[clang][deps] Overload Filesystem::exists in DependencyScanningFilesystem to have it use cached status (llvm#88152)

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`.

(cherry picked from commit 779ba60)

Author: artemcm

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

@@ -381,6 +381,10 @@ class DependencyScanningWorkerFilesystem
   /// false if not (i.e. this entry is not a file or its scan fails).
   bool ensureDirectiveTokensArePopulated(EntryRef 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;
+
 private:
   /// For a filename that's not yet associated with any entry in the caches,
   /// uses the underlying filesystem to either look up the entry based in the

clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp

@@ -306,6 +306,17 @@ DependencyScanningWorkerFilesystem::status(const Twine &Path) {
   return Result->getStatus();
 }
 
+bool DependencyScanningWorkerFilesystem::exists(const Twine &Path) {
+  // While some VFS overlay filesystems may implement more-efficient
+  // mechanisms for `exists` queries, `DependencyScanningWorkerFilesystem`
+  // typically wraps `RealFileSystem` which does not specialize `exists`,
+  // so it is not likely to benefit from such optimizations. Instead,
+  // it is more-valuable to have this query go through the
+  // cached-`status` code-path of the `DependencyScanningWorkerFilesystem`.
+  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

clang/unittests/Tooling/DependencyScanning/DependencyScanningFilesystemTest.cpp

@@ -18,6 +18,7 @@ struct InstrumentingFilesystem
     : llvm::RTTIExtends<InstrumentingFilesystem, llvm::vfs::ProxyFileSystem> {
   unsigned NumStatusCalls = 0;
   unsigned NumGetRealPathCalls = 0;
+  unsigned NumExistsCalls = 0;
 
   using llvm::RTTIExtends<InstrumentingFilesystem,
                           llvm::vfs::ProxyFileSystem>::RTTIExtends;
@@ -32,6 +33,11 @@ struct InstrumentingFilesystem
     ++NumGetRealPathCalls;
     return ProxyFileSystem::getRealPath(Path, Output);
   }
+
+  bool exists(const llvm::Twine &Path) override {
+    ++NumExistsCalls;
+    return ProxyFileSystem::exists(Path);
+  }
 };
 } // namespace
 
@@ -147,3 +153,24 @@ TEST(DependencyScanningFilesystem, RealPathAndStatusInvariants) {
     DepFS.status("/bar");
   }
 }
+
+TEST(DependencyScanningFilesystem, CacheStatOnExists) {
+  auto InMemoryFS = llvm::makeIntrusiveRefCnt<llvm::vfs::InMemoryFileSystem>();
+  auto InstrumentingFS =
+      llvm::makeIntrusiveRefCnt<InstrumentingFilesystem>(InMemoryFS);
+  InMemoryFS->setCurrentWorkingDirectory("/");
+  InMemoryFS->addFile("/foo", 0, llvm::MemoryBuffer::getMemBuffer(""));
+  InMemoryFS->addFile("/bar", 0, llvm::MemoryBuffer::getMemBuffer(""));
+  DependencyScanningFilesystemSharedCache SharedCache;
+  DependencyScanningWorkerFilesystem DepFS(SharedCache, InstrumentingFS);
+
+  DepFS.status("/foo");
+  DepFS.status("/foo");
+  DepFS.status("/bar");
+  EXPECT_EQ(InstrumentingFS->NumStatusCalls, 2u);
+
+  DepFS.exists("/foo");
+  DepFS.exists("/bar");
+  EXPECT_EQ(InstrumentingFS->NumStatusCalls, 2u);
+  EXPECT_EQ(InstrumentingFS->NumExistsCalls, 0u);
+}