Unify writev and no-writev code paths in lsp_pipe_writer

lsp_pipe_writer has two code paths:

* writev: convert the message byte_buffer into a byte_buffer_iovec, and
  use the writev syscall to write data to the pipe
* no-writev: convert the message byte_buffer into a string8, and use the
  write/WriteFile syscall to write data to the pipe

Having two code paths complicates future changes, such as scheduling
WriteFile on a separate thread [1]. Settle on one code path as much as
possible:

* writev: convert the message byte_buffer into a byte_buffer_iovec, and
  use the writev syscall to write data to the pipe
* no-writev: convert the message byte_buffer into a byte_buffer_iovec
  (same code as if writev is supported), and use the write/WriteFile
  syscall to write data to the pipe

This commit possibly harms performance by calling WriteFile more (with
smaller buffers) on Windows. I didn't attempt to measure this
performance difference.

[1]: #229

src/lsp-pipe-writer.cpp

@@ -36,52 +36,36 @@ char8* make_header(std::size_t message_size, char8* out) {
 lsp_pipe_writer::lsp_pipe_writer(platform_file_ref pipe) : pipe_(pipe) {}
 
 void lsp_pipe_writer::send_message(byte_buffer&& message) {
-#if QLJS_HAVE_WRITEV
   std::array<char8, max_header_size> header;
   char8* header_end = make_header(message.size(), header.data());
   message.prepend_copy(string8_view(
       header.data(), narrow_cast<std::size_t>(header_end - header.data())));
   this->write(std::move(message).to_iovec());
-#else
-  // TODO(strager): Avoid std::basic_string's bloat. (SSO will never kick in.)
-  string8 data;
-  std::size_t message_size = message.size();
-  data.resize(message_size + max_header_size);
-  char8* out = data.data();
-
-  out = make_header(message_size, out);
-  message.copy_to(out);
-  out += message_size;
-
-  this->write(
-      string8_view(data.data(), narrow_cast<std::size_t>(out - data.data())));
-#endif
 }
 
-void lsp_pipe_writer::write(string8_view data) {
-  while (!data.empty()) {
-    std::optional<int> bytes_written =
-        this->pipe_.write(data.data(), narrow_cast<int>(data.size()));
-    if (!bytes_written.has_value()) {
-      QLJS_UNIMPLEMENTED();
-    }
-    data = data.substr(narrow_cast<std::size_t>(*bytes_written));
-  }
-}
-
-#if QLJS_HAVE_WRITEV
 void lsp_pipe_writer::write(byte_buffer_iovec&& data) {
   while (data.iovec_count() != 0) {
-    ::ssize_t bytes_written =
+#if QLJS_HAVE_WRITEV
+    ::ssize_t raw_bytes_written =
         ::writev(this->pipe_.get(), data.iovec(), data.iovec_count());
-    if (bytes_written < 0) {
+    if (raw_bytes_written < 0) {
+      QLJS_UNIMPLEMENTED();
+    }
+    std::size_t bytes_written = narrow_cast<std::size_t>(raw_bytes_written);
+#else
+    const byte_buffer_chunk& chunk = data.iovec()[0];
+    QLJS_ASSERT(chunk.size != 0);  // Writing can hang if given size 0.
+    std::optional<int> raw_bytes_written =
+        this->pipe_.write(chunk.data, narrow_cast<int>(chunk.size));
+    if (!raw_bytes_written.has_value()) {
       QLJS_UNIMPLEMENTED();
     }
+    std::size_t bytes_written = narrow_cast<std::size_t>(*raw_bytes_written);
+#endif
     QLJS_ASSERT(bytes_written != 0);
-    data.remove_front(narrow_cast<std::size_t>(bytes_written));
+    data.remove_front(bytes_written);
   }
 }
-#endif
 }
 
 // quick-lint-js finds bugs in JavaScript programs.

src/quick-lint-js/lsp-pipe-writer.h

@@ -22,10 +22,7 @@ class lsp_pipe_writer {
   void send_message(byte_buffer&&);
 
  private:
-  void write(string8_view);
-#if QLJS_HAVE_WRITEV
   void write(byte_buffer_iovec&&);
-#endif
 
   platform_file_ref pipe_;
 };