[lldb] [Host/ConnectionFileDescriptor] Do not use non-blocking mode

[mgorny]

Summary

Disable non-blocking mode that's enabled only for file:// and serial://
protocols. We do not seem to have a really good reason for it, and it
only complicates handling partial writes.

[labath]

All read should be going through the select(2) in ConnectionFileDescriptor::BytesAvaliable, which effectively erases (non-)blocking mode differences in reading. And we do want to perform writes in blocking mode.

lldb/source/Host/posix/ConnectionFileDescriptorPosix.cpp

if (::isatty(fd)) {
 
     // Make sure only one character is needed to return from a read
     options.c_cc[VMIN] = 1;
     options.c_cc[VTIME] = 0;
 
     llvm::sys::RetryAfterSignal(-1, ::tcsetattr, fd, TCSANOW, &options);
   }
 
-  int flags = ::fcntl(fd, F_GETFL, 0);
-  if (flags >= 0) {
-    if ((flags & O_NONBLOCK) == 0) {
-      flags |= O_NONBLOCK;
-      ::fcntl(fd, F_SETFL, flags);
-    }
-  }
   m_io_sp =
       std::make_shared<NativeFile>(fd, File::eOpenOptionReadWrite, true);
   return eConnectionStatusSuccess;
 #endif // LLDB_ENABLE_POSIX
   llvm_unreachable("this function should be only called w/ LLDB_ENABLE_POSIX");
 }
 
 ConnectionStatus
#if LLDB_ENABLE_POSIX
 
   int fd = llvm::sys::RetryAfterSignal(-1, ::open, path.str().c_str(), O_RDWR);
   if (fd == -1) {
     if (error_ptr)
       error_ptr->SetErrorToErrno();
     return eConnectionStatusError;
   }
 
-  int flags = ::fcntl(fd, F_GETFL, 0);
-  if (flags >= 0) {
-    if ((flags & O_NONBLOCK) == 0) {
-      flags |= O_NONBLOCK;
-      ::fcntl(fd, F_SETFL, flags);
-    }
-  }
-
   llvm::Expected<std::unique_ptr<SerialPort>> serial_sp = SerialPort::Create(
       fd, File::eOpenOptionReadWrite, serial_options.get(), true);
   if (!serial_sp) {
     if (error_ptr)
       *error_ptr = serial_sp.takeError();
     else
       llvm::consumeError(serial_sp.takeError());
     return eConnectionStatusError;