Context package to support context-aware logging

The new context package supports context-aware logging, integrating with
logrus. Several utilities are provided to associate http requests with a
context, ensuring that one can trace log messages all the way through a
context-aware call stack.

A full description of this functionality is available in doc.go.

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

context/util.go

@@ -0,0 +1,34 @@
+package context
+
+import (
+	"time"
+
+	"golang.org/x/net/context"
+)
+
+// Since looks up key, which should be a time.Time, and returns the duration
+// since that time. If the key is not found, the value returned will be zero.
+// This is helpful when inferring metrics related to context execution times.
+func Since(ctx context.Context, key interface{}) time.Duration {
+	startedAtI := ctx.Value(key)
+	if startedAtI != nil {
+		if startedAt, ok := startedAtI.(time.Time); ok {
+			return time.Since(startedAt)
+		}
+	}
+
+	return 0
+}
+
+// GetStringValue returns a string value from the context. The empty string
+// will be returned if not found.
+func GetStringValue(ctx context.Context, key string) (value string) {
+	stringi := ctx.Value(key)
+	if stringi != nil {
+		if valuev, ok := stringi.(string); ok {
+			value = valuev
+		}
+	}
+
+	return value
+}