…callback The `TypeSystemMap::m_mutex` guards against concurrent modifications of members of `TypeSystemMap`. In particular, `m_map`. `TypeSystemMap::ForEach` iterates through the entire `m_map` calling a user-specified callback for each entry. This is all done while `m_mutex` is locked. However, there's nothing that guarantees that the callback itself won't call back into `TypeSystemMap` APIs on the same thread. This lead to double-locking `m_mutex`, which is undefined behaviour. We've seen this cause a deadlock in the swift plugin with following backtrace: ``` int main() { std::unique_ptr<int> up = std::make_unique<int>(5); volatile int val = *up; return val; } clang++ -std=c++2a -g -O1 main.cpp ./bin/lldb -o “br se -p return” -o run -o “v *up” -o “expr *up” -b ``` ``` frame #4: std::lock_guard<std::mutex>::lock_guard frame #5: lldb_private::TypeSystemMap::GetTypeSystemForLanguage <<<< Lock #2 frame #6: lldb_private::TypeSystemMap::GetTypeSystemForLanguage frame #7: lldb_private::Target::GetScratchTypeSystemForLanguage ... frame #26: lldb_private::SwiftASTContext::LoadLibraryUsingPaths frame #27: lldb_private::SwiftASTContext::LoadModule frame #30: swift::ModuleDecl::collectLinkLibraries frame #31: lldb_private::SwiftASTContext::LoadModule frame #34: lldb_private::SwiftASTContext::GetCompileUnitImportsImpl frame #35: lldb_private::SwiftASTContext::PerformCompileUnitImports frame #36: lldb_private::TypeSystemSwiftTypeRefForExpressions::GetSwiftASTContext frame #37: lldb_private::TypeSystemSwiftTypeRefForExpressions::GetPersistentExpressionState frame #38: lldb_private::Target::GetPersistentSymbol frame #41: lldb_private::TypeSystemMap::ForEach <<<< Lock #1 frame #42: lldb_private::Target::GetPersistentSymbol frame #43: lldb_private::IRExecutionUnit::FindInUserDefinedSymbols frame #44: lldb_private::IRExecutionUnit::FindSymbol frame #45: lldb_private::IRExecutionUnit::MemoryManager::GetSymbolAddressAndPresence frame #46: lldb_private::IRExecutionUnit::MemoryManager::findSymbol frame #47: non-virtual thunk to lldb_private::IRExecutionUnit::MemoryManager::findSymbol frame #48: llvm::LinkingSymbolResolver::findSymbol frame #49: llvm::LegacyJITSymbolResolver::lookup frame #50: llvm::RuntimeDyldImpl::resolveExternalSymbols frame #51: llvm::RuntimeDyldImpl::resolveRelocations frame #52: llvm::MCJIT::finalizeLoadedModules frame #53: llvm::MCJIT::finalizeObject frame #54: lldb_private::IRExecutionUnit::ReportAllocations frame #55: lldb_private::IRExecutionUnit::GetRunnableInfo frame #56: lldb_private::ClangExpressionParser::PrepareForExecution frame #57: lldb_private::ClangUserExpression::TryParse frame #58: lldb_private::ClangUserExpression::Parse ``` Our solution is to simply iterate over a local copy of `m_map`. **Testing** * Confirmed on manual reproducer (would reproduce 100% of the time before the patch) Differential Revision: https://reviews.llvm.org/D149949

Need to apply b159987 #55

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