diff --git a/clang-tools-extra/clangd/ClangdServer.h b/clang-tools-extra/clangd/ClangdServer.h
--- a/clang-tools-extra/clangd/ClangdServer.h
+++ b/clang-tools-extra/clangd/ClangdServer.h
@@ -185,10 +185,6 @@
     /// regions in the document.
     bool PublishInactiveRegions = false;
 
-    /// Whether to run preamble indexing asynchronously in an independent
-    /// thread.
-    bool AsyncPreambleIndexing = false;
-
     explicit operator TUScheduler::Options() const;
   };
   // Sensible default options for use in tests.
diff --git a/clang-tools-extra/clangd/ClangdServer.cpp b/clang-tools-extra/clangd/ClangdServer.cpp
--- a/clang-tools-extra/clangd/ClangdServer.cpp
+++ b/clang-tools-extra/clangd/ClangdServer.cpp
@@ -67,11 +67,10 @@
   UpdateIndexCallbacks(FileIndex *FIndex,
                        ClangdServer::Callbacks *ServerCallbacks,
                        const ThreadsafeFS &TFS, AsyncTaskRunner *Tasks,
-                       bool CollectInactiveRegions,
-                       const ClangdServer::Options &Opts)
-      : FIndex(FIndex), ServerCallbacks(ServerCallbacks),
-        TFS(TFS), Stdlib{std::make_shared<StdLibSet>()}, Tasks(Tasks),
-        CollectInactiveRegions(CollectInactiveRegions), Opts(Opts) {}
+                       bool CollectInactiveRegions)
+      : FIndex(FIndex), ServerCallbacks(ServerCallbacks), TFS(TFS),
+        Stdlib{std::make_shared<StdLibSet>()}, Tasks(Tasks),
+        CollectInactiveRegions(CollectInactiveRegions) {}
 
   void onPreambleAST(
       PathRef Path, llvm::StringRef Version, CapturedASTCtx ASTCtx,
@@ -94,7 +93,7 @@
                              ASTCtx.getPreprocessor(), *CanonIncludes);
     };
 
-    if (Opts.AsyncPreambleIndexing && Tasks) {
+    if (Tasks) {
       Tasks->runAsync("Preamble indexing for:" + Path + Version,
                       std::move(Task));
     } else
@@ -164,7 +163,6 @@
   std::shared_ptr<StdLibSet> Stdlib;
   AsyncTaskRunner *Tasks;
   bool CollectInactiveRegions;
-  const ClangdServer::Options &Opts;
 };
 
 class DraftStoreFS : public ThreadsafeFS {
@@ -229,7 +227,7 @@
                         std::make_unique<UpdateIndexCallbacks>(
                             DynamicIdx.get(), Callbacks, TFS,
                             IndexTasks ? &*IndexTasks : nullptr,
-                            PublishInactiveRegions, Opts));
+                            PublishInactiveRegions));
   // Adds an index to the stack, at higher priority than existing indexes.
   auto AddIndex = [&](SymbolIndex *Idx) {
     if (this->Index != nullptr) {
diff --git a/clang-tools-extra/clangd/Preamble.cpp b/clang-tools-extra/clangd/Preamble.cpp
--- a/clang-tools-extra/clangd/Preamble.cpp
+++ b/clang-tools-extra/clangd/Preamble.cpp
@@ -11,6 +11,8 @@
 #include "Compiler.h"
 #include "Config.h"
 #include "Diagnostics.h"
+#include "FS.h"
+#include "FeatureModule.h"
 #include "Headers.h"
 #include "Protocol.h"
 #include "SourceCode.h"
@@ -21,8 +23,10 @@
 #include "support/ThreadsafeFS.h"
 #include "support/Trace.h"
 #include "clang/AST/DeclTemplate.h"
+#include "clang/AST/Type.h"
 #include "clang/Basic/Diagnostic.h"
 #include "clang/Basic/DiagnosticLex.h"
+#include "clang/Basic/DiagnosticOptions.h"
 #include "clang/Basic/LangOptions.h"
 #include "clang/Basic/SourceLocation.h"
 #include "clang/Basic/SourceManager.h"
@@ -51,12 +55,17 @@
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/ErrorOr.h"
 #include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/VirtualFileSystem.h"
 #include "llvm/Support/raw_ostream.h"
+#include <cassert>
+#include <chrono>
 #include <cstddef>
+#include <cstdint>
+#include <cstdlib>
 #include <functional>
 #include <memory>
 #include <optional>
@@ -616,7 +625,8 @@
       });
   llvm::IntrusiveRefCntPtr<DiagnosticsEngine> PreambleDiagsEngine =
       CompilerInstance::createDiagnostics(&CI.getDiagnosticOpts(),
-                                          &PreambleDiagnostics, false);
+                                          &PreambleDiagnostics,
+                                          /*ShouldOwnClient=*/false);
   const Config &Cfg = Config::current();
   PreambleDiagnostics.setLevelAdjuster([&](DiagnosticsEngine::Level DiagLevel,
                                            const clang::Diagnostic &Info) {
@@ -663,8 +673,12 @@
   auto BuiltPreamble = PrecompiledPreamble::Build(
       CI, ContentsBuffer.get(), Bounds, *PreambleDiagsEngine,
       Stats ? TimedFS : StatCacheFS, std::make_shared<PCHContainerOperations>(),
-      StoreInMemory, /*StoragePath=*/StringRef(), CapturedInfo);
+      StoreInMemory, /*StoragePath=*/"", CapturedInfo);
   PreambleTimer.stopTimer();
+  // Reset references to ref-counted-ptrs before executing the callbacks, to
+  // prevent resetting them concurrently.
+  PreambleDiagsEngine.reset();
+  CI.DiagnosticOpts.reset();
 
   // When building the AST for the main file, we do want the function
   // bodies.