Introduce Environment to webrtc codebase

Bug: webrtc:15656
Change-Id: I6da85b8dbd22627d04a2f2605ce93d578b12bd4b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/327560
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#41192}

api/environment/BUILD.gn

@@ -0,0 +1,20 @@
+# Copyright (c) 2023 The WebRTC project authors. All Rights Reserved.
+#
+# Use of this source code is governed by a BSD-style license
+# that can be found in the LICENSE file in the root of the source
+# tree. An additional intellectual property rights grant can be found
+# in the file PATENTS.  All contributing project authors may
+# be found in the AUTHORS file in the root of the source tree.
+
+import("../../webrtc.gni")
+
+rtc_source_set("environment") {
+  visibility = [ "*" ]
+  sources = [ "environment.h" ]
+  deps = [
+    "..:refcountedbase",
+    "..:scoped_refptr",
+    "../../rtc_base/system:rtc_export",
+  ]
+  absl_deps = [ "//third_party/abseil-cpp/absl/base:nullability" ]
+}

api/environment/OWNERS

@@ -0,0 +1,15 @@
+# Environment has a limited visibility for stronger control what utilities are
+# exposed through it.
+# Utilities exposed through environemnt
+# - should be helpful for various WebRTC sub components.
+# - should be thread safe.
+# - should have a default implementation.
+# - should provide functionality different to existing utilities in the
+#   environemnt.
+# - should need at most one instance per peer connection.
+set noparent
+include ../../OWNERS_INFRA
+
+danilchap@webrtc.org
+hta@webrtc.org
+mbonadei@webrtc.org

api/environment/environment.h

@@ -0,0 +1,141 @@
+/*
+ *  Copyright 2023 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+// This header file provides wrapper for common WebRTC utilities.
+// Different application may need different implementations of these utilities,
+// Moreover, single application may need to use WebRTC for multiple purposes,
+// and thus would need to provide different utilities implementations for
+// different peer connections.
+// The main purpose of the `Environment` class below is to propagate references
+// to those utilities to all WebRTC classes that need them.
+
+#ifndef API_ENVIRONMENT_ENVIRONMENT_H_
+#define API_ENVIRONMENT_ENVIRONMENT_H_
+
+#include <utility>
+
+#include "absl/base/nullability.h"
+#include "api/ref_counted_base.h"
+#include "api/scoped_refptr.h"
+#include "rtc_base/system/rtc_export.h"
+
+namespace webrtc {
+
+// These classes are forward declared to keep Environment dependencies
+// lightweight. Users who need any of the types below should include their
+// header explicitely.
+class Clock;
+class TaskQueueFactory;
+class FieldTrialsView;
+class RtcEventLog;
+
+// Contains references to WebRTC utilities. Object of this class should be
+// passed as a construction parameter and saved by value in each class that
+// needs it. Most classes shouldn't create a new instance of the `Environment`,
+// but instead should use a propagated copy.
+// Example:
+//    class PeerConnection {
+//     public:
+//      PeerConnection(const Environment& env, ...)
+//          : env_(env),
+//            rtp_manager_(env, ...),
+//            ...
+//
+//      const FieldTrialsView& trials() const { return env_.field_trials(); }
+//
+//      scoped_refptr<RtpTransceiverInterface> AddTransceiver(...) {
+//        return make_ref_counted<RtpTransceiverImpl>(env_, ...);
+//      }
+//
+//     private:
+//      const Environment env_;
+//      RtpTransmissionManager rtp_manager_;
+//    };
+// This class is thread safe.
+class RTC_EXPORT Environment final {
+ public:
+  // Default constructor is deleted in favor of creating this object using
+  // `EnvironmentFactory`. To create the default environment use
+  // `EnvironmentFactory().Create()`.
+  Environment() = delete;
+
+  Environment(const Environment&) = default;
+  Environment(Environment&&) = default;
+  Environment& operator=(const Environment&) = default;
+  Environment& operator=(Environment&&) = default;
+
+  ~Environment() = default;
+
+  // Provides means to alter behavior, mostly for A/B testing new features.
+  // See ../../g3doc/field-trials.md
+  const FieldTrialsView& field_trials() const;
+
+  // Provides an interface to query current time.
+  // See ../../g3doc/implementation_basics.md#time
+  Clock& clock() const;
+
+  // Provides a factory for task queues, WebRTC threading primitives.
+  // See ../../g3doc/implementation_basics.md#threads
+  TaskQueueFactory& task_queue_factory() const;
+
+  // Provides an interface for collecting structured logs.
+  // See ../../logging/g3doc/rtc_event_log.md
+  RtcEventLog& event_log() const;
+
+ private:
+  friend class EnvironmentFactory;
+  Environment(scoped_refptr<const rtc::RefCountedBase> storage,
+              absl::Nonnull<const FieldTrialsView*> field_trials,
+              absl::Nonnull<Clock*> clock,
+              absl::Nonnull<TaskQueueFactory*> task_queue_factory,
+              absl::Nonnull<RtcEventLog*> event_log)
+      : storage_(std::move(storage)),
+        field_trials_(field_trials),
+        clock_(clock),
+        task_queue_factory_(task_queue_factory),
+        event_log_(event_log) {}
+
+  // Container that keeps ownership of the utilities below.
+  // Defining this as a RefCountedBase allows `Environment` to share this
+  // storage with another `Environment`, in particular allows `Environment` to
+  // be copyable. It is up to the `EnvironmentFactory` to provide an object that
+  // ensures references to utilties below are valid while object in the
+  // `storage_` is alive.
+  scoped_refptr<const rtc::RefCountedBase> storage_;
+
+  absl::Nonnull<const FieldTrialsView*> field_trials_;
+  absl::Nonnull<Clock*> clock_;
+  absl::Nonnull<TaskQueueFactory*> task_queue_factory_;
+  absl::Nonnull<RtcEventLog*> event_log_;
+};
+
+//------------------------------------------------------------------------------
+// Implementation details follow
+//------------------------------------------------------------------------------
+
+inline const FieldTrialsView& Environment::field_trials() const {
+  return *field_trials_;
+}
+
+inline Clock& Environment::clock() const {
+  return *clock_;
+}
+
+inline TaskQueueFactory& Environment::task_queue_factory() const {
+  return *task_queue_factory_;
+}
+
+inline RtcEventLog& Environment::event_log() const {
+  return *event_log_;
+}
+
+}  // namespace webrtc
+
+#endif  // API_ENVIRONMENT_ENVIRONMENT_H_