[lldb] Improve isolation between Process plugins and OS plugins (llvm…

…#125302)

Generally speaking, process plugins (e.g. ProcessGDBRemote) should not
be aware of OS plugin threads. However, ProcessGDBRemote attempts to
check for the existence of OS threads when calculating stop info. When
OS threads are present, it sets the stop info directly on the OS plugin
thread and leaves the ThreadGDBRemote without a StopInfo.

This is problematic for a few reasons:

1. No other process plugins do this, as they shouldn't. They should set
the stop info for their own process threads, and let the abstractions
built on top propagate StopInfos.

2. This conflicts with the expectations of ThreadMemory, which checks
for the backing threads's info, and then attempts to propagate it (in
the future, it should probably ask the plugin itself too...). We see
this happening in the code below. The `if` condition will not trigger,
because `backing_stop_info_sp` will be null (remember, ProcessGDB remote
is ignoring its own threads), and then this method returns false.

```
bool ThreadMemory::CalculateStopInfo() {
...
  lldb::StopInfoSP backing_stop_info_sp(
      m_backing_thread_sp->GetPrivateStopInfo());
  if (backing_stop_info_sp &&
      backing_stop_info_sp->IsValidForOperatingSystemThread(*this)) {
    backing_stop_info_sp->SetThread(shared_from_this());
```

```
Thread::GetPrivateStopInfo
...
        if (!CalculateStopInfo())
          SetStopInfo(StopInfoSP());
```

To solve this, we change ProcessGDB remote so that it does the
principled thing: it now only sets the stop info of its own threads.
This change by itself breaks the tests TestPythonOSPlugin.py and
TestOSPluginStepping.py and probably explains why ProcessGDB had
originally "violated" this isolation of layers.

To make this work, BreakpointSites must be aware of BackingThreads when
answering the question: "Is this breakpoint valid for this thread?".
Why? Breakpoints are created on top of the OS threads (that's what the
user sees), but breakpoints are hit by process threads. In the presence
of OS threads, a TID-specific breakpoint is valid for a process thread
if it is backing an OS thread with that TID.

lldb/source/Breakpoint/BreakpointSite.cpp

@@ -12,6 +12,7 @@
 
 #include "lldb/Breakpoint/Breakpoint.h"
 #include "lldb/Breakpoint/BreakpointLocation.h"
+#include "lldb/Target/Thread.h"
 #include "lldb/Utility/Stream.h"
 
 using namespace lldb;
@@ -161,6 +162,8 @@ BreakpointLocationSP BreakpointSite::GetConstituentAtIndex(size_t index) {
 
 bool BreakpointSite::ValidForThisThread(Thread &thread) {
   std::lock_guard<std::recursive_mutex> guard(m_constituents_mutex);
+  if (ThreadSP backed_thread = thread.GetBackedThread())
+    return m_constituents.ValidForThisThread(*backed_thread);
   return m_constituents.ValidForThisThread(thread);
 }
 

lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp

@@ -1726,10 +1726,6 @@ ThreadSP ProcessGDBRemote::SetThreadStopInfo(
 
   if (!thread_sp->StopInfoIsUpToDate()) {
     thread_sp->SetStopInfo(StopInfoSP());
-    // If there's a memory thread backed by this thread, we need to use it to
-    // calculate StopInfo.
-    if (ThreadSP memory_thread_sp = thread_sp->GetBackedThread())
-      thread_sp = memory_thread_sp;
 
     if (exc_type != 0) {
       // For thread plan async interrupt, creating stop info on the

lldb/unittests/OperatingSystem/CMakeLists.txt

@@ -0,0 +1,21 @@
+add_lldb_unittest(ProcessGdbRemoteTests
+  GDBRemoteClientBaseTest.cpp
+  GDBRemoteCommunicationClientTest.cpp
+  GDBRemoteCommunicationServerLLGSTest.cpp
+  GDBRemoteCommunicationServerTest.cpp
+  GDBRemoteCommunicationTest.cpp
+  GDBRemoteTestUtils.cpp
+
+  LINK_LIBS
+    LLVMTestingSupport
+    lldbCore
+    lldbHost
+    lldbInterpreter
+    lldbPluginProcessUtility
+    lldbSymbol
+    lldbTarget
+    lldbValueObject
+
+  LINK_COMPONENTS
+    Support
+  )

lldb/unittests/OperatingSystem/OperatingSystemPlugin.h

@@ -0,0 +1,59 @@
+//===-- OperatingSystemPlugin.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
+//
+//===----------------------------------------------------------------------===//
+
+#include "lldb/Core/PluginManager.h"
+#include "lldb/Target/OperatingSystem.h"
+#include "lldb/Target/Thread.h"
+#include "lldb/Target/ThreadList.h"
+
+/// An operating system plugin that does nothing: simply keeps the thread lists
+/// as they are.
+class OperatingSystemIdentityMap : public lldb_private::OperatingSystem {
+public:
+  OperatingSystemIdentityMap(lldb_private::Process *process)
+      : OperatingSystem(process) {}
+
+  static OperatingSystem *CreateInstance(lldb_private::Process *process,
+                                         bool force) {
+    return new OperatingSystemIdentityMap(process);
+  }
+  static llvm::StringRef GetPluginNameStatic() { return "identity map"; }
+  static llvm::StringRef GetPluginDescriptionStatic() { return ""; }
+
+  static void Initialize() {
+    lldb_private::PluginManager::RegisterPlugin(GetPluginNameStatic(),
+                                                GetPluginDescriptionStatic(),
+                                                CreateInstance, nullptr);
+  }
+  static void Terminate() {
+    lldb_private::PluginManager::UnregisterPlugin(CreateInstance);
+  }
+  llvm::StringRef GetPluginName() override { return GetPluginNameStatic(); }
+
+  // Simply adds the threads from real_thread_list into new_thread_list.
+  bool UpdateThreadList(lldb_private::ThreadList &old_thread_list,
+                        lldb_private::ThreadList &real_thread_list,
+                        lldb_private::ThreadList &new_thread_list) override {
+    for (const auto &real_thread : real_thread_list.Threads())
+      new_thread_list.AddThread(real_thread);
+    return true;
+  }
+
+  void ThreadWasSelected(lldb_private::Thread *thread) override {}
+
+  lldb::RegisterContextSP
+  CreateRegisterContextForThread(lldb_private::Thread *thread,
+                                 lldb::addr_t reg_data_addr) override {
+    return thread->GetRegisterContext();
+  }
+
+  lldb::StopInfoSP
+  CreateThreadStopReason(lldb_private::Thread *thread) override {
+    return thread->GetStopInfo();
+  }
+};

lldb/unittests/OperatingSystem/TestThreadSpecificBreakpoints.cpp

@@ -0,0 +1,10 @@
+//===-- OperatingSystemPlugin.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
+//
+//===----------------------------------------------------------------------===//
+
+#include "OperatingSystemPlugin.h"
+LLDB_PLUGIN_DEFINE(OperatingSystemIdentityMap)