[trace][intel pt] Create a common accessor for live and postmortem data

[wallace]

Summary

Some parts of the code have to distinguish between live and postmortem threads
to figure out how to get some data, e.g. thread trace buffers. This makes the
code less generic and more error prone. An example of that is that we have
two different decoders: LiveThreadDecoder and PostMortemThreadDecoder. They
exist because getting the trace bufer is different for each case.

The problem doesn't stop there. Soon we'll have even more kinds of data, like
the context switch trace, whose fetching will be different for live and post-
mortem processes.

As a way to fix this, I'm creating a common API for accessing thread data,
which is able to figure out how to handle the postmortem and live cases on
behalf of the caller. As a result of that, I was able to eliminate the two
decoders and unify them into a simpler one. Not only that, our TraceSave
functionality only worked for live threads, but now it can also work for
postmortem processes, which might be useful now, but it might in the future.

This common API is OnThreadBinaryDataRead. More information in the inline
documentation.

[wallace]

nits

[jj10306]

lldb/include/lldb/Target/Trace.h

265

typedef the callback to be cleaner and make the intention more clear?

385

Can you explain what "kind" represents and why we need the nested map? Also, I see that for live tracing we have a map for both processes and threads, why is this not the case for post mortem?

lldb/source/Plugins/Trace/intel-pt/LibiptDecoder.cpp

272–273

The couple minor changes to Libiptdecoder aren't related to this diff, maybe move these to D123106.

lldb/source/Target/Trace.cpp

265

is guaranteed that the UP will be non-null?

[wallace]

lldb/include/lldb/Target/Trace.h

265

good idea

385

those "kinds" are just arbitrarily tagged data. If you see the jLLDBTraceGetState, there's a mention to those data kinds. This is the client side of those data kinds. The difference in usage is that lldb-server will return it as a data blob in memory, and in the post-mortem case the data will be in files.

I should make those maps DenseMaps, now that I think of.

We still don't have a map for post mortem process because we haven't needed it yet, but we might soon.

lldb/source/Plugins/Trace/intel-pt/LibiptDecoder.cpp

272–273

these are needed because ArrayRef and MutableArrayRef have different const modifiers for their pointers. I like the fact that we are now passing an ArrayRef here instead of a mutable one for safety

lldb/source/Target/Trace.cpp

265

yes, otherwise there would have been an error in the previous if

[wallace]

address comments

[jj10306]

lldb/include/lldb/Target/Trace.h

380–387

Why not change all the maps to DenseMap while we're at it?

[wallace]

lldb/include/lldb/Target/Trace.h

380–387

interestingly DenseMap doesn't know how to work with std::strings as key, only with const char *. Maybe they want to force a comparison by pointer and not by string content.

lldb/include/lldb/Target/Trace.h

public:
   /// \param[in] request
   ///     The information determining which threads or process to stop tracing.
   ///
   /// \return
   ///     \a llvm::Error::success if the operation was successful, or
   ///     \a llvm::Error otherwise.
   llvm::Error Stop();
 
-  /// Get the trace file of the given post mortem thread.
-  llvm::Expected<const FileSpec &> GetPostMortemTraceFile(lldb::tid_t tid);
-
   /// \return
   ///     The stop ID of the live process being traced, or an invalid stop ID
   ///     if the trace is in an error or invalid state.
   uint32_t GetStopID();
 
+  /// Fetch binary data associated with a thread, either live or postmortem, and
+  /// pass it to the given callback. The reason of having a callback is to free
+  /// the caller from having to manage the life cycle of the data and to hide
+  /// the different data fetching procedures that exist for live and post mortem
+  /// threads.
+  ///
+  /// The fetched data is not persisted after the callback is invoked.
+  ///
+  /// \param[in] tid
+  ///     The tid who owns the data.
+  ///
+  /// \param[in] kind
+  ///     The kind of data to read.
+  ///
+  /// \param[in] callback
+  ///     The callback to be invoked once the data was successfully read. Its
+  ///     return value, which is an \a llvm::Error, is returned by this
+  ///     function.
+  ///
+  /// \return
+  ///     An \a llvm::Error if the data couldn't be fetched, or the return value
+  ///     of the callback, otherwise.
+  llvm::Error OnThreadBinaryDataRead(
+      lldb::tid_t tid, llvm::StringRef kind,
+      std::function<llvm::Error(llvm::MutableArrayRef<uint8_t> data)> callback);
+
 protected:
+  /// Implementation of \a OnThreadBinaryDataRead() for live threads.
+  llvm::Error OnLiveThreadBinaryDataRead(
+      lldb::tid_t tid, llvm::StringRef kind,
+      std::function<llvm::Error(llvm::MutableArrayRef<uint8_t> data)> callback);
+
+  /// Implementation of \a OnThreadBinaryDataRead() for post mortem threads.
+  llvm::Error OnPostMortemThreadBinaryDataRead(
+      lldb::tid_t tid, llvm::StringRef kind,
+      std::function<llvm::Error(llvm::MutableArrayRef<uint8_t> data)> callback);
+
+  /// Get the file path containing data of a postmortem thread given a data
+  /// identifier.
+  ///
+  /// \param[in] tid
+  ///     The thread whose data is requested.
+  ///
+  /// \param[in] kind
+  ///     The kind of data requested.
+  ///
+  /// \return
+  ///     The file spec containing the requested data, or an \a llvm::Error in
+  ///     case of failures.
+  llvm::Expected<FileSpec> GetPostMortemThreadDataFile(lldb::tid_t tid,
+                                                       llvm::StringRef kind);
+
+  /// Associate a given thread with a data file using a data identifier.
+  ///
+  /// \param[in] tid
+  ///     The thread associated with the data file.
+  ///
+  /// \param[in] kind
+  ///     The kind of data being registered.
+  ///
+  /// \param[in] file_spec
+  ///     The path of the data file.
+  void SetPostMortemThreadDataFile(lldb::tid_t tid, llvm::StringRef kind,
+                                   FileSpec file_spec);
+
   /// Get binary data of a live thread given a data identifier.
   ///
   /// \param[in] tid
   ///     The thread whose data is requested.
   ///
   /// \param[in] kind
   ///     The kind of data requested.
   ///
protected:
   ///
   /// The result is cached through the same process stop.
   void RefreshLiveProcessState();
 
   uint32_t m_stop_id = LLDB_INVALID_STOP_ID;
   /// Process traced by this object if doing live tracing. Otherwise it's null.
   Process *m_live_process = nullptr;
   /// tid -> data kind -> size
   std::map<lldb::tid_t, std::unordered_map<std::string, size_t>>
       m_live_thread_data;
   /// data kind -> size
   std::unordered_map<std::string, size_t> m_live_process_data;
+  /// tid -> data kind -> file
+  std::map<lldb::tid_t, std::unordered_map<std::string, FileSpec>>
+      m_postmortem_thread_data;
 };
 
 } // namespace lldb_private
 
 #endif // LLDB_TARGET_TRACE_H

lldb/source/Plugins/Trace/common/TraceSessionSaver.h

public:
 
   /// Build the processes section of the trace session description file. Besides
   /// returning the processes information, this method saves to disk all modules
   /// and raw traces corresponding to the traced threads of the given process.
   ///
   /// \param[in] live_process
   ///     The process being traced.
   ///
-  /// \param[in] raw_trace_fetcher
-  ///     Callback function that receives a thread ID and returns its raw trace.
-  ///     This callback should return \a None if the thread is not being traced.
-  ///     Otherwise, it should return the raw trace in bytes or an
-  ///     \a llvm::Error in case of failures.
+  /// \param[in] raw_thread_trace_data_kind
+  ///     Identifier for the data kind of the raw trace for each thread.
   ///
   /// \param[in] directory
   ///     The directory where files will be saved when building the processes
   ///     section.
   ///
   /// \return
   ///     The processes section or \a llvm::Error in case of failures.
-  static llvm::Expected<JSONTraceSessionBase> BuildProcessesSection(
-      Process &live_process,
-      std::function<
-          llvm::Expected<llvm::Optional<std::vector<uint8_t>>>(lldb::tid_t tid)>
-          raw_trace_fetcher,
+  static llvm::Expected<JSONTraceSessionBase>
+  BuildProcessesSection(Process &live_process,
+                        llvm::StringRef raw_thread_trace_data_kind,
                         FileSpec directory);
 
   /// Build the threads sub-section of the trace session description file.
   /// For each traced thread, its raw trace is also written to the file
   /// \a thread_id_.trace inside of the given directory.
   ///
   /// \param[in] live_process
   ///     The process being traced.
   ///
-  /// \param[in] raw_trace_fetcher
-  ///     Callback function that receives a thread ID and returns its raw trace.
-  ///     This callback should return \a None if the thread is not being traced.
-  ///     Otherwise, it should return the raw trace in bytes or an
-  ///     \a llvm::Error in case of failures.
+  /// \param[in] raw_thread_trace_data_kind
+  ///     Identifier for the data kind of the raw trace for each thread.
   ///
   /// \param[in] directory
   ///     The directory where files will be saved when building the threads
   ///     section.
   ///
   /// \return
   ///     The threads section or \a llvm::Error in case of failures.
-  static llvm::Expected<std::vector<JSONThread>> BuildThreadsSection(
-      Process &live_process,
-      std::function<
-          llvm::Expected<llvm::Optional<std::vector<uint8_t>>>(lldb::tid_t tid)>
-          raw_trace_fetcher,
+  static llvm::Expected<std::vector<JSONThread>>
+  BuildThreadsSection(Process &live_process,
+                      llvm::StringRef raw_thread_trace_data_kind,
                       FileSpec directory);
 
   /// Build modules sub-section of the trace's session. The original modules
   /// will be copied over to the \a <directory/modules> folder. Invalid modules
   /// are skipped.
   /// Copying the modules has the benefit of making these trace session
   /// directories self-contained, as the raw traces and modules are part of the
   /// output directory and can be sent to another machine, where lldb can load
   /// them and replicate exactly the same trace session.

lldb/source/Plugins/Trace/common/TraceSessionSaver.cpp

llvm::Error TraceSessionSaver::WriteSessionToFile(
   if (!os)
     return createStringError(inconvertibleErrorCode(),
                              formatv("couldn't write to the file {0}",
                                      trace_path.GetPath().c_str()));
   return Error::success();
 }
 
 llvm::Expected<JSONTraceSessionBase> TraceSessionSaver::BuildProcessesSection(
-    Process &live_process,
-    std::function<
-        llvm::Expected<llvm::Optional<std::vector<uint8_t>>>(lldb::tid_t tid)>
-        raw_trace_fetcher,
+    Process &live_process, llvm::StringRef raw_thread_trace_data_kind,
     FileSpec directory) {
 
   JSONTraceSessionBase json_session_description;
   Expected<std::vector<JSONThread>> json_threads =
-      BuildThreadsSection(live_process, raw_trace_fetcher, directory);
+      BuildThreadsSection(live_process, raw_thread_trace_data_kind, directory);
   if (!json_threads)
     return json_threads.takeError();
 
   Expected<std::vector<JSONModule>> json_modules =
       BuildModulesSection(live_process, directory);
   if (!json_modules)
     return json_modules.takeError();
 
   json_session_description.processes.push_back(JSONProcess{
       static_cast<int64_t>(live_process.GetID()),
       live_process.GetTarget().GetArchitecture().GetTriple().getTriple(),
       json_threads.get(), json_modules.get()});
   return json_session_description;
 }
 
 llvm::Expected<std::vector<JSONThread>> TraceSessionSaver::BuildThreadsSection(
-    Process &live_process,
-    std::function<
-        llvm::Expected<llvm::Optional<std::vector<uint8_t>>>(lldb::tid_t tid)>
-        raw_trace_fetcher,
+    Process &live_process, llvm::StringRef raw_thread_trace_data_kind,
     FileSpec directory) {
   std::vector<JSONThread> json_threads;
   for (ThreadSP thread_sp : live_process.Threads()) {
+    TraceSP trace_sp = live_process.GetTarget().GetTrace();
+    lldb::tid_t tid = thread_sp->GetID();
+    if (!trace_sp->IsTraced(tid))
+      continue;
+
     // resolve the directory just in case
     FileSystem::Instance().Resolve(directory);
     FileSpec raw_trace_path = directory;
-    raw_trace_path.AppendPathComponent(std::to_string(thread_sp->GetID()) +
-                                       ".trace");
-    json_threads.push_back(JSONThread{static_cast<int64_t>(thread_sp->GetID()),
+    raw_trace_path.AppendPathComponent(std::to_string(tid) + ".trace");
+    json_threads.push_back(JSONThread{static_cast<int64_t>(tid),
                                       raw_trace_path.GetPath().c_str()});
 
-    llvm::Expected<llvm::Optional<std::vector<uint8_t>>> raw_trace =
-        raw_trace_fetcher(thread_sp->GetID());
-
-    if (!raw_trace)
-      return raw_trace.takeError();
-    if (!raw_trace.get())
-      continue;
-
-    std::basic_fstream<char> raw_trace_fs = std::fstream(
-        raw_trace_path.GetPath().c_str(), std::ios::out | std::ios::binary);
-    raw_trace_fs.write(reinterpret_cast<const char *>(&raw_trace.get()->at(0)),
-                       raw_trace.get()->size() * sizeof(uint8_t));
+    llvm::Error err =
+        live_process.GetTarget().GetTrace()->OnThreadBinaryDataRead(
+            tid, raw_thread_trace_data_kind,
+            [&](llvm::MutableArrayRef<uint8_t> data) -> llvm::Error {
+              std::basic_fstream<char> raw_trace_fs =
+                  std::fstream(raw_trace_path.GetPath().c_str(),
+                               std::ios::out | std::ios::binary);
+              raw_trace_fs.write(reinterpret_cast<const char *>(&data[0]),
+                                 data.size() * sizeof(uint8_t));
               raw_trace_fs.close();
-    if (!raw_trace_fs) {
-      return createStringError(inconvertibleErrorCode(),
-                               formatv("couldn't write to the file {0}",
-                                       raw_trace_path.GetPath().c_str()));
-    }
+              if (!raw_trace_fs)
+                return createStringError(
+                    inconvertibleErrorCode(),
+              return Error::success();
+            });
+    if (err)
+      return std::move(err);
   }
   return json_threads;
 }
 
 llvm::Expected<std::vector<JSONModule>>
 TraceSessionSaver::BuildModulesSection(Process &live_process,
                                        FileSpec directory) {
   std::vector<JSONModule> json_modules;

lldb/source/Plugins/Trace/intel-pt/LibiptDecoder.cpp

CreateInstructionDecoder(DecodedThread &decoded_thread,
   pt_config_init(&config);
   config.cpu = *cpu_info;
   int status = pte_ok;
 
   if (IsLibiptError(status = pt_cpu_errata(&config.errata, &config.cpu)))
     return make_error<IntelPTError>(status);
 
   config.begin = buffer.data();
   config.end = buffer.data() + buffer.size();
 
   pt_insn_decoder *decoder_ptr = pt_insn_alloc_decoder(&config);
   if (!decoder_ptr)
     return make_error<IntelPTError>(-pte_nomem);
   PtInsnDecoderUP decoder_up(decoder_ptr, DecoderDeleter);
 
   pt_image *image = pt_insn_get_image(decoder_ptr);
   Process *process = decoded_thread.GetThread()->GetProcess().get();
 
   if (IsLibiptError(
           status = pt_image_set_callback(image, ReadProcessMemory, process)))
     return make_error<IntelPTError>(status);
   return decoder_up;
 }
 
 void lldb_private::trace_intel_pt::DecodeInMemoryTrace(
     DecodedThread &decoded_thread, TraceIntelPT &trace_intel_pt,
     MutableArrayRef<uint8_t> buffer) {
+  decoded_thread.SetRawTraceSize(buffer.size());
+
   Expected<PtInsnDecoderUP> decoder_up =
       CreateInstructionDecoder(decoded_thread, trace_intel_pt, buffer);
   if (!decoder_up)
     return decoded_thread.AppendError(decoder_up.takeError());
 
   LibiptDecoder libipt_decoder(*decoder_up.get(), decoded_thread);
   libipt_decoder.DecodeUntilEndOfTrace();
 }

lldb/source/Plugins/Trace/intel-pt/ThreadDecoder.h

 #include "DecodedThread.h"
 #include "forward-declarations.h"
 #include "lldb/Target/Process.h"
 #include "lldb/Utility/FileSpec.h"
 
 namespace lldb_private {
 namespace trace_intel_pt {
 
-/// Base class that handles the decoding of a thread and caches the result.
+/// Class that handles the decoding of a thread and caches the result.
 class ThreadDecoder {
 public:
-  virtual ~ThreadDecoder() = default;
-
-  ThreadDecoder() = default;
+  /// \param[in] thread_sp
+  ///     The thread whose trace buffer will be decoded.
+  ///
+  /// \param[in] trace
+  ///     The main Trace object who owns this decoder and its data.
+  ThreadDecoder(const lldb::ThreadSP &thread_sp, TraceIntelPT &trace);
 
   /// Decode the thread and store the result internally, to avoid
   /// recomputations.
   ///
   /// \return
   ///     A \a DecodedThread instance.
   DecodedThreadSP Decode();
 
   ThreadDecoder(const ThreadDecoder &other) = delete;
   ThreadDecoder &operator=(const ThreadDecoder &other) = delete;
 
-protected:
-  /// Decode the thread.
-  ///
-  /// \return
-  ///     A \a DecodedThread instance.
-  virtual DecodedThreadSP DoDecode() = 0;
-
-  llvm::Optional<DecodedThreadSP> m_decoded_thread;
-};
-
-/// Decoder implementation for \a lldb_private::ThreadPostMortemTrace, which are
-/// non-live processes that come trace session files.
-class PostMortemThreadDecoder : public ThreadDecoder {
-public:
-  /// \param[in] trace_thread
-  ///     The thread whose trace file will be decoded.
-  ///
-  /// \param[in] trace
-  ///     The main Trace object who owns this decoder and its data.
-  PostMortemThreadDecoder(const lldb::ThreadPostMortemTraceSP &trace_thread,
-                          TraceIntelPT &trace);
-
 private:
-  DecodedThreadSP DoDecode() override;
-
-  lldb::ThreadPostMortemTraceSP m_trace_thread;
-  TraceIntelPT &m_trace;
-};
-
-class LiveThreadDecoder : public ThreadDecoder {
-public:
-  /// \param[in] thread
-  ///     The thread whose traces will be decoded.
-  ///
-  /// \param[in] trace
-  ///     The main Trace object who owns this decoder and its data.
-  LiveThreadDecoder(Thread &thread, TraceIntelPT &trace);
-
-private:
-  DecodedThreadSP DoDecode() override;
+  DecodedThreadSP DoDecode();
 
   lldb::ThreadSP m_thread_sp;
   TraceIntelPT &m_trace;
+  llvm::Optional<DecodedThreadSP> m_decoded_thread;
 };
 
 } // namespace trace_intel_pt
 } // namespace lldb_private
 
 #endif // LLDB_SOURCE_PLUGINS_TRACE_THREAD_DECODER_H

lldb/source/Plugins/Trace/intel-pt/ThreadDecoder.cpp

 
 #include <utility>
 
 using namespace lldb;
 using namespace lldb_private;
 using namespace lldb_private::trace_intel_pt;
 using namespace llvm;
 
-// ThreadDecoder ====================
+ThreadDecoder::ThreadDecoder(const ThreadSP &thread_sp, TraceIntelPT &trace)
+    : m_thread_sp(thread_sp), m_trace(trace) {}
 
 DecodedThreadSP ThreadDecoder::Decode() {
   if (!m_decoded_thread.hasValue())
     m_decoded_thread = DoDecode();
   return *m_decoded_thread;
 }
 
-// LiveThreadDecoder ====================
-
-LiveThreadDecoder::LiveThreadDecoder(Thread &thread, TraceIntelPT &trace)
-    : m_thread_sp(thread.shared_from_this()), m_trace(trace) {}
-
-DecodedThreadSP LiveThreadDecoder::DoDecode() {
+DecodedThreadSP ThreadDecoder::DoDecode() {
   DecodedThreadSP decoded_thread_sp =
       std::make_shared<DecodedThread>(m_thread_sp);
 
-  Expected<std::vector<uint8_t>> buffer =
-      m_trace.GetLiveThreadBuffer(m_thread_sp->GetID());
-  if (!buffer) {
-    decoded_thread_sp->AppendError(buffer.takeError());
-    return decoded_thread_sp;
-  }
-
-  decoded_thread_sp->SetRawTraceSize(buffer->size());
-  DecodeInMemoryTrace(*decoded_thread_sp, m_trace,
-                      MutableArrayRef<uint8_t>(*buffer));
-  return decoded_thread_sp;
-}
-
-// PostMortemThreadDecoder =======================
-
-PostMortemThreadDecoder::PostMortemThreadDecoder(
-    const lldb::ThreadPostMortemTraceSP &trace_thread, TraceIntelPT &trace)
-    : m_trace_thread(trace_thread), m_trace(trace) {}
-
-DecodedThreadSP PostMortemThreadDecoder::DoDecode() {
-  DecodedThreadSP decoded_thread_sp =
-      std::make_shared<DecodedThread>(m_trace_thread);
-
-  ErrorOr<std::unique_ptr<MemoryBuffer>> trace_or_error =
-      MemoryBuffer::getFile(m_trace_thread->GetTraceFile().GetPath());
-  if (std::error_code err = trace_or_error.getError()) {
-    decoded_thread_sp->AppendError(errorCodeToError(err));
-    return decoded_thread_sp;
-  }
-
-  MemoryBuffer &trace = **trace_or_error;
-  MutableArrayRef<uint8_t> trace_data(
-      // The libipt library does not modify the trace buffer, hence the
-      // following cast is safe.
-      reinterpret_cast<uint8_t *>(const_cast<char *>(trace.getBufferStart())),
-      trace.getBufferSize());
-  decoded_thread_sp->SetRawTraceSize(trace_data.size());
-
-  DecodeInMemoryTrace(*decoded_thread_sp, m_trace, trace_data);
+  Error err = m_trace.OnThreadBufferRead(
+      m_thread_sp->GetID(), [&](llvm::MutableArrayRef<uint8_t> data) {
+        DecodeInMemoryTrace(*decoded_thread_sp, m_trace, data);
+        return Error::success();
+      });
+  if (err)
+    decoded_thread_sp->AppendError(std::move(err));
   return decoded_thread_sp;
 }

lldb/source/Plugins/Trace/intel-pt/TraceIntelPT.h

public:
   llvm::Error Start(llvm::ArrayRef<lldb::tid_t> tids, size_t thread_buffer_size,
                     bool enable_tsc, llvm::Optional<size_t> psb_period);
 
   /// \copydoc Trace::Start
   llvm::Error Start(llvm::ArrayRef<lldb::tid_t> tids,
                     StructuredData::ObjectSP configuration =
                         StructuredData::ObjectSP()) override;
 
-  /// Get the thread buffer content for a live thread
-  llvm::Expected<std::vector<uint8_t>> GetLiveThreadBuffer(lldb::tid_t tid);
+  /// See \a Trace::OnThreadBinaryDataRead().
+  llvm::Error OnThreadBufferRead(
+      lldb::tid_t tid,
+      std::function<llvm::Error(llvm::MutableArrayRef<uint8_t> callback)>
+          callback);
 
   llvm::Expected<pt_cpu> GetCPUInfo();
 
   /// Get the current traced live process.
   ///
   /// \return
   ///     The current traced live process. If it's not a live process,
   ///     return \a nullptr.

lldb/source/Plugins/Trace/intel-pt/TraceIntelPT.cpp

Expected<TraceSP> TraceIntelPT::CreateInstanceForLiveProcess(Process &process) {
   process.GetTarget().SetTrace(instance);
   return instance;
 }
 
 TraceIntelPT::TraceIntelPT(
     const pt_cpu &cpu_info,
     const std::vector<ThreadPostMortemTraceSP> &traced_threads)
     : m_cpu_info(cpu_info) {
-  for (const ThreadPostMortemTraceSP &thread : traced_threads)
-    m_thread_decoders.emplace(
-        thread->GetID(),
-        std::make_unique<PostMortemThreadDecoder>(thread, *this));
+  for (const ThreadPostMortemTraceSP &thread : traced_threads) {
+    m_thread_decoders.emplace(thread->GetID(),
+                              std::make_unique<ThreadDecoder>(thread, *this));
+    SetPostMortemThreadDataFile(thread->GetID(), "threadTraceBuffer",
+                                thread->GetTraceFile());
+  }
 }
 
 DecodedThreadSP TraceIntelPT::Decode(Thread &thread) {
   RefreshLiveProcessState();
   if (m_live_refresh_error.hasValue())
     return std::make_shared<DecodedThread>(
         thread.shared_from_this(),
         createStringError(inconvertibleErrorCode(), *m_live_refresh_error));
void TraceIntelPT::DoRefreshLiveProcessState(
   m_thread_decoders.clear();
 
   if (!state) {
     m_live_refresh_error = toString(state.takeError());
     return;
   }
 
   for (const TraceThreadState &thread_state : state->tracedThreads) {
-    Thread &thread =
-        *m_live_process->GetThreadList().FindThreadByID(thread_state.tid);
+    ThreadSP thread_sp =
+        m_live_process->GetThreadList().FindThreadByID(thread_state.tid);
     m_thread_decoders.emplace(
-        thread_state.tid, std::make_unique<LiveThreadDecoder>(thread, *this));
+        thread_state.tid, std::make_unique<ThreadDecoder>(thread_sp, *this));
   }
 }
 
 bool TraceIntelPT::IsTraced(lldb::tid_t tid) {
   RefreshLiveProcessState();
   return m_thread_decoders.count(tid);
 }
 
if (StructuredData::Dictionary *dict = configuration->GetAsDictionary()) {
       return createStringError(inconvertibleErrorCode(),
                                "configuration object is not a dictionary");
     }
   }
 
   return Start(tids, thread_buffer_size, enable_tsc, psb_period);
 }
 
-Expected<std::vector<uint8_t>>
-TraceIntelPT::GetLiveThreadBuffer(lldb::tid_t tid) {
-  return Trace::GetLiveThreadBinaryData(tid, "threadTraceBuffer");
+Error TraceIntelPT::OnThreadBufferRead(
+    lldb::tid_t tid,
+    std::function<llvm::Error(llvm::MutableArrayRef<uint8_t> callback)>
+        callback) {
+  return OnThreadBinaryDataRead(tid, "threadTraceBuffer", callback);
 }

lldb/source/Plugins/Trace/intel-pt/TraceIntelPTSessionSaver.cpp

if (std::error_code ec =
     return llvm::errorCodeToError(ec);
 
   llvm::Expected<JSONTraceIntelPTTrace> json_intel_pt_trace =
       BuildTraceSection(trace_ipt);
   if (!json_intel_pt_trace)
     return json_intel_pt_trace.takeError();
 
   llvm::Expected<JSONTraceSessionBase> json_session_description =
-      TraceSessionSaver::BuildProcessesSection(
-          *live_process,
-          [&](lldb::tid_t tid)
-              -> llvm::Expected<llvm::Optional<std::vector<uint8_t>>> {
-            if (!trace_ipt.IsTraced(tid))
-              return None;
-            return trace_ipt.GetLiveThreadBuffer(tid);
-          },
-          directory);
+      TraceSessionSaver::BuildProcessesSection(*live_process,
+                                               "threadTraceBuffer", directory);
 
   if (!json_session_description)
     return json_session_description.takeError();
 
   JSONTraceIntelPTSession json_intel_pt_session{json_intel_pt_trace.get(),
                                                 json_session_description.get()};
 
   return TraceSessionSaver::WriteSessionToFile(

lldb/source/Target/Trace.cpp

void Trace::RefreshLiveProcessState() {
 
   DoRefreshLiveProcessState(std::move(live_process_state));
 }
 
 uint32_t Trace::GetStopID() {
   RefreshLiveProcessState();
   return m_stop_id;
 }
+
+llvm::Expected<FileSpec>
+Trace::GetPostMortemThreadDataFile(lldb::tid_t tid, llvm::StringRef kind) {
+  auto NotFoundError = [&]() {
+    return createStringError(
+        inconvertibleErrorCode(),
+        formatv("The thread with tid={0} doesn't have the tracing data {1}",
+                tid, kind));
+  };
+
+  auto it = m_postmortem_thread_data.find(tid);
+  if (it == m_postmortem_thread_data.end())
+    return NotFoundError();
+
+  std::unordered_map<std::string, FileSpec> &data_kind_to_file_spec_map =
+      it->second;
+  auto it2 = data_kind_to_file_spec_map.find(kind.str());
+  if (it2 == data_kind_to_file_spec_map.end())
+    return NotFoundError();
+  return it2->second;
+}
+
+void Trace::SetPostMortemThreadDataFile(lldb::tid_t tid, llvm::StringRef kind,
+                                        FileSpec file_spec) {
+  m_postmortem_thread_data[tid][kind.str()] = file_spec;
+}
+
+llvm::Error Trace::OnLiveThreadBinaryDataRead(
+    lldb::tid_t tid, llvm::StringRef kind,
+    std::function<llvm::Error(llvm::MutableArrayRef<uint8_t> data)> callback) {
+  Expected<std::vector<uint8_t>> data = GetLiveThreadBinaryData(tid, kind);
+  if (!data)
+    return data.takeError();
+  return callback(*data);
+}
+
+llvm::Error Trace::OnPostMortemThreadBinaryDataRead(
+    lldb::tid_t tid, llvm::StringRef kind,
+    std::function<llvm::Error(llvm::MutableArrayRef<uint8_t> data)> callback) {
+  Expected<FileSpec> file = GetPostMortemThreadDataFile(tid, kind);
+  if (!file)
+    return file.takeError();
+  ErrorOr<std::unique_ptr<MemoryBuffer>> trace_or_error =
+      MemoryBuffer::getFile(file->GetPath());
+  if (std::error_code err = trace_or_error.getError())
+    return errorCodeToError(err);
+
+  MemoryBuffer &trace = **trace_or_error;
+  MutableArrayRef<uint8_t> trace_data(
+      // The libipt library does not modify the trace buffer, hence the
+      // following cast is safe.
+      reinterpret_cast<uint8_t *>(const_cast<char *>(trace.getBufferStart())),
+      trace.getBufferSize());
+  return callback(trace_data);
+}
+
+llvm::Error Trace::OnThreadBinaryDataRead(
+    lldb::tid_t tid, llvm::StringRef kind,
+    std::function<llvm::Error(llvm::MutableArrayRef<uint8_t> data)> callback) {
+  if (m_live_process)
+    return OnLiveThreadBinaryDataRead(tid, kind, callback);
+  else
+    return OnPostMortemThreadBinaryDataRead(tid, kind, callback);
+}