Initial commit.

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

@@ -220,6 +220,14 @@ class DependencyScanningFilesystemSharedCache {
   CacheShard &getShardForFilename(StringRef Filename) const;
   CacheShard &getShardForUID(llvm::sys::fs::UniqueID UID) const;
 
+  /// Visits all cached entries and re-stat an entry using the UnderlyingFS if
+  /// it is negatively stat cached. If the re-stat succeeds, print diagnostics
+  /// information to OS indicating that negative stat caching has been
+  /// invalidated.
+  void
+  diagnoseNegativeStatCachedPaths(llvm::raw_ostream &OS,
+                                  llvm::vfs::FileSystem &UnderlyingFS) const;
+
 private:
   std::unique_ptr<CacheShard[]> CacheShards;
   unsigned NumShards;

clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp

@@ -108,6 +108,32 @@ DependencyScanningFilesystemSharedCache::getShardForUID(
   return CacheShards[Hash % NumShards];
 }
 
+void DependencyScanningFilesystemSharedCache::diagnoseNegativeStatCachedPaths(
+    llvm::raw_ostream &OS, llvm::vfs::FileSystem &UnderlyingFS) const {
+  // Iterate through all shards and look for cached stat errors.
+  for (unsigned i = 0; i < NumShards; i++) {
+    const CacheShard &Shard = CacheShards[i];
+    for (const auto &Path : Shard.CacheByFilename.keys()) {
+      auto CachedPairIt = Shard.CacheByFilename.find(Path);
+      auto CachedPair = CachedPairIt->getValue();
+      const CachedFileSystemEntry *Entry = CachedPair.first;
+
+      if (Entry->getError()) {
+        // Only examine cached errors.
+        llvm::ErrorOr<llvm::vfs::Status> Stat = UnderlyingFS.status(Path);
+        if (Stat) {
+          // This is the case where we have negatively cached the non-existence
+          // of the file at Path, but the cache is later invalidated. Report
+          // diagnostics.
+          OS << Path << " did not exist when it was first searched "
+             << "but was created later. This may have led to a build failure "
+                "due to missing files.\n";
+        }
+      }
+    }
+  }
+}
+
 const CachedFileSystemEntry *
 DependencyScanningFilesystemSharedCache::CacheShard::findEntryByFilename(
     StringRef Filename) const {

clang/unittests/Tooling/DependencyScanning/DependencyScanningFilesystemTest.cpp

@@ -178,3 +178,31 @@ TEST(DependencyScanningFilesystem, CacheStatFailures) {
   DepFS.exists("/cache/a.pcm");
   EXPECT_EQ(InstrumentingFS->NumStatusCalls, 5u);
 }
+
+TEST(DependencyScanningFilesystem, DiagnoseStaleStatFailures) {
+  auto InMemoryFS = llvm::makeIntrusiveRefCnt<llvm::vfs::InMemoryFileSystem>();
+  InMemoryFS->setCurrentWorkingDirectory("/");
+  InMemoryFS->addFile("/dir", 0, llvm::MemoryBuffer::getMemBuffer(""));
+
+  DependencyScanningFilesystemSharedCache SharedCache;
+  DependencyScanningWorkerFilesystem DepFS(SharedCache, InMemoryFS);
+
+  bool Path1Exists = DepFS.exists("/path1");
+  EXPECT_EQ(Path1Exists, false);
+
+  // Adding a file that has been stat-ed,
+  InMemoryFS->addFile("/path1", 0, llvm::MemoryBuffer::getMemBuffer(""));
+  Path1Exists = DepFS.exists("/path1");
+  // Due to caching in SharedCache, path1 should not exist in
+  // DepFS's eyes.
+  EXPECT_EQ(Path1Exists, false);
+
+  std::string Diags;
+  llvm::raw_string_ostream DiagsStream(Diags);
+  SharedCache.diagnoseNegativeStatCachedPaths(DiagsStream, *InMemoryFS.get());
+
+  ASSERT_STREQ(
+      "/path1 did not exist when it was first searched but was created later. "
+      "This may have led to a build failure due to missing files.\n",
+      Diags.c_str());
+}