pw_metric/metric_service_pwpb.cc

// Copyright 2022 The Pigweed Authors
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may not
// use this file except in compliance with the License. You may obtain a copy of
// the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
// License for the specific language governing permissions and limitations under
// the License.

#include "pw_metric/metric_service_pwpb.h"

#include <cstring>

#include "pw_assert/check.h"
#include "pw_containers/vector.h"
#include "pw_metric/metric.h"
#include "pw_metric_proto/metric_service.pwpb.h"
#include "pw_preprocessor/util.h"
#include "pw_rpc/raw/server_reader_writer.h"
#include "pw_span/span.h"
#include "pw_status/status.h"
#include "pw_status/try.h"

namespace pw::metric {

// TODO: Make this follow the metric_service.options configuration.
constexpr size_t kMaxNumPackedEntries = 3;

namespace {

class MetricWriter {
 public:
  MetricWriter(span<std::byte> response, rpc::RawServerWriter& response_writer)
      : response_(response),
        response_writer_(response_writer),
        encoder_(response) {}

  // TODO(keir): Figure out a pw_rpc mechanism to fill a streaming packet based
  // on transport MTU, rather than having this as a static knob. For example,
  // some transports may be able to fit 30 metrics; others, only 5.
  Status Write(const Metric& metric, const Vector<Token>& path) {
    {  // Scope to control proto_encoder lifetime.

      // Grab the next available Metric slot to write to in the response.
      proto::Metric::StreamEncoder proto_encoder = encoder_.GetMetricsEncoder();
      PW_TRY(proto_encoder.WriteTokenPath(path));
      // Encode the metric value.
      if (metric.is_float()) {
        PW_TRY(proto_encoder.WriteAsFloat(metric.as_float()));
      } else {
        PW_TRY(proto_encoder.WriteAsInt(metric.as_int()));
      }

      metrics_count++;
    }

    if (metrics_count == kMaxNumPackedEntries) {
      return Flush();
    }
    return OkStatus();
  }

  Status Flush() {
    Status status;
    if (metrics_count) {
      status = response_writer_.Write(encoder_);
      // Different way to clear MemoryEncoder. Copy constructor is disabled
      // for memory encoder, and there is no "clear()" method.
      encoder_.~MemoryEncoder();
      new (&encoder_) proto::MetricRequest::MemoryEncoder(response_);
      metrics_count = 0;
    }
    return status;
  }

 private:
  span<std::byte> response_;
  // This RPC stream writer handle must be valid for the metric writer
  // lifetime.
  rpc::RawServerWriter& response_writer_;
  proto::MetricRequest::MemoryEncoder encoder_;
  size_t metrics_count = 0;
};

// Walk a metric tree recursively; passing metrics with their path (names) to a
// metric writer which can consume them.
//
// TODO(keir): Generalize this to support a generic visitor.
class MetricWalker {
 public:
  MetricWalker(MetricWriter& writer) : writer_(writer) {}

  Status Walk(const IntrusiveList<Metric>& metrics) {
    for (const auto& m : metrics) {
      ScopedName scoped_name(m.name(), *this);
      PW_TRY(writer_.Write(m, path_));
    }
    return OkStatus();
  }

  Status Walk(const IntrusiveList<Group>& groups) {
    for (const auto& g : groups) {
      PW_TRY(Walk(g));
    }
    return OkStatus();
  }

  Status Walk(const Group& group) {
    ScopedName scoped_name(group.name(), *this);
    PW_TRY(Walk(group.children()));
    PW_TRY(Walk(group.metrics()));
    return OkStatus();
  }

 private:
  // Exists to safely push/pop parent groups from the explicit stack.
  struct ScopedName {
    ScopedName(Token name, MetricWalker& rhs) : walker(rhs) {
      PW_CHECK_INT_LT(walker.path_.size(),
                      walker.path_.capacity(),
                      "Metrics are too deep; bump path_ capacity");
      walker.path_.push_back(name);
    }
    ~ScopedName() { walker.path_.pop_back(); }
    MetricWalker& walker;
  };

  Vector<Token, 4 /* max depth */> path_;
  MetricWriter& writer_;
};

}  // namespace

void MetricService::Get(ConstByteSpan /*request*/,
                        rpc::RawServerWriter& raw_response) {
  // For now, ignore the request and just stream all the metrics back.
  // TODO: Make this follow the metric_service.options configuration.
  constexpr size_t kSizeOfOneMetric =
      pw::metric::proto::MetricResponse::kMaxEncodedSizeBytes +
      pw::metric::proto::Metric::kMaxEncodedSizeBytes;
  constexpr size_t kEncodeBufferSize = kMaxNumPackedEntries * kSizeOfOneMetric;

  std::array<std::byte, kEncodeBufferSize> encode_buffer;

  MetricWriter writer(encode_buffer, raw_response);
  MetricWalker walker(writer);

  // This will stream all the metrics in the span of this Get() method call.
  // This will have the effect of blocking the RPC thread until all the metrics
  // are sent. That is likely to cause problems if there are many metrics, or
  // if other RPCs are higher priority and should complete first.
  //
  // In the future, this should be replaced with an optional async solution
  // that puts the application in control of when the response batches are sent.

  // Propagate status through walker.
  auto status = OkStatus();
  status.Update(walker.Walk(metrics_));
  status.Update(walker.Walk(groups_));
  status.Update(writer.Flush());
  raw_response.Finish(status).IgnoreError();
}
}  // namespace pw::metric