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

[Artem Chikin via cfe-commits]

https://github.com/artemcm created https://github.com/llvm/llvm-project/pull/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`.

>From 8be194ac570d702b5df2fdcd42868122b0ae16dd Mon Sep 17 00:00:00 2001
From: Artem Chikin <achikin at apple.com>
Date: Tue, 9 Apr 2024 09:37:09 -0700
Subject: [PATCH] [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'.
---
 .../DependencyScanning/DependencyScanningFilesystem.h       | 4 ++++
 .../DependencyScanning/DependencyScanningFilesystem.cpp     | 6 ++++++
 2 files changed, 10 insertions(+)

diff --git a/clang/include/clang/Tooling/DependencyScanning/DependencyScanningFilesystem.h b/clang/include/clang/Tooling/DependencyScanning/DependencyScanningFilesystem.h
index 9a522a3e2fe252..4fdfebada5b7f7 100644
--- a/clang/include/clang/Tooling/DependencyScanning/DependencyScanningFilesystem.h
+++ b/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.
diff --git a/clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp b/clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp
index 9b7812a1adb9e3..174edc98da5877 100644
--- a/clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp
+++ b/clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp
@@ -270,6 +270,12 @@ DependencyScanningWorkerFilesystem::status(const Twine &Path) {
   return Result->getStatus();
 }
 
+bool
+DependencyScanningWorkerFilesystem::exists(const Twine &Path) {
+  auto Status = status(Path);
+  return Status && Status->exists();
+}
+
 namespace {
 
 /// The VFS that is used by clang consumes the \c CachedFileSystemEntry using