log: provide simple context logging

We consolidate simple context logging for use across packages. For now,
one still needs logrus, but we can take this further and avoid the
import at a later time.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

log/context.go

@@ -0,0 +1,37 @@
+package log
+
+import (
+	"github.com/Sirupsen/logrus"
+	"golang.org/x/net/context"
+)
+
+var (
+	// G is an alias for GetLogger.
+	//
+	// We may want to define this locally to a package to get package tagged log
+	// messages.
+	G = GetLogger
+
+	// L is an alias for the the standard logger.
+	L = logrus.NewEntry(logrus.StandardLogger())
+)
+
+type loggerKey struct{}
+
+// WithLogger returns a new context with the provided logger. Use in
+// combination with logger.WithField(s) for great effect.
+func WithLogger(ctx context.Context, logger *logrus.Entry) context.Context {
+	return context.WithValue(ctx, loggerKey{}, logger)
+}
+
+// GetLogger retrieves the current logger from the context. If no logger is
+// available, the default logger is returned.
+func GetLogger(ctx context.Context) *logrus.Entry {
+	logger := ctx.Value(loggerKey{})
+
+	if logger == nil {
+		return L
+	}
+
+	return logger.(*logrus.Entry)
+}