[clang][deps] Overload 'Filesystem::exists' in 'DependencyScanningFilesystem' to have it use cached status

As-is, calls to `exists()` fallback on the implementation in 'ProxyFileSystem::exists' which explicitly calls out to the underlying `FS`, which for the 'DependencyScanningFilesystem' (overlay) is the real underlying filesystem.

Instead, directly overloading 'exists' allows us to have it rely on the cached `status` behavior used elsewhere by the 'DependencyScanningFilesystem'.

Author: artemcm

clang/include/clang/Tooling/DependencyScanning/DependencyScanningFilesystem.h

@@ -365,6 +365,10 @@ class DependencyScanningWorkerFilesystem
     return LocalCache.insertEntryForFilename(Filename, Entry);
   }
 
+  /// Check whether \p Path exists. By default checks cached result of \c status(),
+  /// and falls back on FS if unable to do so.
+  bool exists(const Twine &Path) override;
+
   /// Returns entry associated with the filename in the shared cache if there is
   /// some. Otherwise, constructs new one with the given error code, associates
   /// it with the filename and returns the result.

clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp

@@ -270,6 +270,12 @@ DependencyScanningWorkerFilesystem::status(const Twine &Path) {
   return Result->getStatus();
 }
 
+bool
+DependencyScanningWorkerFilesystem::exists(const Twine &Path) {
+  llvm::ErrorOr<llvm::vfs::Status> Status = status(Path);
+  return Status && Status->exists();
+}
+
 namespace {
 
 /// The VFS that is used by clang consumes the \c CachedFileSystemEntry using

llvm/unittests/Support/VirtualFileSystemTest.cpp

@@ -3560,3 +3560,27 @@ TEST(RedirectingFileSystemTest, ExistsRedirectOnly) {
   EXPECT_FALSE(Redirecting->exists("/b"));
   EXPECT_TRUE(Redirecting->exists("/vfile"));
 }
+
+TEST(DependencyScanningWorkerFilesystemTest, ExistsUsesStatCache) {
+  struct StatCountingProxyFS : llvm::vfs::ProxyFileSystem {
+    int StatCount;
+
+    StatCountingProxyFS(IntrusiveRefCntPtr<FileSystem> UnderlyingFS)
+    : ProxyFileSystem(UnderlyingFS), StatCount(0) {}
+
+    llvm::ErrorOr<llvm::vfs::Status> status(const Twine &Path) override {
+      StatCount++;
+      return ProxyFileSystem::status(Path);
+    }
+  };
+  auto BaseFS = IntrusiveRefCntPtr<DummyFileSystem>(new DummyFileSystem());
+  auto ScanFS = makeIntrusiveRefCnt<StatCountingProxyFS>(BaseFS);
+  BaseFS->addRegularFile("/a");
+  BaseFS->addRegularFile("/b");
+  ScanFS->status("/a");
+  ScanFS->status("/b");
+  EXPECT_TRUE(ScanFS->StatCount == 2);
+  ScanFS->exists("/a");
+  ScanFS->exists("/b");
+  EXPECT_TRUE(ScanFS->StatCount == 2);
+}