[clangd] Cache FS stat() calls when building preamble.linaro-local/ci/tcwg-llvm-kernel-baseline-armv8l-release-stablelinaro-local/ci/tcwg-llvm-kernel-baseline-armv8l-release-nextlinaro-local/ci/tcwg-llvm-kernel-baseline-armv8l-release-ltslinaro-local/ci/tcwg-llvm-kernel-baseline-aarch64-release-nextlinaro-local/ci/llvm-kernel-armv8l-baselinelinaro-local/ci/llvm-kernel-aarch64-baseline

Summary:
The file stats can be reused when preamble is reused (e.g. code
completion). It's safe to assume that cached status is not outdated as we
assume preamble files to remain unchanged.

On real file system, this made code completion ~20% faster on a measured file
(with big preamble). The preamble build time doesn't change much.

Reviewers: sammccall, ilya-biryukov

Reviewed By: sammccall

Subscribers: mgorny, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D52419

12 files changed, 319 insertions, 33 deletions

diff --git a/clang-tools-extra/clangd/CMakeLists.txt b/clang-tools-extra/clangd/CMakeLists.txt
index 0894a927ead..bbe8df00720 100644
--- a/clang-tools-extra/clangd/CMakeLists.txt
+++ b/clang-tools-extra/clangd/CMakeLists.txt
@@ -21,6 +21,7 @@ add_clang_library(clangDaemon
   DraftStore.cpp
   FindSymbols.cpp
   FileDistance.cpp
+  FS.cpp
   FuzzyMatch.cpp
   GlobalCompilationDatabase.cpp
   Headers.cpp
diff --git a/clang-tools-extra/clangd/ClangdServer.cpp b/clang-tools-extra/clangd/ClangdServer.cpp
index 153dce29484..01fbe34fa87 100644
--- a/clang-tools-extra/clangd/ClangdServer.cpp
+++ b/clang-tools-extra/clangd/ClangdServer.cpp
@@ -181,8 +181,6 @@ void ClangdServer::codeComplete(PathRef File, Position Pos,
     if (isCancelled())
       return CB(llvm::make_error<CancelledError>());
 
-    auto PreambleData = IP->Preamble;
-
     llvm::Optional<SpeculativeFuzzyFind> SpecFuzzyFind;
     if (CodeCompleteOpts.Index && CodeCompleteOpts.SpeculativeIndexRequest) {
       SpecFuzzyFind.emplace();
@@ -195,10 +193,8 @@ void ClangdServer::codeComplete(PathRef File, Position Pos,
     // FIXME(ibiryukov): even if Preamble is non-null, we may want to check
     // both the old and the new version in case only one of them matches.
     CodeCompleteResult Result = clangd::codeComplete(
-        File, IP->Command, PreambleData ? &PreambleData->Preamble : nullptr,
-        PreambleData ? PreambleData->Includes : IncludeStructure(),
-        IP->Contents, Pos, FS, PCHs, CodeCompleteOpts,
-        SpecFuzzyFind ? SpecFuzzyFind.getPointer() : nullptr);
+        File, IP->Command, IP->Preamble, IP->Contents, Pos, FS, PCHs,
+        CodeCompleteOpts, SpecFuzzyFind ? SpecFuzzyFind.getPointer() : nullptr);
     {
       clang::clangd::trace::Span Tracer("Completion results callback");
       CB(std::move(Result));
@@ -231,9 +227,8 @@ void ClangdServer::signatureHelp(PathRef File, Position Pos,
       return CB(IP.takeError());
 
     auto PreambleData = IP->Preamble;
-    CB(clangd::signatureHelp(File, IP->Command,
-                             PreambleData ? &PreambleData->Preamble : nullptr,
-                             IP->Contents, Pos, FS, PCHs, Index));
+    CB(clangd::signatureHelp(File, IP->Command, PreambleData, IP->Contents, Pos,
+                             FS, PCHs, Index));
   };
 
   // Unlike code completion, we wait for an up-to-date preamble here.
diff --git a/clang-tools-extra/clangd/ClangdUnit.cpp b/clang-tools-extra/clangd/ClangdUnit.cpp
index 6c184ff06b0..2ad6dc69cc8 100644
--- a/clang-tools-extra/clangd/ClangdUnit.cpp
+++ b/clang-tools-extra/clangd/ClangdUnit.cpp
@@ -246,9 +246,10 @@ const IncludeStructure &ParsedAST::getIncludeStructure() const {
 }
 
 PreambleData::PreambleData(PrecompiledPreamble Preamble,
-                           std::vector<Diag> Diags, IncludeStructure Includes)
+                           std::vector<Diag> Diags, IncludeStructure Includes,
+                           std::unique_ptr<PreambleFileStatusCache> StatCache)
     : Preamble(std::move(Preamble)), Diags(std::move(Diags)),
-      Includes(std::move(Includes)) {}
+      Includes(std::move(Includes)), StatCache(std::move(StatCache)) {}
 
 ParsedAST::ParsedAST(std::shared_ptr<const PreambleData> Preamble,
                      std::unique_ptr<CompilerInstance> Clang,
@@ -334,9 +335,12 @@ std::shared_ptr<const PreambleData> clangd::buildPreamble(
     // We proceed anyway, our lit-tests rely on results for non-existing working
     // dirs.
   }
+
+  auto StatCache = llvm::make_unique<PreambleFileStatusCache>();
   auto BuiltPreamble = PrecompiledPreamble::Build(
-      CI, ContentsBuffer.get(), Bounds, *PreambleDiagsEngine, Inputs.FS, PCHs,
-      StoreInMemory, SerializedDeclsCollector);
+      CI, ContentsBuffer.get(), Bounds, *PreambleDiagsEngine,
+      StatCache->getProducingFS(Inputs.FS), PCHs, StoreInMemory,
+      SerializedDeclsCollector);
 
   // When building the AST for the main file, we do want the function
   // bodies.
@@ -347,7 +351,7 @@ std::shared_ptr<const PreambleData> clangd::buildPreamble(
          FileName);
     return std::make_shared<PreambleData>(
         std::move(*BuiltPreamble), PreambleDiagnostics.take(),
-        SerializedDeclsCollector.takeIncludes());
+        SerializedDeclsCollector.takeIncludes(), std::move(StatCache));
   } else {
     elog("Could not build a preamble for file {0}", FileName);
     return nullptr;
@@ -361,15 +365,19 @@ llvm::Optional<ParsedAST> clangd::buildAST(
   trace::Span Tracer("BuildAST");
   SPAN_ATTACH(Tracer, "File", FileName);
 
-  if (Inputs.FS->setCurrentWorkingDirectory(Inputs.CompileCommand.Directory)) {
+  auto VFS = Inputs.FS;
+  if (Preamble && Preamble->StatCache)
+    VFS = Preamble->StatCache->getConsumingFS(std::move(VFS));
+  if (VFS->setCurrentWorkingDirectory(Inputs.CompileCommand.Directory)) {
     log("Couldn't set working directory when building the preamble.");
     // We proceed anyway, our lit-tests rely on results for non-existing working
     // dirs.
   }
 
-  return ParsedAST::build(
-      llvm::make_unique<CompilerInvocation>(*Invocation), Preamble,
-      llvm::MemoryBuffer::getMemBufferCopy(Inputs.Contents), PCHs, Inputs.FS);
+  return ParsedAST::build(llvm::make_unique<CompilerInvocation>(*Invocation),
+                          Preamble,
+                          llvm::MemoryBuffer::getMemBufferCopy(Inputs.Contents),
+                          PCHs, std::move(VFS));
 }
 
 SourceLocation clangd::getBeginningOfIdentifier(ParsedAST &Unit,
diff --git a/clang-tools-extra/clangd/ClangdUnit.h b/clang-tools-extra/clangd/ClangdUnit.h
index f739270a104..184a7a6c517 100644
--- a/clang-tools-extra/clangd/ClangdUnit.h
+++ b/clang-tools-extra/clangd/ClangdUnit.h
@@ -11,6 +11,7 @@
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_CLANGDUNIT_H
 
 #include "Diagnostics.h"
+#include "FS.h"
 #include "Function.h"
 #include "Headers.h"
 #include "Path.h"
@@ -45,7 +46,8 @@ namespace clangd {
 // Stores Preamble and associated data.
 struct PreambleData {
   PreambleData(PrecompiledPreamble Preamble, std::vector<Diag> Diags,
-               IncludeStructure Includes);
+               IncludeStructure Includes,
+               std::unique_ptr<PreambleFileStatusCache> StatCache);
 
   tooling::CompileCommand CompileCommand;
   PrecompiledPreamble Preamble;
@@ -53,6 +55,9 @@ struct PreambleData {
   // Processes like code completions and go-to-definitions will need #include
   // information, and their compile action skips preamble range.
   IncludeStructure Includes;
+  // Cache of FS operations performed when building the preamble.
+  // When reusing a preamble, this cache can be consumed to save IO.
+  std::unique_ptr<PreambleFileStatusCache> StatCache;
 };
 
 /// Information required to run clang, e.g. to parse AST or do code completion.
diff --git a/clang-tools-extra/clangd/CodeComplete.cpp b/clang-tools-extra/clangd/CodeComplete.cpp
index 48e495afaab..9e2f162b449 100644
--- a/clang-tools-extra/clangd/CodeComplete.cpp
+++ b/clang-tools-extra/clangd/CodeComplete.cpp
@@ -20,6 +20,7 @@
 
 #include "CodeComplete.h"
 #include "AST.h"
+#include "ClangdUnit.h"
 #include "CodeCompletionStrings.h"
 #include "Compiler.h"
 #include "Diagnostics.h"
@@ -986,7 +987,7 @@ private:
 struct SemaCompleteInput {
   PathRef FileName;
   const tooling::CompileCommand &Command;
-  PrecompiledPreamble const *Preamble;
+  const PreambleData *Preamble;
   StringRef Contents;
   Position Pos;
   IntrusiveRefCntPtr<vfs::FileSystem> VFS;
@@ -1010,12 +1011,15 @@ bool semaCodeComplete(std::unique_ptr<CodeCompleteConsumer> Consumer,
     // working dirs.
   }
 
+  IntrusiveRefCntPtr<vfs::FileSystem> VFS = Input.VFS;
+  if (Input.Preamble && Input.Preamble->StatCache)
+    VFS = Input.Preamble->StatCache->getConsumingFS(std::move(VFS));
   IgnoreDiagnostics DummyDiagsConsumer;
   auto CI = createInvocationFromCommandLine(
       ArgStrs,
       CompilerInstance::createDiagnostics(new DiagnosticOptions,
                                           &DummyDiagsConsumer, false),
-      Input.VFS);
+      VFS);
   if (!CI) {
     elog("Couldn't create CompilerInvocation");
     return false;
@@ -1054,8 +1058,10 @@ bool semaCodeComplete(std::unique_ptr<CodeCompleteConsumer> Consumer,
   // NOTE: we must call BeginSourceFile after prepareCompilerInstance. Otherwise
   // the remapped buffers do not get freed.
   auto Clang = prepareCompilerInstance(
-      std::move(CI), CompletingInPreamble ? nullptr : Input.Preamble,
-      std::move(ContentsBuffer), std::move(Input.PCHs), std::move(Input.VFS),
+      std::move(CI),
+      (Input.Preamble && !CompletingInPreamble) ? &Input.Preamble->Preamble
+                                                : nullptr,
+      std::move(ContentsBuffer), std::move(Input.PCHs), std::move(VFS),
       DummyDiagsConsumer);
   Clang->getPreprocessorOpts().SingleFileParseMode = CompletingInPreamble;
   Clang->setCodeCompletionConsumer(Consumer.release());
@@ -1565,19 +1571,20 @@ speculateCompletionFilter(llvm::StringRef Content, Position Pos) {
 
 CodeCompleteResult
 codeComplete(PathRef FileName, const tooling::CompileCommand &Command,
-             PrecompiledPreamble const *Preamble,
-             const IncludeStructure &PreambleInclusions, StringRef Contents,
-             Position Pos, IntrusiveRefCntPtr<vfs::FileSystem> VFS,
+             const PreambleData *Preamble, StringRef Contents, Position Pos,
+             IntrusiveRefCntPtr<vfs::FileSystem> VFS,
              std::shared_ptr<PCHContainerOperations> PCHs,
              CodeCompleteOptions Opts, SpeculativeFuzzyFind *SpecFuzzyFind) {
-  return CodeCompleteFlow(FileName, PreambleInclusions, SpecFuzzyFind, Opts)
+  return CodeCompleteFlow(FileName,
+                          Preamble ? Preamble->Includes : IncludeStructure(),
+                          SpecFuzzyFind, Opts)
       .run({FileName, Command, Preamble, Contents, Pos, VFS, PCHs});
 }
 
 SignatureHelp signatureHelp(PathRef FileName,
                             const tooling::CompileCommand &Command,
-                            PrecompiledPreamble const *Preamble,
-                            StringRef Contents, Position Pos,
+                            const PreambleData *Preamble, StringRef Contents,
+                            Position Pos,
                             IntrusiveRefCntPtr<vfs::FileSystem> VFS,
                             std::shared_ptr<PCHContainerOperations> PCHs,
                             const SymbolIndex *Index) {
diff --git a/clang-tools-extra/clangd/CodeComplete.h b/clang-tools-extra/clangd/CodeComplete.h
index 92fa2279a5c..d0c8a70673f 100644
--- a/clang-tools-extra/clangd/CodeComplete.h
+++ b/clang-tools-extra/clangd/CodeComplete.h
@@ -16,6 +16,7 @@
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_CODECOMPLETE_H
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_CODECOMPLETE_H
 
+#include "ClangdUnit.h"
 #include "Headers.h"
 #include "Logger.h"
 #include "Path.h"
@@ -221,8 +222,7 @@ struct SpeculativeFuzzyFind {
 /// destroyed when the async request finishes.
 CodeCompleteResult codeComplete(PathRef FileName,
                                 const tooling::CompileCommand &Command,
-                                PrecompiledPreamble const *Preamble,
-                                const IncludeStructure &PreambleInclusions,
+                                const PreambleData *Preamble,
                                 StringRef Contents, Position Pos,
                                 IntrusiveRefCntPtr<vfs::FileSystem> VFS,
                                 std::shared_ptr<PCHContainerOperations> PCHs,
@@ -232,8 +232,8 @@ CodeCompleteResult codeComplete(PathRef FileName,
 /// Get signature help at a specified \p Pos in \p FileName.
 SignatureHelp signatureHelp(PathRef FileName,
                             const tooling::CompileCommand &Command,
-                            PrecompiledPreamble const *Preamble,
-                            StringRef Contents, Position Pos,
+                            const PreambleData *Preamble, StringRef Contents,
+                            Position Pos,
                             IntrusiveRefCntPtr<vfs::FileSystem> VFS,
                             std::shared_ptr<PCHContainerOperations> PCHs,
                             const SymbolIndex *Index);
diff --git a/clang-tools-extra/clangd/FS.cpp b/clang-tools-extra/clangd/FS.cpp
new file mode 100644
index 00000000000..ce62a59271f
--- /dev/null
+++ b/clang-tools-extra/clangd/FS.cpp
@@ -0,0 +1,92 @@
+//===--- FS.cpp - File system related utils ----------------------*- C++-*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "FS.h"
+#include "clang/Basic/VirtualFileSystem.h"
+#include "llvm/ADT/None.h"
+
+namespace clang {
+namespace clangd {
+
+void PreambleFileStatusCache::update(const vfs::FileSystem &FS, vfs::Status S) {
+  SmallString<32> PathStore(S.getName());
+  if (auto Err = FS.makeAbsolute(PathStore))
+    return;
+  // Stores the latest status in cache as it can change in a preamble build.
+  StatCache.insert({PathStore, std::move(S)});
+}
+
+llvm::Optional<vfs::Status>
+PreambleFileStatusCache::lookup(llvm::StringRef File) const {
+  auto I = StatCache.find(File);
+  if (I != StatCache.end())
+    return I->getValue();
+  return llvm::None;
+}
+
+IntrusiveRefCntPtr<vfs::FileSystem> PreambleFileStatusCache::getProducingFS(
+    IntrusiveRefCntPtr<vfs::FileSystem> FS) {
+  // This invalidates old status in cache if files are re-`open()`ed or
+  // re-`stat()`ed in case file status has changed during preamble build.
+  class CollectFS : public vfs::ProxyFileSystem {
+  public:
+    CollectFS(IntrusiveRefCntPtr<vfs::FileSystem> FS,
+              PreambleFileStatusCache &StatCache)
+        : ProxyFileSystem(std::move(FS)), StatCache(StatCache) {}
+
+    llvm::ErrorOr<std::unique_ptr<vfs::File>>
+    openFileForRead(const Twine &Path) override {
+      auto File = getUnderlyingFS().openFileForRead(Path);
+      if (!File || !*File)
+        return File;
+      // Eagerly stat opened file, as the followup `status` call on the file
+      // doesn't necessarily go through this FS. This puts some extra work on
+      // preamble build, but it should be worth it as preamble can be reused
+      // many times (e.g. code completion) and the repeated status call is
+      // likely to be cached in the underlying file system anyway.
+      if (auto S = File->get()->status())
+        StatCache.update(getUnderlyingFS(), std::move(*S));
+      return File;
+    }
+
+    llvm::ErrorOr<vfs::Status> status(const Twine &Path) override {
+      auto S = getUnderlyingFS().status(Path);
+      if (S)
+        StatCache.update(getUnderlyingFS(), *S);
+      return S;
+    }
+
+  private:
+    PreambleFileStatusCache &StatCache;
+  };
+  return IntrusiveRefCntPtr<CollectFS>(new CollectFS(std::move(FS), *this));
+}
+
+IntrusiveRefCntPtr<vfs::FileSystem> PreambleFileStatusCache::getConsumingFS(
+    IntrusiveRefCntPtr<vfs::FileSystem> FS) const {
+  class CacheVFS : public vfs::ProxyFileSystem {
+  public:
+    CacheVFS(IntrusiveRefCntPtr<vfs::FileSystem> FS,
+             const PreambleFileStatusCache &StatCache)
+        : ProxyFileSystem(std::move(FS)), StatCache(StatCache) {}
+
+    llvm::ErrorOr<vfs::Status> status(const Twine &Path) override {
+      if (auto S = StatCache.lookup(Path.str()))
+        return *S;
+      return getUnderlyingFS().status(Path);
+    }
+
+  private:
+    const PreambleFileStatusCache &StatCache;
+  };
+  return IntrusiveRefCntPtr<CacheVFS>(new CacheVFS(std::move(FS), *this));
+}
+
+} // namespace clangd
+} // namespace clang
diff --git a/clang-tools-extra/clangd/FS.h b/clang-tools-extra/clangd/FS.h
new file mode 100644
index 00000000000..5f0fe62c6c0
--- /dev/null
+++ b/clang-tools-extra/clangd/FS.h
@@ -0,0 +1,65 @@
+//===--- FS.h - File system related utils ------------------------*- C++-*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_FS_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_FS_H
+
+#include "clang/Basic/VirtualFileSystem.h"
+#include "llvm/ADT/Optional.h"
+
+namespace clang {
+namespace clangd {
+
+/// Records status information for files open()ed or stat()ed during preamble
+/// build, so we can avoid stat()s on the underlying FS when reusing the
+/// preamble. For example, code completion can re-stat files when getting FileID
+/// for source locations stored in preamble (e.g. checking whether a location is
+/// in the main file).
+///
+/// The cache is keyed by absolute path of file name in cached status, as this
+/// is what preamble stores.
+///
+/// The cache is not thread-safe when updates happen, so the use pattern should
+/// be:
+///   - One FS writes to the cache from one thread (or several but strictly
+///   sequenced), e.g. when building preamble.
+///   - Sequence point (no writes after this point, no reads before).
+///   - Several FSs can read from the cache, e.g. code completions.
+///
+/// Note that the cache is only valid when reusing preamble.
+class PreambleFileStatusCache {
+public:
+  void update(const vfs::FileSystem &FS, vfs::Status S);
+  /// \p Path is a path stored in preamble.
+  llvm::Optional<vfs::Status> lookup(llvm::StringRef Path) const;
+
+  /// Returns a VFS that collects file status.
+  /// Only cache stats for files that exist because
+  ///   1) we only care about existing files when reusing preamble, unlike
+  ///   building preamble.
+  ///   2) we use the file name in the Status as the cache key.
+  ///
+  /// Note that the returned VFS should not outlive the cache.
+  IntrusiveRefCntPtr<vfs::FileSystem>
+  getProducingFS(IntrusiveRefCntPtr<vfs::FileSystem> FS);
+
+  /// Returns a VFS that uses the cache collected.
+  ///
+  /// Note that the returned VFS should not outlive the cache.
+  IntrusiveRefCntPtr<vfs::FileSystem>
+  getConsumingFS(IntrusiveRefCntPtr<vfs::FileSystem> FS) const;
+
+private:
+  llvm::StringMap<vfs::Status> StatCache;
+};
+
+} // namespace clangd
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANGD_FS_H
diff --git a/clang-tools-extra/unittests/clangd/CMakeLists.txt b/clang-tools-extra/unittests/clangd/CMakeLists.txt
index a870c2990a9..1c98347be55 100644
--- a/clang-tools-extra/unittests/clangd/CMakeLists.txt
+++ b/clang-tools-extra/unittests/clangd/CMakeLists.txt
@@ -21,6 +21,7 @@ add_extra_unittest(ClangdTests
   FileDistanceTests.cpp
   FileIndexTests.cpp
   FindSymbolsTests.cpp
+  FSTests.cpp
   FuzzyMatchTests.cpp
   GlobalCompilationDatabaseTests.cpp
   HeadersTests.cpp
diff --git a/clang-tools-extra/unittests/clangd/ClangdTests.cpp b/clang-tools-extra/unittests/clangd/ClangdTests.cpp
index ace2cef4b45..21300e906c8 100644
--- a/clang-tools-extra/unittests/clangd/ClangdTests.cpp
+++ b/clang-tools-extra/unittests/clangd/ClangdTests.cpp
@@ -963,6 +963,71 @@ TEST_F(ClangdVFSTest, ChangedHeaderFromISystem) {
                                        Field(&CodeCompletion::Name, "baz")));
 }
 
+// Check that running code completion doesn't stat() a bunch of files from the
+// preamble again. (They should be using the preamble's stat-cache)
+TEST(ClangdTests, PreambleVFSStatCache) {
+  class ListenStatsFSProvider : public FileSystemProvider {
+  public:
+    ListenStatsFSProvider(llvm::StringMap<unsigned> &CountStats)
+        : CountStats(CountStats) {}
+
+    IntrusiveRefCntPtr<vfs::FileSystem> getFileSystem() override {
+      class ListenStatVFS : public vfs::ProxyFileSystem {
+      public:
+        ListenStatVFS(IntrusiveRefCntPtr<vfs::FileSystem> FS,
+                      llvm::StringMap<unsigned> &CountStats)
+            : ProxyFileSystem(std::move(FS)), CountStats(CountStats) {}
+
+        llvm::ErrorOr<std::unique_ptr<vfs::File>>
+        openFileForRead(const Twine &Path) override {
+          ++CountStats[llvm::sys::path::filename(Path.str())];
+          return ProxyFileSystem::openFileForRead(Path);
+        }
+        llvm::ErrorOr<vfs::Status> status(const Twine &Path) override {
+          ++CountStats[llvm::sys::path::filename(Path.str())];
+          return ProxyFileSystem::status(Path);
+        }
+
+      private:
+        llvm::StringMap<unsigned> &CountStats;
+      };
+
+      return IntrusiveRefCntPtr<ListenStatVFS>(
+          new ListenStatVFS(buildTestFS(Files), CountStats));
+    }
+
+    // If relative paths are used, they are resolved with testPath().
+    llvm::StringMap<std::string> Files;
+    llvm::StringMap<unsigned> &CountStats;
+  };
+
+  llvm::StringMap<unsigned> CountStats;
+  ListenStatsFSProvider FS(CountStats);
+  ErrorCheckingDiagConsumer DiagConsumer;
+  MockCompilationDatabase CDB;
+  ClangdServer Server(CDB, FS, DiagConsumer, ClangdServer::optsForTest());
+
+  auto SourcePath = testPath("foo.cpp");
+  auto HeaderPath = testPath("foo.h");
+  FS.Files[HeaderPath] = "struct TestSym {};";
+  Annotations Code(R"cpp(
+    #include "foo.h"
+
+    int main() {
+      TestSy^
+    })cpp");
+
+  runAddDocument(Server, SourcePath, Code.code());
+
+  EXPECT_EQ(CountStats["foo.h"], 1u);
+  auto Completions = cantFail(runCodeComplete(Server, SourcePath, Code.point(),
+                                              clangd::CodeCompleteOptions()))
+                         .Completions;
+  EXPECT_EQ(CountStats["foo.h"], 1u);
+  EXPECT_THAT(Completions,
+              ElementsAre(Field(&CodeCompletion::Name, "TestSym")));
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang
diff --git a/clang-tools-extra/unittests/clangd/FSTests.cpp b/clang-tools-extra/unittests/clangd/FSTests.cpp
new file mode 100644
index 00000000000..3b028426016
--- /dev/null
+++ b/clang-tools-extra/unittests/clangd/FSTests.cpp
@@ -0,0 +1,46 @@
+//===-- FSTests.cpp - File system related tests -----------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "FS.h"
+#include "TestFS.h"
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+namespace clang {
+namespace clangd {
+namespace {
+
+TEST(FSTests, PreambleStatusCache) {
+  llvm::StringMap<std::string> Files;
+  Files["x"] = "";
+  Files["y"] = "";
+  auto FS = buildTestFS(Files);
+  FS->setCurrentWorkingDirectory(testRoot());
+
+  PreambleFileStatusCache StatCache;
+  auto ProduceFS = StatCache.getProducingFS(FS);
+  EXPECT_TRUE(ProduceFS->openFileForRead("x"));
+  EXPECT_TRUE(ProduceFS->status("y"));
+
+  EXPECT_TRUE(StatCache.lookup(testPath("x")).hasValue());
+  EXPECT_TRUE(StatCache.lookup(testPath("y")).hasValue());
+
+  vfs::Status S("fake", llvm::sys::fs::UniqueID(0, 0),
+                std::chrono::system_clock::now(), 0, 0, 1024,
+                llvm::sys::fs::file_type::regular_file, llvm::sys::fs::all_all);
+  StatCache.update(*FS, S);
+  auto ConsumeFS = StatCache.getConsumingFS(FS);
+  auto Cached = ConsumeFS->status(testPath("fake"));
+  EXPECT_TRUE(Cached);
+  EXPECT_EQ(Cached->getName(), S.getName());
+}
+
+} // namespace
+} // namespace clangd
+} // namespace clang
diff --git a/clang-tools-extra/unittests/clangd/TestFS.cpp b/clang-tools-extra/unittests/clangd/TestFS.cpp
index d3112b92278..bc8c5479a93 100644
--- a/clang-tools-extra/unittests/clangd/TestFS.cpp
+++ b/clang-tools-extra/unittests/clangd/TestFS.cpp
@@ -23,6 +23,7 @@ buildTestFS(llvm::StringMap<std::string> const &Files,
             llvm::StringMap<time_t> const &Timestamps) {
   IntrusiveRefCntPtr<vfs::InMemoryFileSystem> MemFS(
       new vfs::InMemoryFileSystem);
+  MemFS->setCurrentWorkingDirectory(testRoot());
   for (auto &FileAndContents : Files) {
     StringRef File = FileAndContents.first();
     MemFS->addFile(