[lldb-dap] Partially reverting OutputRedirector changes. (#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 873426b but keeping the
refactored structure and thread management.

Author: ashgti

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();
   }
 }

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;
 };