[Lldb-commits] [lldb] [lldb-dap] Partially reverting OutputRedirector changes. (PR #125136)

[John Harrison via lldb-commits]

https://github.com/ashgti created https://github.com/llvm/llvm-project/pull/125136

I just noticed with these changes lldb-dap was using 200% of my CPU and root causing the issue it seems that lldb_private::Pipe::Read() (without a timeout) is using a timeout of `std::chrono::microseconds::zero()` which ends up setting the SelectHelper  timeout to `now + 0` (see https://github.com/llvm/llvm-project/blob/7ceef1b1824073fcfd4724539f5942442da1a9c2/lldb/source/Host/posix/PipePosix.cpp#L314 and https://github.com/llvm/llvm-project/blob/7ceef1b1824073fcfd4724539f5942442da1a9c2/lldb/source/Utility/SelectHelper.cpp#L46) which causes SelectHelper to return immediately with a timeout error. As a result the `lldb_dap::OutputRedirector::RedirectTo()` to turn into a busy loop.

Additionally, the 'read' call is waiting until the output buffer is full before returning, which prevents any partial output (see https://github.com/llvm/llvm-project/blob/7ceef1b1824073fcfd4724539f5942442da1a9c2/lldb/source/Host/posix/PipePosix.cpp#L325C9-L326C17).

This is not the desired behavior for lldb-dap. Instead we want a write to the FD to result in a callback to send the DAP Output event, which mean we want partial output.

To mitigate this, I'm reverting the reading operation to the previous behavior before 873426bea3dd67d80dd10650e64e91c69796614f but keeping the refactored structure and thread management.

>From d5feadb1e2188886065f2205e161c174e3cad344 Mon Sep 17 00:00:00 2001
From: John Harrison <harjohn at google.com>
Date: Thu, 30 Jan 2025 14:39:37 -0800
Subject: [PATCH] [lldb-dap] Partially reverting OutputRedirector changes.

I just noticed with these changes lldb-dap was using 200% of my CPU and root causing the issue it seems that lldb_private::Pipe::Read() (without a timeout) is using a timeout of `std::chrono::microseconds::zero()` which ends up setting the SelectHelper timeout to `now + 0`. This causes the `lldb_dap::OutputRedirector::RedirectTo()` to turn into a busy loop. Additionally, the 'Read' call is not peforming parital reads and will only invoke the redirect callback once the output buffer is filled.

This is not the desired behavior for lldb-dap. Instead we want a write to the FD to result in a callback to send the DAP Output event, which mean we want partial output.

To mitigate this, I'm reverting the reading operation to the previous behavior before 873426bea3dd67d80dd10650e64e91c69796614f but keeping the refactored structure and thread management.
---
 lldb/tools/lldb-dap/OutputRedirector.cpp | 66 +++++++++++++++---------
 lldb/tools/lldb-dap/OutputRedirector.h   |  6 ++-
 2 files changed, 46 insertions(+), 26 deletions(-)

diff --git a/lldb/tools/lldb-dap/OutputRedirector.cpp b/lldb/tools/lldb-dap/OutputRedirector.cpp
index 8fcbcfec99c4431..7935e17a653bec3 100644
--- a/lldb/tools/lldb-dap/OutputRedirector.cpp
+++ b/lldb/tools/lldb-dap/OutputRedirector.cpp
@@ -6,6 +6,9 @@
 //
 //===----------------------------------------------------------------------===/
 
+#include "OutputRedirector.h"
+#include "DAP.h"
+#include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Error.h"
 #include <system_error>
 #if defined(_WIN32)
@@ -15,45 +18,59 @@
 #include <unistd.h>
 #endif
 
-#include "DAP.h"
-#include "OutputRedirector.h"
-#include "llvm/ADT/StringRef.h"
-
 using lldb_private::Pipe;
-using lldb_private::Status;
 using llvm::createStringError;
 using llvm::Error;
 using llvm::Expected;
+using llvm::inconvertibleErrorCode;
 using llvm::StringRef;
 
 namespace lldb_dap {
 
+int OutputRedirector::kInvalidDescriptor = -1;
+
+OutputRedirector::OutputRedirector() : m_fd(kInvalidDescriptor) {}
+
 Expected<int> OutputRedirector::GetWriteFileDescriptor() {
-  if (!m_pipe.CanWrite())
+  if (m_fd == kInvalidDescriptor)
     return createStringError(std::errc::bad_file_descriptor,
                              "write handle is not open for writing");
-  return m_pipe.GetWriteFileDescriptor();
+  return m_fd;
 }
 
 Error OutputRedirector::RedirectTo(std::function<void(StringRef)> callback) {
-  Status status = m_pipe.CreateNew(/*child_process_inherit=*/false);
-  if (status.Fail())
-    return status.takeError();
+  assert(m_fd == kInvalidDescriptor && "Output readirector already started.");
+  int new_fd[2];
 
-  m_forwarder = std::thread([this, callback]() {
-    char buffer[OutputBufferSize];
-    while (m_pipe.CanRead() && !m_stopped) {
-      size_t bytes_read;
-      Status status = m_pipe.Read(&buffer, sizeof(buffer), bytes_read);
-      if (status.Fail())
-        continue;
+#if defined(_WIN32)
+  if (::_pipe(new_fd, OutputBufferSize, O_TEXT) == -1) {
+#else
+  if (::pipe(new_fd) == -1) {
+#endif
+    int error = errno;
+    return createStringError(inconvertibleErrorCode(),
+                             "Couldn't create new pipe %s", strerror(error));
+  }
 
-      // EOF detected
-      if (bytes_read == 0 || m_stopped)
+  int read_fd = new_fd[0];
+  m_fd = new_fd[1];
+  m_forwarder = std::thread([this, callback, read_fd]() {
+    char buffer[OutputBufferSize];
+    while (!m_stopped) {
+      ssize_t bytes_count = ::read(read_fd, &buffer, sizeof(buffer));
+      // EOF detected.
+      if (bytes_count == 0)
+        break;
+      if (bytes_count == -1) {
+        // Skip non-fatal errors.
+        if (errno == EAGAIN || errno == EINTR || errno == EWOULDBLOCK)
+          continue;
         break;
+      }
 
-      callback(StringRef(buffer, bytes_read));
+      callback(StringRef(buffer, bytes_count));
     }
+    ::close(read_fd);
   });
 
   return Error::success();
@@ -62,14 +79,15 @@ Error OutputRedirector::RedirectTo(std::function<void(StringRef)> callback) {
 void OutputRedirector::Stop() {
   m_stopped = true;
 
-  if (m_pipe.CanWrite()) {
+  if (m_fd != kInvalidDescriptor) {
+    int fd = m_fd;
+    m_fd = kInvalidDescriptor;
     // Closing the pipe may not be sufficient to wake up the thread in case the
     // write descriptor is duplicated (to stdout/err or to another process).
     // Write a null byte to ensure the read call returns.
     char buf[] = "\0";
-    size_t bytes_written;
-    m_pipe.Write(buf, sizeof(buf), bytes_written);
-    m_pipe.CloseWriteFileDescriptor();
+    ::write(fd, buf, sizeof(buf));
+    ::close(fd);
     m_forwarder.join();
   }
 }
diff --git a/lldb/tools/lldb-dap/OutputRedirector.h b/lldb/tools/lldb-dap/OutputRedirector.h
index 41ea05c22c69195..d2bd39797f3d75b 100644
--- a/lldb/tools/lldb-dap/OutputRedirector.h
+++ b/lldb/tools/lldb-dap/OutputRedirector.h
@@ -20,6 +20,8 @@ namespace lldb_dap {
 
 class OutputRedirector {
 public:
+  static int kInvalidDescriptor;
+
   /// Creates writable file descriptor that will invoke the given callback on
   /// each write in a background thread.
   ///
@@ -33,13 +35,13 @@ class OutputRedirector {
 
   ~OutputRedirector() { Stop(); }
 
-  OutputRedirector() = default;
+  OutputRedirector();
   OutputRedirector(const OutputRedirector &) = delete;
   OutputRedirector &operator=(const OutputRedirector &) = delete;
 
 private:
   std::atomic<bool> m_stopped = false;
-  lldb_private::Pipe m_pipe;
+  int m_fd;
   std::thread m_forwarder;
 };