proposal: log: Logger should be able to create "child" logger

[poy]

Problem

Creating a new logger is a useful pattern for using different prefixes to separate logs within a process (e.g., gRPC vs user-code).

If the output is always known (e.g., os.Stderr), this is not an issue as the developer could always use log.New(os.Stderr, "[NEW-PREFIX] ", log.LstdFlags).

However, the problem arises when instead of a known/generic value like os.Stderr, the developer wants the logger to derive from something like a Stackdriver logger.

Proposed Solution

Add a method to the Logger struct to allow a new Logger object be created with its own prefix and flags but the original writer. In the previously described example with a Stackdriver logger, the developer could propagate the logger around and create new loggers as necessary:

func Foo(ctx context.Context, param int, logger *log.Logger) {
  logger.Println("In Foo")
  Bar(ctx, logger.FromWriter("[BAR] ", log.LstdFlags))
}

[ianlancetaylor]

Would it be sufficient to add a method to Logger that returns the out field?

[poy]

That would work, however I wonder if there is any reason why it was not exported in the first place. I'm not sure if the intent was to prevent someone from writing to it directly.

[rsc]

Logger has SetFlags, SetPrefix, and SetOutput, and it has Flags and Prefix to get the flags and prefix back. Unfortunately Output does not return the writer passed to SetOutput - it is more like a Write method. But if we added a method to return the writer, it seems like that would be enough, as Ian said.

It would be nicer from an API perspective to just expose the writer and let you do what you need, instead of trying to define exactly what a "child logger" is and add that new complexity here.

We could add GetOutput but in general we avoid GetFoo names. Maybe just a new Writer() io.Writer method?

[rsc]

/cc @robpike

[poy]

@rsc That seems reasonable to me. If there is no concern about "leaking" the writer, this solves my given use-case.

[robpike]

I like the idea of a Writer() io.Writer method.

[gopherbot]

Change https://golang.org/cl/144757 mentions this issue: log: adds Writer method to Logger