2001
From: Michael Buch <michaelbuch12 at gmail.com>
Date: Thu, 17 Oct 2024 17:40:35 +0100
Subject: [PATCH 1/2] [lldb][ClangASTImporter][NFC] Emit a log message when we
 break MapImported invariant

---
 .../Clang/ClangASTImporter.cpp                | 27 ++++++++++++++-----
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/lldb/source/Plugins/ExpressionParser/Clang/ClangASTImporter.cpp b/lldb/source/Plugins/ExpressionParser/Clang/ClangASTImporter.cpp
index 630ad7e20ab7e0..9a0e354daff712 100644
--- a/lldb/source/Plugins/ExpressionParser/Clang/ClangASTImporter.cpp
+++ b/lldb/source/Plugins/ExpressionParser/Clang/ClangASTImporter.cpp
@@ -1136,6 +1136,25 @@ ClangASTImporter::ASTImporterDelegate::ImportImpl(Decl *From) {
 
 void ClangASTImporter::ASTImporterDelegate::ImportDefinitionTo(
     clang::Decl *to, clang::Decl *from) {
+  Log *log = GetLog(LLDBLog::Expressions);
+
+  auto getDeclName = [](Decl const *decl) {
+    std::string name_string;
+    if (auto const *from_named_decl = dyn_cast<clang::NamedDecl>(decl)) {
+      llvm::raw_string_ostream name_stream(name_string);
+      from_named_decl->printName(name_stream);
+    }
+
+    return name_string;
+  };
+
+  if (auto *D = GetAlreadyImportedOrNull(from); D && D != to)
+    LLDB_LOG(log,
+             "[ClangASTImporter] WARNING: overwriting an already imported decl "
+             "'{0:x}' ('{1}') from '{2:x}' with {3:x}. Likely due to a name "
+             "conflict when importing {1}.",
+             D, getDeclName(from), from, to);
+
   // We might have a forward declaration from a shared library that we
   // gave external lexical storage so that Clang asks us about the full
   // definition when it needs it. In this case the ASTImporter isn't aware
@@ -1145,8 +1164,6 @@ void ClangASTImporter::ASTImporterDelegate::ImportDefinitionTo(
   // tell the ASTImporter that 'to' was imported from 'from'.
   MapImported(from, to);
 
-  Log *log = GetLog(LLDBLog::Expressions);
-
   if (llvm::Error err = ImportDefinition(from)) {
     LLDB_LOG_ERROR(log, std::move(err),
                    "[ClangASTImporter] Error during importing definition: {0}");
@@ -1158,11 +1175,7 @@ void ClangASTImporter::ASTImporterDelegate::ImportDefinitionTo(
       to_tag->setCompleteDefinition(from_tag->isCompleteDefinition());
 
       if (Log *log_ast = GetLog(LLDBLog::AST)) {
-        std::string name_string;
-        if (NamedDecl *from_named_decl = dyn_cast<clang::NamedDecl>(from)) {
-          llvm::raw_string_ostream name_stream(name_string);
-          from_named_decl->printName(name_stream);
-        }
+        std::string name_string = getDeclName(from);
         LLDB_LOG(log_ast,
                  "==== [ClangASTImporter][TUDecl: {0:x}] Imported "
                  "({1}Decl*){2:x}, named {3} (from "

>From c1e78158074549d83448733a6560fa53fa843041 Mon Sep 17 00:00:00 2001
From: Michael Buch <michaelbuch12 at gmail.com>
Date: Thu, 17 Oct 2024 17:52:07 +0100
Subject: [PATCH 2/2] fixup! add quotes

---
 .../Plugins/ExpressionParser/Clang/ClangASTImporter.cpp       | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/lldb/source/Plugins/ExpressionParser/Clang/ClangASTImporter.cpp b/lldb/source/Plugins/ExpressionParser/Clang/ClangASTImporter.cpp
index 9a0e354daff712..2fbd8e6ca688fc 100644
--- a/lldb/source/Plugins/ExpressionParser/Clang/ClangASTImporter.cpp
+++ b/lldb/source/Plugins/ExpressionParser/Clang/ClangASTImporter.cpp
@@ -1151,8 +1151,8 @@ void ClangASTImporter::ASTImporterDelegate::ImportDefinitionTo(
   if (auto *D = GetAlreadyImportedOrNull(from); D && D != to)
     LLDB_LOG(log,
              "[ClangASTImporter] WARNING: overwriting an already imported decl "
-             "'{0:x}' ('{1}') from '{2:x}' with {3:x}. Likely due to a name "
-             "conflict when importing {1}.",
+             "'{0:x}' ('{1}') from '{2:x}' with '{3:x}'. Likely due to a name "
+             "conflict when importing '{1}'.",
              D, getDeclName(from), from, to);
 
   // We might have a forward declaration from a shared library that we