log: improve module-oriented logging

To improve the output of module logging, we now include a path that
represents the modules that a context has passed through. This makes it
easy to tell if a log line came from a particular module hierarchy.

A helper function, `WithModule`, provides an easy way to mark a context
with a module and inject that module into the logger.

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

log/context.go

@@ -1,6 +1,8 @@
 package log
 
 import (
+	"path"
+
 	"github.com/Sirupsen/logrus"
 	"golang.org/x/net/context"
 )
@@ -16,7 +18,10 @@ var (
 	L = logrus.NewEntry(logrus.StandardLogger())
 )
 
-type loggerKey struct{}
+type (
+	loggerKey struct{}
+	moduleKey struct{}
+)
 
 // WithLogger returns a new context with the provided logger. Use in
 // combination with logger.WithField(s) for great effect.
@@ -35,3 +40,42 @@ func GetLogger(ctx context.Context) *logrus.Entry {
 
 	return logger.(*logrus.Entry)
 }
+
+// WithModule adds the module to the context, appending it with a slash if a
+// module already exists. A module is just an roughly correlated defined by the
+// call tree for a given context.
+//
+// As an example, we might have a "node" module already part of a context. If
+// this function is called with "tls", the new value of module will be
+// "node/tls".
+//
+// Modules represent the call path. If the new module and last module are the
+// same, a new module entry will not be created. If the new module and old
+// older module are the same but separated by other modules, the cycle will be
+// represented by the module path.
+func WithModule(ctx context.Context, module string) context.Context {
+	parent := GetModulePath(ctx)
+
+	if parent != "" {
+		// don't re-append module when module is the same.
+		if path.Base(parent) == module {
+			return ctx
+		}
+
+		module = path.Join(parent, module)
+	}
+
+	ctx = WithLogger(ctx, GetLogger(ctx).WithField("module", module))
+	return context.WithValue(ctx, moduleKey{}, module)
+}
+
+// GetModulePath returns the module path for the provided context. If no module
+// is set, an empty string is returned.
+func GetModulePath(ctx context.Context) string {
+	module := ctx.Value(moduleKey{})
+	if module == nil {
+		return ""
+	}
+
+	return module.(string)
+}