From 7de8310ad811b4451a7b2288ceace270b61253c2 Mon Sep 17 00:00:00 2001
From: Paul Taykalo <tt.kilew@gmail.com>
Date: Wed, 20 Nov 2019 00:19:20 +0200
Subject: [PATCH] Use kevent queue for waiting for file unlocking

---
 llvm/include/llvm/Support/LockFileManager.h |  2 +
 llvm/lib/Support/LockFileManager.cpp        | 98 ++++++++++++++++++++-
 2 files changed, 99 insertions(+), 1 deletion(-)

diff --git a/llvm/include/llvm/Support/LockFileManager.h b/llvm/include/llvm/Support/LockFileManager.h
index 57e4fbd84cd95..16c943a7bbc09 100644
--- a/llvm/include/llvm/Support/LockFileManager.h
+++ b/llvm/include/llvm/Support/LockFileManager.h
@@ -64,6 +64,8 @@ class LockFileManager {
   static Optional<std::pair<std::string, int> >
   readLockFile(StringRef LockFileName);
 
+  bool waitForUnlockUsingSystemEvents(WaitForUnlockResult *);
+
   static bool processStillExecuting(StringRef Hostname, int PID);
 
 public:
diff --git a/llvm/lib/Support/LockFileManager.cpp b/llvm/lib/Support/LockFileManager.cpp
index 10181192afbd8..a549a43e0d8f4 100644
--- a/llvm/lib/Support/LockFileManager.cpp
+++ b/llvm/lib/Support/LockFileManager.cpp
@@ -32,14 +32,20 @@
 
 #if defined(__APPLE__) && defined(__MAC_OS_X_VERSION_MIN_REQUIRED) && (__MAC_OS_X_VERSION_MIN_REQUIRED > 1050)
 #define USE_OSX_GETHOSTUUID 1
+#define USE_OSX_FILEWATCHER 1
 #else
 #define USE_OSX_GETHOSTUUID 0
+#define USE_OSX_FILEWATCHER 0
 #endif
 
 #if USE_OSX_GETHOSTUUID
 #include <uuid/uuid.h>
 #endif
 
+#if USE_OSX_FILEWATCHER
+#include <fcntl.h>
+#include <sys/event.h>
+#endif
 using namespace llvm;
 
 /// Attempt to read the lock file with the given name, if it exists.
@@ -290,10 +296,101 @@ LockFileManager::~LockFileManager() {
   sys::DontRemoveFileOnSignal(UniqueLockFileName);
 }
 
+bool LockFileManager::waitForUnlockUsingSystemEvents(
+    LockFileManager::WaitForUnlockResult *Result) {
+#if !USE_OSX_FILEWATCHER
+  return false;
+#else
+
+  // Class that employs RAII to save a file descriptor
+  class FileDescriptorKeeper {
+    int FileDescriptor;
+
+  public:
+    FileDescriptorKeeper(int Descriptor) { FileDescriptor = Descriptor; }
+    ~FileDescriptorKeeper() { close(FileDescriptor); }
+  };
+
+
+  const unsigned MaxSeconds = 90;
+  struct timespec timeout = {1, 0};
+
+  int EventQueue;
+  int LockFileDescriptor;
+  int EventResult;
+
+  if ((EventQueue = kqueue()) == -1)
+    return false;
+
+  FileDescriptorKeeper queueRAII = FileDescriptorKeeper(EventQueue);
+
+  // Opening file for lock descriptor
+  if ((LockFileDescriptor = open(LockFileName.c_str(), O_RDONLY)) == -1)
+    return false;
+
+  FileDescriptorKeeper fileRAII = FileDescriptorKeeper(LockFileDescriptor);
+
+  struct kevent FileRemovingEvent;
+  EV_SET(&FileRemovingEvent, LockFileDescriptor, EVFILT_VNODE,
+         EV_ADD | EV_ENABLE | EV_ONESHOT, NOTE_DELETE, 0, 0);
+
+  unsigned Iterations = 0;
+  while (Iterations < MaxSeconds) {
+
+    struct kevent ReceivedEvent;
+    EventResult =
+        kevent(EventQueue, &FileRemovingEvent, 1, &ReceivedEvent, 1, &timeout);
+
+    // Unexpected event result, something went wrong, let's back to the polling
+    if (EventResult == -1)
+      return false;
+
+    // File was succesfully deleted
+    if (EventResult > 0 && ReceivedEvent.fflags & NOTE_DELETE) {
+      *Result = Res_Success;
+      return true;
+    }
+
+    // Check if file was deleted
+    if (sys::fs::access(LockFileName.c_str(), sys::fs::AccessMode::Exist) ==
+        errc::no_such_file_or_directory) {
+      // If the original file wasn't created, somone thought the lock was
+      // dead.
+      if (!sys::fs::exists(FileName)) {
+        *Result = Res_OwnerDied;
+        return true;
+      }
+      *Result = Res_Success;
+      return true;
+    }
+
+    // If the process owning the lock died without cleaning up, just bail
+    // out.
+    if (!processStillExecuting((*Owner).first, (*Owner).second)) {
+      *Result = Res_Success;
+      return true;
+    }
+
+    Iterations++;
+  }
+
+  // For some reason we haven't received correct event from the file system
+  // And lock file is still present and owner process is still alive so we will
+  // fallback to default implementation
+  *Result = Res_Timeout;
+  return true;
+#endif
+}
+
 LockFileManager::WaitForUnlockResult LockFileManager::waitForUnlock() {
   if (getState() != LFS_Shared)
     return Res_Success;
 
+  WaitForUnlockResult SystemEventsResult;
+  if (waitForUnlockUsingSystemEvents(&SystemEventsResult)) {
+    return SystemEventsResult;
+  }
+
 #ifdef _WIN32
   unsigned long Interval = 1;
 #else
@@ -345,7 +442,6 @@ LockFileManager::WaitForUnlockResult LockFileManager::waitForUnlock() {
            Interval.tv_sec < (time_t)MaxSeconds
 #endif
            );
-
   // Give up.
   return Res_Timeout;
 }