Instantiate http context before dispatch

Ensure that the status is logged in the context by instantiating before the
request is routed to handlers. While this requires some level of hacking to
acheive, the result is that the context value of "http.request.status" is as
accurate as possible for each request.

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

docs/handlers/context.go

@@ -3,6 +3,7 @@ package handlers
 import (
 	"fmt"
 	"net/http"
+	"sync"
 
 	"github.com/docker/distribution"
 	ctxu "github.com/docker/distribution/context"
@@ -88,3 +89,62 @@ func getUserName(ctx context.Context, r *http.Request) string {
 
 	return username
 }
+
+// contextManager allows us to associate net/context.Context instances with a
+// request, based on the memory identity of http.Request. This prepares http-
+// level context, which is not application specific. If this is called,
+// (*contextManager).release must be called on the context when the request is
+// completed.
+//
+// Providing this circumvents a lot of necessity for dispatchers with the
+// benefit of instantiating the request context much earlier.
+//
+// TODO(stevvooe): Consider making this facility a part of the context package.
+type contextManager struct {
+	contexts map[*http.Request]context.Context
+	mu       sync.Mutex
+}
+
+// defaultContextManager is just a global instance to register request contexts.
+var defaultContextManager = newContextManager()
+
+func newContextManager() *contextManager {
+	return &contextManager{
+		contexts: make(map[*http.Request]context.Context),
+	}
+}
+
+// context either returns a new context or looks it up in the manager.
+func (cm *contextManager) context(parent context.Context, w http.ResponseWriter, r *http.Request) context.Context {
+	cm.mu.Lock()
+	defer cm.mu.Unlock()
+
+	ctx, ok := cm.contexts[r]
+	if ok {
+		return ctx
+	}
+
+	if parent == nil {
+		parent = ctxu.Background()
+	}
+
+	ctx = ctxu.WithRequest(parent, r)
+	ctx, w = ctxu.WithResponseWriter(ctx, w)
+	ctx = ctxu.WithLogger(ctx, ctxu.GetRequestLogger(ctx))
+	cm.contexts[r] = ctx
+
+	return ctx
+}
+
+// releases frees any associated with resources from request.
+func (cm *contextManager) release(ctx context.Context) {
+	cm.mu.Lock()
+	defer cm.mu.Unlock()
+
+	r, err := ctxu.GetRequest(ctx)
+	if err != nil {
+		ctxu.GetLogger(ctx).Errorf("no request found in context during release")
+		return
+	}
+	delete(cm.contexts, r)
+}