Mojo Log service and a thread-safe client library.

mojo/services/log/cpp/log_client.h

Index: mojo/services/log/cpp/log_client.h
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+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef MOJO_SERVICES_LOG_CPP_LOG_CLIENT_H_
+#define MOJO_SERVICES_LOG_CPP_LOG_CLIENT_H_
+
+#include "mojo/public/c/environment/logger.h"
+#include "mojo/services/log/interfaces/log.mojom.h"
+
+namespace mojo {
+namespace log {
+
+// This is the client library that constructs a MojoLogger that talks to the

[viettrungluu, 2015/11/20 23:21:51]

*a* client library, not *the* client library

[vardhan, 2015/12/02 00:06:13]

Done.

+// Mojo logging service (see ../interfaces/log.mojo). It provides a |MojoLogger|
+// implementation for logging, which you may supply to your |mojo::Environment|.
+//
+// LogClient's life-cycle is similar to |mojo::Environment|. It is only allowed
+// to be instantiated once, and remain so for the duration of its use. The
+// constructor will initialize some internal state and take ownership of the
+// supplied |log_service|. If the pipe to the supplied |log_service| fails,
+// |LogClient| will fallback to using the supplied |fallback_logger|.
+//
+// This class itself is not thread-safe, but the MojoLogger returned by
+// LogClient::GetLogger() is thread-safe for the life time of LogClient.
+//
+// Example on how to replace the Environment's default logger with LogClient:
+//   LogPtr log_service;
+//   // .. bind log_service to proxy ..
+//   LogClient logclient(std::move(log_service),
+//                       Environment::GetDefaultLogger());
+//   Environment::SetDefaultLogger(LogClient::GetLogger());

[viettrungluu, 2015/11/20 23:21:51]

This example code seems rather bad to me.

In particular, it's not obvious that the LogClient outlives the Environment.

Now that I think of it, that static Environment::SetDefaultLogger seems like a
bad idea.

It should be up to the particular implementation of |Environment| to decide how
the default logger should be set.

[vardhan, 2015/12/02 00:06:13]

I updated the comments to use ApplicationRunner instead of referring to
Environment's logger methods.

(we also discussed offline that we should move Environment's
Get/SetDefaultLogger methods out of Environment, as public global functions in
the mojo namespace.)

+//   // At this point, |MOJO_LOG(..) <<| will start talking to the logging
+//   // service.
+class LogClient {
+ public:
+  LogClient(LogPtr log_service, const MojoLogger* fallback_logger);
+  ~LogClient();
+
+  // You can retrieve the logger before any LogClient instance is created, but
+  // any uses of the logger may assert-fail.
+  static const MojoLogger* GetLogger();
+};
+
+}  // namespace log
+}  // namespace mojo
+
+#endif  // MOJO_SERVICES_LOG_CPP_LOG_CLIENT_H_