encoder: reduce memory allocations and copying (#28704)

* reduce memory allocations and copying

* #include <vector>

system/loggerd/encoder/encoder.cc

@@ -27,8 +27,11 @@ void VideoEncoder::publisher_publish(VideoEncoder *e, int segment_num, uint32_t
   edat.setData(dat);
   if (flags & V4L2_BUF_FLAG_KEYFRAME) edat.setHeader(header);
 
-  auto words = new kj::Array<capnp::word>(capnp::messageToFlatArray(msg));
-  auto bytes = words->asBytes();
-  e->pm->send(e->encoder_info.publish_name, bytes.begin(), bytes.size());
-  delete words;
+  uint32_t bytes_size = capnp::computeSerializedSizeInWords(msg) * sizeof(capnp::word);
+  if (e->msg_cache.size() < bytes_size) {
+    e->msg_cache.resize(bytes_size);
+  }
+  kj::ArrayOutputStream output_stream(kj::ArrayPtr<capnp::byte>(e->msg_cache.data(), bytes_size));
+  capnp::writeMessage(output_stream, msg);
+  e->pm->send(e->encoder_info.publish_name, e->msg_cache.data(), bytes_size);
 }

system/loggerd/encoder/encoder.h

@@ -4,6 +4,7 @@
 #include <cstdint>
 #include <memory>
 #include <thread>
+#include <vector>
 
 #include "cereal/messaging/messaging.h"
 #include "cereal/visionipc/visionipc.h"
@@ -23,7 +24,6 @@ public:
 
   static void publisher_publish(VideoEncoder *e, int segment_num, uint32_t idx, VisionIpcBufExtra &extra, unsigned int flags, kj::ArrayPtr<capnp::byte> header, kj::ArrayPtr<capnp::byte> dat);
 
-
 protected:
   int in_width, in_height;
   const EncoderInfo encoder_info;
@@ -32,4 +32,5 @@ private:
   // total frames encoded
   int cnt = 0;
   std::unique_ptr<PubMaster> pm;
+  std::vector<capnp::byte> msg_cache;
 };