commit 76f1baa7875acd88bdd4b431eed6e2d2decfc0fe
Author: Saleem Abdulrasool <compnerd@compnerd.org>
Date:   Fri Jun 11 19:05:42 2021 -0700

    Revert "Revert "DirectoryWatcher: add an implementation for Windows""
    
    This reverts commit 0ec1cf13f2a4e31aa2c5ccc665c5fbdcd3a94577.
    
    Restore the implementation with some minor tweaks:
    - Use std::unique_ptr for the path instead of std::vector
      * Stylistic improvement as the buffer is already heap allocated, this
        just makes it clearer.
    - Correct the notification buffer allocation size
      * Memory usage fix: we were allocating 4x the computed size
    - Correct the passing of the buffer size to RDC
      * Memory usage fix: we were reporting 1/4th of the size
    - Convert the operation event to auto-reset
      * Bug Fix: we never reset the event
    - Remove `FILE_NOTIFY_CHANGE_LAST_ACCESS` from RDC events
      * Memory usage fix: we never needed this notification
    - Fold events for the notification action
      * Stylistic improvement to be clear how the events map
    - Update comment
      * Stylistic improvement to be clear what the RAII controls
    - Fix the race condition that was uncovered previously
      * We would return from the construction before the watcher thread
        began execution.  The test would then proceed to begin execution,
        and we would miss the initial notifications.  We now ensure that the
        watcher thread is initialized before we return.  This ensures that
        we do not miss the initial notifications.
    
    Running the test on a SSD was able to uncover the access pattern.  This
    now seems to pass reliably where it was previously flaky locally.

diff --git a/clang/lib/DirectoryWatcher/windows/DirectoryWatcher-windows.cpp b/clang/lib/DirectoryWatcher/windows/DirectoryWatcher-windows.cpp
index 25cbcf536388..6bcfb86e7f99 100644
--- a/clang/lib/DirectoryWatcher/windows/DirectoryWatcher-windows.cpp
+++ b/clang/lib/DirectoryWatcher/windows/DirectoryWatcher-windows.cpp
@@ -6,19 +6,12 @@
 //
 //===----------------------------------------------------------------------===//
 
-// TODO: This is not yet an implementation, but it will make it so Windows
-//       builds don't fail.
-
 #include "DirectoryScanner.h"
 #include "clang/DirectoryWatcher/DirectoryWatcher.h"
-
 #include "llvm/ADT/STLExtras.h"
-#include "llvm/ADT/ScopeExit.h"
-#include "llvm/Support/AlignOf.h"
-#include "llvm/Support/Errno.h"
-#include "llvm/Support/Mutex.h"
+#include "llvm/Support/ConvertUTF.h"
 #include "llvm/Support/Path.h"
-#include <atomic>
+#include "llvm/Support/Windows/WindowsSupport.h"
 #include <condition_variable>
 #include <mutex>
 #include <queue>
@@ -28,23 +21,271 @@
 
 namespace {
 
+using DirectoryWatcherCallback =
+    std::function<void(llvm::ArrayRef<clang::DirectoryWatcher::Event>, bool)>;
+
 using namespace llvm;
 using namespace clang;
 
 class DirectoryWatcherWindows : public clang::DirectoryWatcher {
+  OVERLAPPED Overlapped;
+
+  std::vector<DWORD> Notifications;
+
+  std::thread WatcherThread;
+  std::thread HandlerThread;
+  std::function<void(ArrayRef<DirectoryWatcher::Event>, bool)> Callback;
+  SmallString<MAX_PATH> Path;
+  HANDLE Terminate;
+
+  std::mutex Mutex;
+  bool WatcherActive = false;
+  std::condition_variable Ready;
+
+  class EventQueue {
+    std::mutex M;
+    std::queue<DirectoryWatcher::Event> Q;
+    std::condition_variable CV;
+
+  public:
+    void emplace(DirectoryWatcher::Event::EventKind Kind, StringRef Path) {
+      {
+        std::unique_lock<std::mutex> L(M);
+        Q.emplace(Kind, Path);
+      }
+      CV.notify_one();
+    }
+
+    DirectoryWatcher::Event pop_front() {
+      std::unique_lock<std::mutex> L(M);
+      while (true) {
+        if (!Q.empty()) {
+          DirectoryWatcher::Event E = Q.front();
+          Q.pop();
+          return E;
+        }
+        CV.wait(L, [this]() { return !Q.empty(); });
+      }
+    }
+  } Q;
+
 public:
-  ~DirectoryWatcherWindows() override { }
-  void InitialScan() { }
-  void EventReceivingLoop() { }
-  void StopWork() { }
+  DirectoryWatcherWindows(HANDLE DirectoryHandle, bool WaitForInitialSync,
+                          DirectoryWatcherCallback Receiver);
+
+  ~DirectoryWatcherWindows() override;
+
+  void InitialScan();
+  void WatcherThreadProc(HANDLE DirectoryHandle);
+  void NotifierThreadProc(bool WaitForInitialSync);
 };
+
+DirectoryWatcherWindows::DirectoryWatcherWindows(
+    HANDLE DirectoryHandle, bool WaitForInitialSync,
+    DirectoryWatcherCallback Receiver)
+    : Callback(Receiver), Terminate(INVALID_HANDLE_VALUE) {
+  // Pre-compute the real location as we will be handing over the directory
+  // handle to the watcher and performing synchronous operations.
+  {
+    DWORD Size = GetFinalPathNameByHandleW(DirectoryHandle, NULL, 0, 0);
+    std::unique_ptr<WCHAR[]> Buffer{new WCHAR[Size]};
+    Size = GetFinalPathNameByHandleW(DirectoryHandle, Buffer.get(), Size, 0);
+    Buffer[Size] = L'\0';
+    llvm::sys::windows::UTF16ToUTF8(Buffer.get(), Size, Path);
+  }
+
+  size_t EntrySize = sizeof(FILE_NOTIFY_INFORMATION) + MAX_PATH * sizeof(WCHAR);
+  Notifications.resize((4 * EntrySize) / sizeof(DWORD));
+
+  memset(&Overlapped, 0, sizeof(Overlapped));
+  Overlapped.hEvent =
+      CreateEventW(NULL, /*bManualReset=*/FALSE, /*bInitialState=*/FALSE, NULL);
+  assert(Overlapped.hEvent && "unable to create event");
+
+  Terminate =
+      CreateEventW(NULL, /*bManualReset=*/TRUE, /*bInitialState=*/FALSE, NULL);
+
+  WatcherThread = std::thread([this, DirectoryHandle]() {
+    this->WatcherThreadProc(DirectoryHandle);
+  });
+
+  if (WaitForInitialSync)
+    InitialScan();
+
+  HandlerThread = std::thread([this, WaitForInitialSync]() {
+    this->NotifierThreadProc(WaitForInitialSync);
+  });
+
+  std::unique_lock<std::mutex> lock(Mutex);
+  Ready.wait(lock, [this] { return this->WatcherActive; });
+}
+
+DirectoryWatcherWindows::~DirectoryWatcherWindows() {
+  // Signal the Watcher to exit.
+  SetEvent(Terminate);
+  HandlerThread.join();
+  WatcherThread.join();
+  CloseHandle(Terminate);
+  CloseHandle(Overlapped.hEvent);
+}
+
+void DirectoryWatcherWindows::InitialScan() {
+  Callback(getAsFileEvents(scanDirectory(Path.data())), /*IsInitial=*/true);
+}
+
+void DirectoryWatcherWindows::WatcherThreadProc(HANDLE DirectoryHandle) {
+  {
+    std::unique_lock<std::mutex> lock(Mutex);
+    WatcherActive = true;
+  }
+  Ready.notify_one();
+
+  while (true) {
+    // We do not guarantee subdirectories, but macOS already provides
+    // subdirectories, might as well as ...
+    BOOL WatchSubtree = TRUE;
+    DWORD NotifyFilter = FILE_NOTIFY_CHANGE_FILE_NAME
+                       | FILE_NOTIFY_CHANGE_DIR_NAME
+                       | FILE_NOTIFY_CHANGE_SIZE
+                       | FILE_NOTIFY_CHANGE_LAST_WRITE
+                       | FILE_NOTIFY_CHANGE_CREATION;
+
+    DWORD BytesTransferred;
+    if (!ReadDirectoryChangesW(DirectoryHandle, Notifications.data(),
+                               Notifications.size() * sizeof(DWORD),
+                               WatchSubtree, NotifyFilter, &BytesTransferred,
+                               &Overlapped, NULL)) {
+      Q.emplace(DirectoryWatcher::Event::EventKind::WatcherGotInvalidated,
+                "");
+      break;
+    }
+
+    HANDLE Handles[2] = { Terminate, Overlapped.hEvent };
+    switch (WaitForMultipleObjects(2, Handles, FALSE, INFINITE)) {
+    case WAIT_OBJECT_0: // Terminate Request
+    case WAIT_FAILED:   // Failure
+      Q.emplace(DirectoryWatcher::Event::EventKind::WatcherGotInvalidated,
+                "");
+      (void)CloseHandle(DirectoryHandle);
+      return;
+    case WAIT_TIMEOUT:  // Spurious wakeup?
+      continue;
+    case WAIT_OBJECT_0 + 1: // Directory change
+      break;
+    }
+
+    if (!GetOverlappedResult(DirectoryHandle, &Overlapped, &BytesTransferred,
+                             FALSE)) {
+      Q.emplace(DirectoryWatcher::Event::EventKind::WatchedDirRemoved,
+                "");
+      Q.emplace(DirectoryWatcher::Event::EventKind::WatcherGotInvalidated,
+                "");
+      break;
+    }
+
+    // There was a buffer underrun on the kernel side.  We may have lost
+    // events, please re-synchronize.
+    if (BytesTransferred == 0) {
+      Q.emplace(DirectoryWatcher::Event::EventKind::WatcherGotInvalidated,
+                "");
+      break;
+    }
+
+    for (FILE_NOTIFY_INFORMATION *I =
+            (FILE_NOTIFY_INFORMATION *)Notifications.data();
+         I;
+         I = I->NextEntryOffset
+              ? (FILE_NOTIFY_INFORMATION *)((CHAR *)I + I->NextEntryOffset)
+              : NULL) {
+      DirectoryWatcher::Event::EventKind Kind =
+          DirectoryWatcher::Event::EventKind::WatcherGotInvalidated;
+      switch (I->Action) {
+      case FILE_ACTION_ADDED:
+      case FILE_ACTION_MODIFIED:
+      case FILE_ACTION_RENAMED_NEW_NAME:
+        Kind = DirectoryWatcher::Event::EventKind::Modified;
+        break;
+      case FILE_ACTION_REMOVED:
+      case FILE_ACTION_RENAMED_OLD_NAME:
+        Kind = DirectoryWatcher::Event::EventKind::Removed;
+        break;
+      }
+
+      SmallString<MAX_PATH> filename;
+      sys::windows::UTF16ToUTF8(I->FileName, I->FileNameLength / sizeof(WCHAR),
+                                filename);
+      Q.emplace(Kind, filename);
+    }
+  }
+
+  (void)CloseHandle(DirectoryHandle);
+}
+
+void DirectoryWatcherWindows::NotifierThreadProc(bool WaitForInitialSync) {
+  // If we did not wait for the initial sync, then we should perform the
+  // scan when we enter the thread.
+  if (!WaitForInitialSync)
+    this->InitialScan();
+
+  while (true) {
+    DirectoryWatcher::Event E = Q.pop_front();
+    Callback(E, /*IsInitial=*/false);
+    if (E.Kind == DirectoryWatcher::Event::EventKind::WatcherGotInvalidated)
+      break;
+  }
+}
+
+auto error(DWORD ErrorCode) {
+  DWORD Flags = FORMAT_MESSAGE_ALLOCATE_BUFFER
+              | FORMAT_MESSAGE_FROM_SYSTEM
+              | FORMAT_MESSAGE_IGNORE_INSERTS;
+
+  LPSTR Buffer;
+  if (!FormatMessageA(Flags, NULL, ErrorCode,
+                      MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), (LPSTR)&Buffer,
+                      0, NULL)) {
+    return make_error<llvm::StringError>("error " + utostr(ErrorCode),
+                                         inconvertibleErrorCode());
+  }
+  std::string Message{Buffer};
+  LocalFree(Buffer);
+  return make_error<llvm::StringError>(Message, inconvertibleErrorCode());
+}
+
 } // namespace
 
 llvm::Expected<std::unique_ptr<DirectoryWatcher>>
-clang::DirectoryWatcher::create(
-    StringRef Path,
-    std::function<void(llvm::ArrayRef<DirectoryWatcher::Event>, bool)> Receiver,
-    bool WaitForInitialSync) {
-  return llvm::Expected<std::unique_ptr<DirectoryWatcher>>(
-      llvm::errorCodeToError(std::make_error_code(std::errc::not_supported)));
+clang::DirectoryWatcher::create(StringRef Path,
+                                DirectoryWatcherCallback Receiver,
+                                bool WaitForInitialSync) {
+  if (Path.empty())
+    llvm::report_fatal_error(
+        "DirectoryWatcher::create can not accept an empty Path.");
+
+  if (!sys::fs::is_directory(Path))
+    llvm::report_fatal_error(
+        "DirectoryWatcher::create can not accept a filepath.");
+
+  SmallVector<wchar_t, MAX_PATH> WidePath;
+  if (sys::windows::UTF8ToUTF16(Path, WidePath))
+    return llvm::make_error<llvm::StringError>(
+        "unable to convert path to UTF-16", llvm::inconvertibleErrorCode());
+
+  DWORD DesiredAccess = FILE_LIST_DIRECTORY;
+  DWORD ShareMode = FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE;
+  DWORD CreationDisposition = OPEN_EXISTING;
+  DWORD FlagsAndAttributes = FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OVERLAPPED;
+
+  HANDLE DirectoryHandle =
+      CreateFileW(WidePath.data(), DesiredAccess, ShareMode,
+                  /*lpSecurityAttributes=*/NULL, CreationDisposition,
+                  FlagsAndAttributes, NULL);
+  if (DirectoryHandle == INVALID_HANDLE_VALUE)
+    return error(GetLastError());
+
+  // NOTE: We use the watcher instance as a RAII object to discard the handles
+  // for the directory in case of an error.  Hence, this is early allocated,
+  // with the state being written directly to the watcher.
+  return std::make_unique<DirectoryWatcherWindows>(
+      DirectoryHandle, WaitForInitialSync, Receiver);
 }
diff --git a/clang/unittests/DirectoryWatcher/CMakeLists.txt b/clang/unittests/DirectoryWatcher/CMakeLists.txt
index 0355525a86b0..84a1a9d40c25 100644
--- a/clang/unittests/DirectoryWatcher/CMakeLists.txt
+++ b/clang/unittests/DirectoryWatcher/CMakeLists.txt
@@ -1,4 +1,4 @@
-if(APPLE OR CMAKE_SYSTEM_NAME MATCHES "Linux")
+if(APPLE OR CMAKE_SYSTEM_NAME MATCHES "Linux" OR CMAKE_SYSTEM_NAME STREQUAL Windows)
 
   set(LLVM_LINK_COMPONENTS
     Support