[lldb-dap] Implement a MemoryMonitor (#129332)

This implements a memory monitor for macOS, Linux and Windows. It
registers a callback that invokes `SBDebugger::MemoryPressureDetected`
when a low memory event is detected. This is motivated by the new
server mode, where the lldb-dap process will live across multiple
debug sessions and will use more memory due to caching.

lldb/include/lldb/Host/MemoryMonitor.h

@@ -0,0 +1,41 @@
+//===-- MemoryMonitor.h ---------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLDB_HOST_MEMORYMONITOR_H
+#define LLDB_HOST_MEMORYMONITOR_H
+
+#include <functional>
+#include <memory>
+
+namespace lldb_private {
+
+class MemoryMonitor {
+public:
+  using Callback = std::function<void()>;
+
+  MemoryMonitor(Callback callback) : m_callback(callback) {}
+  virtual ~MemoryMonitor() = default;
+
+  /// MemoryMonitor is not copyable.
+  /// @{
+  MemoryMonitor(const MemoryMonitor &) = delete;
+  MemoryMonitor &operator=(const MemoryMonitor &) = delete;
+  /// @}
+
+  static std::unique_ptr<MemoryMonitor> Create(Callback callback);
+
+  virtual void Start() = 0;
+  virtual void Stop() = 0;
+
+protected:
+  Callback m_callback;
+};
+
+} // namespace lldb_private
+
+#endif

lldb/source/Host/CMakeLists.txt

@@ -27,6 +27,7 @@ add_host_subdirectory(common
   common/LockFileBase.cpp
   common/LZMA.cpp
   common/MainLoopBase.cpp
+  common/MemoryMonitor.cpp
   common/MonitoringProcessLauncher.cpp
   common/NativeProcessProtocol.cpp
   common/NativeRegisterContext.cpp
@@ -136,7 +137,7 @@ else()
   elseif (CMAKE_SYSTEM_NAME MATCHES "AIX")
     add_host_subdirectory(aix
       aix/HostInfoAIX.cpp
-      )    
+      )
   endif()
 endif()
 

lldb/source/Host/common/MemoryMonitor.cpp

@@ -0,0 +1,109 @@
+//===-- MemoryMonitor.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 "lldb/Host/MemoryMonitor.h"
+#include "lldb/Host/HostThread.h"
+#include "lldb/Host/ThreadLauncher.h"
+#include "lldb/Utility/LLDBLog.h"
+#include "lldb/Utility/Log.h"
+#include "llvm/ADT/ScopeExit.h"
+#include "llvm/Support/Error.h"
+#include <atomic>
+#include <cstddef>
+#include <cstdio>
+#include <cstring>
+
+#if defined(__linux__)
+#include <fcntl.h>
+#include <poll.h>
+#include <unistd.h>
+#endif
+
+#if defined(_WIN32)
+#include <windows.h>
+#endif
+
+using namespace lldb_private;
+
+class MemoryMonitorPoll : public MemoryMonitor {
+public:
+  using MemoryMonitor::MemoryMonitor;
+
+  lldb::thread_result_t MonitorThread() {
+#if defined(__linux__)
+    struct pollfd fds;
+    fds.fd = open("/proc/pressure/memory", O_RDWR | O_NONBLOCK);
+    if (fds.fd < 0)
+      return {};
+    fds.events = POLLPRI;
+
+    auto cleanup = llvm::make_scope_exit([&]() { close(fds.fd); });
+
+    // Detect a 50ms stall in a 2 second time window.
+    const char trig[] = "some 50000 2000000";
+    if (write(fds.fd, trig, strlen(trig) + 1) < 0)
+      return {};
+
+    while (!m_done) {
+      int n = poll(&fds, 1, g_timeout);
+      if (n > 0) {
+        if (fds.revents & POLLERR)
+          return {};
+        if (fds.revents & POLLPRI)
+          m_callback();
+      }
+    }
+#endif
+
+#if defined(_WIN32)
+    HANDLE low_memory_notification =
+        CreateMemoryResourceNotification(LowMemoryResourceNotification);
+    if (!low_memory_notification)
+      return {};
+
+    while (!m_done) {
+      if (WaitForSingleObject(low_memory_notification, g_timeout) ==
+          WAIT_OBJECT_0) {
+        m_callback();
+      }
+    }
+#endif
+
+    return {};
+  }
+
+  void Start() override {
+    llvm::Expected<HostThread> memory_monitor_thread =
+        ThreadLauncher::LaunchThread("lldb.debugger.memory-monitor",
+                                     [this] { return MonitorThread(); });
+    if (memory_monitor_thread) {
+      m_memory_monitor_thread = *memory_monitor_thread;
+    } else {
+      LLDB_LOG_ERROR(GetLog(LLDBLog::Host), memory_monitor_thread.takeError(),
+                     "failed to launch host thread: {0}");
+    }
+  }
+
+  void Stop() override {
+    if (m_memory_monitor_thread.IsJoinable()) {
+      m_done = true;
+      m_memory_monitor_thread.Join(nullptr);
+    }
+  }
+
+private:
+  static constexpr uint32_t g_timeout = 1000;
+  std::atomic<bool> m_done = false;
+  HostThread m_memory_monitor_thread;
+};
+
+#if !defined(__APPLE__)
+std::unique_ptr<MemoryMonitor> MemoryMonitor::Create(Callback callback) {
+  return std::make_unique<MemoryMonitorPoll>(callback);
+}
+#endif

lldb/source/Host/macosx/objcxx/CMakeLists.txt

@@ -6,6 +6,7 @@ add_lldb_library(lldbHostMacOSXObjCXX NO_PLUGIN_DEPENDENCIES
   Host.mm
   HostInfoMacOSX.mm
   HostThreadMacOSX.mm
+  MemoryMonitorMacOSX.mm
 
   LINK_LIBS
     lldbUtility

lldb/source/Host/macosx/objcxx/MemoryMonitorMacOSX.mm

@@ -0,0 +1,51 @@
+//===-- MemoryMonitorMacOSX.mm --------------------------------------------===//
+//
+// 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 "lldb/Host/MemoryMonitor.h"
+#include <cassert>
+#include <dispatch/dispatch.h>
+
+using namespace lldb_private;
+
+class MemoryMonitorMacOSX : public MemoryMonitor {
+  using MemoryMonitor::MemoryMonitor;
+  void Start() override {
+    m_memory_pressure_source = dispatch_source_create(
+        DISPATCH_SOURCE_TYPE_MEMORYPRESSURE, 0,
+        DISPATCH_MEMORYPRESSURE_WARN | DISPATCH_MEMORYPRESSURE_CRITICAL,
+        dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0));
+
+    if (!m_memory_pressure_source)
+      return;
+
+    dispatch_source_set_event_handler(m_memory_pressure_source, ^{
+      dispatch_source_memorypressure_flags_t pressureLevel =
+          dispatch_source_get_data(m_memory_pressure_source);
+      if (pressureLevel &
+          (DISPATCH_MEMORYPRESSURE_WARN | DISPATCH_MEMORYPRESSURE_CRITICAL)) {
+        m_callback();
+      }
+    });
+    dispatch_activate(m_memory_pressure_source);
+    printf("Started\n");
+  }
+
+  void Stop() override {
+    if (m_memory_pressure_source) {
+      dispatch_source_cancel(m_memory_pressure_source);
+      dispatch_release(m_memory_pressure_source);
+    }
+  }
+
+private:
+  dispatch_source_t m_memory_pressure_source;
+};
+
+std::unique_ptr<MemoryMonitor> MemoryMonitor::Create(Callback callback) {
+  return std::make_unique<MemoryMonitorMacOSX>(callback);
+}

lldb/tools/lldb-dap/lldb-dap.cpp

@@ -10,11 +10,13 @@
 #include "EventHelper.h"
 #include "Handler/RequestHandler.h"
 #include "RunInTerminal.h"
+#include "lldb/API/SBDebugger.h"
 #include "lldb/API/SBStream.h"
 #include "lldb/Host/Config.h"
 #include "lldb/Host/File.h"
 #include "lldb/Host/MainLoop.h"
 #include "lldb/Host/MainLoopBase.h"
+#include "lldb/Host/MemoryMonitor.h"
 #include "lldb/Host/Socket.h"
 #include "lldb/Utility/Status.h"
 #include "lldb/Utility/UriParser.h"
@@ -504,9 +506,24 @@ int main(int argc, char *argv[]) {
     return EXIT_FAILURE;
   }
 
+  // Create a memory monitor. This can return nullptr if the host platform is
+  // not supported.
+  std::unique_ptr<lldb_private::MemoryMonitor> memory_monitor =
+      lldb_private::MemoryMonitor::Create([&]() {
+        if (log)
+          *log << "memory pressure detected\n";
+        lldb::SBDebugger::MemoryPressureDetected();
+      });
+
+  if (memory_monitor)
+    memory_monitor->Start();
+
   // Terminate the debugger before the C++ destructor chain kicks in.
-  auto terminate_debugger =
-      llvm::make_scope_exit([] { lldb::SBDebugger::Terminate(); });
+  auto terminate_debugger = llvm::make_scope_exit([&] {
+    if (memory_monitor)
+      memory_monitor->Stop();
+    lldb::SBDebugger::Terminate();
+  });
 
   std::vector<std::string> pre_init_commands;
   for (const std::string &arg :