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

[Jan Svoboda via cfe-commits]

================
@@ -0,0 +1,51 @@
+//===- DependencyScanningFilesystemTest.cpp -------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "clang/Tooling/DependencyScanning/DependencyScanningFilesystem.h"
+#include "llvm/ADT/Twine.h"
+#include "llvm/Support/VirtualFileSystem.h"
+#include "gtest/gtest.h"
+
+using namespace clang::tooling::dependencies;
+
+namespace {
+ struct InstrumentingFilesystem
+     : llvm::RTTIExtends<InstrumentingFilesystem, llvm::vfs::ProxyFileSystem> {
+   unsigned NumStatCalls = 0;
+
+   using llvm::RTTIExtends<InstrumentingFilesystem,
+                           llvm::vfs::ProxyFileSystem>::RTTIExtends;
+
+   llvm::ErrorOr<llvm::vfs::Status> status(const llvm::Twine &Path) override {
+     ++NumStatCalls;
+     return ProxyFileSystem::status(Path);
+   }
+ };
+ } // namespace
+
+
+TEST(DependencyScanningFilesystem, CacheStatOnExists) {
+  auto InMemoryFS = llvm::makeIntrusiveRefCnt<llvm::vfs::InMemoryFileSystem>();
+  InMemoryFS->setCurrentWorkingDirectory("/");
+  InMemoryFS->addFile("/foo", 0, llvm::MemoryBuffer::getMemBuffer(""));
+  InMemoryFS->addFile("/bar", 0, llvm::MemoryBuffer::getMemBuffer(""));
+
+  auto InstrumentingFS =
+    llvm::makeIntrusiveRefCnt<InstrumentingFilesystem>(InMemoryFS);
+  DependencyScanningFilesystemSharedCache SharedCache;
+  DependencyScanningWorkerFilesystem DepFS(SharedCache, InstrumentingFS);
+
+  DepFS.status("/foo");
+  DepFS.status("/foo");
+  DepFS.status("/bar");
+  EXPECT_EQ(InstrumentingFS->NumStatCalls, 2u);
+
+  DepFS.exists("/foo");
+  DepFS.exists("/bar");
+  EXPECT_EQ(InstrumentingFS->NumStatCalls, 2u);  
----------------
jansvoboda11 wrote:

It would be good to explicitly check that we're not calling `exists()` instead of relying on the fact that `InMemoryFileSystem` keeps implementing `exists()` in terms of `status()`.

https://github.com/llvm/llvm-project/pull/88152