[lldb/Core] Fix a race in the Communication class

Summary:
Communication::SynchronizeWithReadThread is called whenever a process
stops to ensure that we process all of its stdout before we report the
stop. If the process exits, we first call this method, and then close
the connection.

However, when the child process exits, the thread reading its stdout
will usually (but not always) read an EOF because the other end of the
pty has been closed. In response to an EOF, the Communication read
thread closes it's end of the connection too.

This can result in a race where the read thread is closing the
connection while the synchronizing thread is attempting to get its
attention via Connection::InterruptRead.

The fix is to hold the synchronization mutex while closing the
connection.

I've found this issue while tracking down a rare flake in some of the
vscode tests. I am not sure this is the cause of those failures (as I
would have expected this issue to manifest itself differently), but it
is an issue nonetheless.

The attached test demonstrates the steps needed to reproduce the race.
It will fail under tsan without this patch.

Reviewers: clayborg, JDevlieghere

Subscribers: mgorny, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D77295

lldb/source/Core/Communication.cpp

@@ -315,16 +315,12 @@ lldb::thread_result_t Communication::ReadThread(lldb::thread_arg_t p) {
   Status error;
   ConnectionStatus status = eConnectionStatusSuccess;
   bool done = false;
+  bool disconnect = false;
   while (!done && comm->m_read_thread_enabled) {
     size_t bytes_read = comm->ReadFromConnection(
         buf, sizeof(buf), std::chrono::seconds(5), status, &error);
-    if (bytes_read > 0)
+    if (bytes_read > 0 || status == eConnectionStatusEndOfFile)
       comm->AppendBytesToCache(buf, bytes_read, true, status);
-    else if ((bytes_read == 0) && status == eConnectionStatusEndOfFile) {
-      if (comm->GetCloseOnEOF())
-        comm->Disconnect();
-      comm->AppendBytesToCache(buf, bytes_read, true, status);
-    }
 
     switch (status) {
     case eConnectionStatusSuccess:
@@ -332,11 +328,12 @@ lldb::thread_result_t Communication::ReadThread(lldb::thread_arg_t p) {
 
     case eConnectionStatusEndOfFile:
       done = true;
+      disconnect = comm->GetCloseOnEOF();
       break;
     case eConnectionStatusError: // Check GetError() for details
       if (error.GetType() == eErrorTypePOSIX && error.GetError() == EIO) {
         // EIO on a pipe is usually caused by remote shutdown
-        comm->Disconnect();
+        disconnect = comm->GetCloseOnEOF();
         done = true;
       }
       if (error.Fail())
@@ -365,9 +362,15 @@ lldb::thread_result_t Communication::ReadThread(lldb::thread_arg_t p) {
   if (log)
     LLDB_LOGF(log, "%p Communication::ReadThread () thread exiting...", p);
 
-  comm->m_read_thread_did_exit = true;
-  // Let clients know that this thread is exiting
   comm->BroadcastEvent(eBroadcastBitNoMorePendingInput);
+  {
+    std::lock_guard<std::mutex> guard(comm->m_synchronize_mutex);
+    comm->m_read_thread_did_exit = true;
+    if (disconnect)
+      comm->Disconnect();
+  }
+
+  // Let clients know that this thread is exiting
   comm->BroadcastEvent(eBroadcastBitReadThreadDidExit);
   return {};
 }

lldb/unittests/Core/CMakeLists.txt

@@ -1,4 +1,5 @@
 add_lldb_unittest(LLDBCoreTests
+  CommunicationTest.cpp
   MangledTest.cpp
   RichManglingContextTest.cpp
   StreamCallbackTest.cpp

lldb/unittests/Core/CommunicationTest.cpp

@@ -0,0 +1,35 @@
+//===-- CommunicationTest.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/Core/Communication.h"
+#include "lldb/Host/ConnectionFileDescriptor.h"
+#include "lldb/Host/Pipe.h"
+#include "llvm/Testing/Support/Error.h"
+#include "gtest/gtest.h"
+
+using namespace lldb_private;
+
+TEST(CommunicationTest, SynchronizeWhileClosing) {
+  // Set up a communication object reading from a pipe.
+  Pipe pipe;
+  ASSERT_THAT_ERROR(pipe.CreateNew(/*child_process_inherit=*/false).ToError(),
+                    llvm::Succeeded());
+
+  Communication comm("test");
+  comm.SetConnection(std::make_unique<ConnectionFileDescriptor>(
+      pipe.ReleaseReadFileDescriptor(), /*owns_fd=*/true));
+  comm.SetCloseOnEOF(true);
+  ASSERT_TRUE(comm.StartReadThread());
+
+  // Ensure that we can safely synchronize with the read thread while it is
+  // closing the read end (in response to us closing the write end).
+  pipe.CloseWriteFileDescriptor();
+  comm.SynchronizeWithReadThread();
+
+  ASSERT_TRUE(comm.StopReadThread());
+}